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Dave repairs his Rigol DP832 Lab Power Supply.
Why did it need repairing? That's the FAIL part Rigol might want to look into...
UPDATE: I have an old revision board. Newer ones contain a much beefier MOSFET:
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[@battletoad6896]

It is 2021, almost 5 years after this video: Silly me shorted the rails of my breadboard design. Result: - A spark occured during the short - An opamp got fried (to be expected) - Channel 1 on my DP832 didn't work anymore (not to be expected) I remembered this video from years ago and rewatched it with regard to my case: - Same error indications - Same faulty FET and fuse - Replacement of both parts repaired my DP832! Thanks Dave that you shared this video which helped me out with the troubleshooting and a cheap repair. As if you had known that I was going to goof up my project five years later :-).

[@brucewoods9377]

Dave as a technician for over fourth years the saying in the trade was.... "Engineers can design, but have no idea of repair." The Fuse blew, that was the protection, what caused it to blow? You simply replaced the fuse without tracking down the cause. You never even measured the FET so you had to go back in again.

[@craigs5212]

Dave, I had the same issue with a 2N3055 supply I built years ago. The issue was caused by the response time for the current limit circuit. A quick short caused the device to operate out of its safe area of operation VI region for a short period of time. It was quite device dependent, my channel 2 device never had the problem but I killed 3 channel 1 pass transistors on one week. Craig

[@ericclark9770]

Will be interesting to see what the folks at Rigol will have to say about this documented component failure or the "dodgy" solder job on that component. Keep us updated, Dave!

[@fuzzy1dk]

you should always screw transistor on a heatsink before soldering to avoid putting stress on the solder joints, so it makes sense if the transistor was soldered on manually after the big heatsink and transistor had been mounted

[@bohelsted7093]

Was just thinking the same; and Dave said it when he replaced the faulty FET. They can't run the board through soldering with screws and heat sinks on. Another thing: What's the fuzz about the flux? Could be an issue in high impedance design, but the gate looks into the output of a low impedance op-amp (or the like). No worries from my point of view.

[@FranLab]

Love that FAIL button!

[@kyoudaiken]

+Fran Blanche Oh its Fran! Again. In the comment box. :D Good on you Fran!

[@EEVblog]

+BloodySword Everyone loves Fran!

[@pvc988]

Damn you Dave. It's 4 AM, I was just going to sleep but you had to upload new video.

[@MegaFPVFlyer]

Watching this reminded me just how amazing my PSU is. It's one of the $50 eBay 30v 5A linear power supplies. I tried leaving it with a 5A load to see how it reacted, I saw a thermal switch on the heat sink (a single plate of metal) and assumed that would shut off the output. It actually turned on the fan when the plate got above 80 degrees C or something. Not that that kept the temp from rising further. My guess is the thing would let out the smoke after 15 minutes at full load. *Now that's quality*

[@MegaFPVFlyer]

Oh and my first PSU was a switching one of similar quality. I killed it by powering a DC-DC converter. Somehow. I'm pretty sure I managed to repair it by replacing a blown MOSFET, but then it died again and it had to go in the bin.

[@MrOpenGL]

I have a "LONG WEI" 30V/5A supply I got for 45€. It had the fan that ran forever, so I put an 80°C thermal switch on the heatsink and a 55°C switch on the transformer (so if either of these get hot it will turn on the fan). I also put a 95°C switch on the heatsink so that if it ever gets THAT hot, it will blow the fuse (so that I am force to troubleshoot why it got so hot). I also partially covered the vents on the side leaving about 2cm open towards the front on both sides so it passes all the air around the transformer, and changed the JWCO caps with Elna 105°C ones. Best 45€ ever spent. This thing is EXTREMELY robust. I used it to charge a 250Ah/12V battery bank (with the current pot set to maximum!!!). Of course it took 4 days to do that, but during those 4 days it was on 24/7, running at 5A and 14.4V and it never blew the fuse! I have also abused it in several other ways, including using it to power self-oscillating relays, shorting the output accidentally too many times, using it to overrev DC motors, powering "dirty" circuits like flyback inverters etc. It is still going strong after 4 years of abuse. I usually also leave it on overnight to charge batteries at low currents (so the fan doesn't come on) without any worries. The only thing that happens is that when it gets hot the voltage display drifts a bit. The regulation is rock solid however, so I usually set it when it's cold and then leave it like this.

[@martinda7446]

Having worked on many audio amps, lots with MOSFET output stages, I have witnessed the occasional failure from previously stressed components. Possibly, with the clue here that the soldering was done by hand, this could have been a dodgy FET that found its way into a production unit? However these devices became ''weakened'' , occasionally, they seem to perform ok until asked to do a bit of heavy work ie, a fast transient in a difficult load whereupon they catastrophically fail. Maybe the gate had partially broken down or almost had a hole blown through, but survived to a few atoms - who knows. Maybe it was just one of those.

[@phillyphakename1255]

I'm guessing they did factory testing of a known issue. Sure, half of this batch was dodgy, we'll factory replace it if it fails the stress test. Oh, it failed? Replace it. Test it again. It passed? Ship it! Only now you have a component that is known to be borderline by nature of the batch, and has been stressed by the factory testing, and now Dave puts sparks on it. It's kinda no wonder why it failed.

[@ratdude747]

I bet since that heatsink was screwed to the board, they chose to hand solder the pass MOSFET and heatsink assembly as a later step as I bet the heatsink-to-board screws wouldn't like wave soldering. Not to mention that without the screws that heatsink would put all of the mechanical load on the MOSFET leads. The other heatsink (the BJT) has solder tabs for mounting, and since it's a "wrap around" flush-to-board type heatsink (and a smaller one too), it can be safely placed on the board before wave soldering. Did the old board also have hand-soldering evidence before you pulled the MOSFET? Also, did the relay have a flyback diode/resistor built in? If not, that could be the issue. A coil (relay, solenoid, etc.) without a flyback/snub diode/resistor will blow all sorts of things to bits, even BJTs (or so I was taught in college).

[@ethanpoole3443]

Except that the only inductance would be in the leads between the relay contacts and the power supply. It was not the channel powering the relay coil but the relay contacts alone that blew, so no coils involved, just a few feet of wire for the alligator clips, so not much inductance there. Plus there is several hundred microfarads on the power supply output, so even if there were an inductive spike, that is a lot of capacitance to overcome before reaching harmful voltage levels. At this point I would be more suspicious of inductance in the power supply's own transformer, though, again, there is an awful lot of input filter capacitance to overcome, which ought to make that an unlikely source as well. It is very possible, even highly likely, that the MOSFET was previously damaged by ESD in manufacturing or assembly (prior to final incorporation into the PCB) and had already been heavily damaged and the repeated shorts just finished off what good silicon remained.

[@fallenaege4403]

+Ethan Poole yep, and they didn't even bother to clean up the flux.

[@EEVblog]

+Larry Bolan As Ethan said. It's the contact channel that blew, only lead indictance involved. It's most likely a control loop response time thing taking out a MOSFET that couldn't handle it.

[@ratdude747]

+Ethan Poole +EEVBLOG Doh! Wrong side of the relay... I feel really stupid. Knowing that, I concur with dave that the control loop wasn't designed to handle such spikes. That's the trap with digital "advanced" DC supply design; if you just use a plain feedback 1-transistor (or FET) supply circuit, you have no short circuit protection and rely on the speed of your microcontroller to perform current limiting. There is a three-transistor circuit that I remember from school that does have short-circuit protection built in; however, in a case like this that may not be the desired functionality.

[@morte3252]

3:18 黑：black 红：red It's probably good for assembly.

[@asj3419]

It is strange that they decided to use chinese caracters instead of something more universal (+ , -). I guess whatever works.

[@TheModerGuy]

sorry to disturb the dead but, they use black and red labels because the underpaid Chinese assembly workers don't know anything about electronics. they simply just put the square peg in the square hole and the round peg in the round hole; likewise the red cable in the spade labeled "red"

[@dougmcartin3881]

Inductive kickback from the collapsing field of the coil. Sounds like a trap for beginners. Lots of volts there.

[@animefreak5757]

+Doug McArtin the coil was on channel 2, which didn't fail

[@garthhowe297]

This was one of my favourite videos. I really enjoy watching (and learning) your process of troubleshooting.

[@user-tr3qt3qs9t]

I believe that this fault has nothing to do with the relay or the load you applied because I have seen a pretty similar problem with an other PSU. The source of a problem was just an unwashed rosin-based flux believe it or not. It looks like after some time rosin tends to recrystallize and absorb moisture. I have seen like a similar blob of flux between pads like this apeared to measure only a couple of kOhms. In your case I think the MOSFET just got fully opened by the leakage to the gate loosing any means of regulation and producing too much heat and in the end it just died before it could be saved by the fuse. And the worst part is that the damn thing will work just after production. It will pass any automated testing. And once you reheat the flux on the joint with a soldering iron the conductivity vanishes and the problem goes away again. I had to cut the device I had issues with of its pins co confirm that it really was the case (I was "lucky" to have a couple of similar failed chineese PSU modules). The moral of this story is that leaving crap and junk like this on a board contaning any high impedance circuits is just asking for a troube to happen after a couple of moths. So just wash your boards and everything should be fine.

[@ham4ham71]

+Кирилл Рагузин I agree and have seen this happen to me.

[@BenjaminEsposti]

I've seen the same thing happen. In fact, the flux conducted enough to start burning the circuitboard, creating a carbon track! O.o

[@krisztianszirtes5414]

+Benjamin “Ozias” Esposti Gee, carbon tracks are pretty dangerous once we are talking about mains. I hope it was not under mains.

[@BenjaminEsposti]

+Krisztián Szirtes Nope, it was 12V!

[@krisztianszirtes5414]

+Benjamin “Ozias” Esposti Well that's a lucky case then. If it becomes graphite it can do some nasty little ( and lethal) things.

[@bryanneo]

At 13:22 the chinese words are 黑 and 红 which means black and red respectively.....

[@ivolol]

To tell the assemblers how to assemble :D

[@mcconkeyb]

I'm a bit surprised by your comments about "a lab supply of this price and quality". If you had worked in any electronics design team (...and I mean any, including the most prestigious military ones too) you'd know that now a days there is no thing as careful design. It is all driven by cost and schedule and the over riding paradigm is get it done yesterday for 10% less than tomorrow. The only reason things are expensive is mostly due to branding mark-up. This $1200 unit most likely cost around $100 or less to produce. So there should be no surprises when it fails due to (most likely) inadequate product and/or production testing.

[@niceguy60]

If your going to use your power supply to trip mechanical relays at least only do so by applying and removing the lead instead, do not deactivate the relay by simply hitting the power switch on the power supply as that sends a massive voltage spike back through the output of the supply which is what happened here. Or you can simply buy mechanical relays with flywheels built into them.

[@herbertsusmann986]

I always put a 15v zener diode on those MOSFETs between Gate and Source. I put it right at the transistor also. I didn't see one on that board. All it takes is a fast voltage spike to punch through the G-S insulation and powee, there goes your transistor! Maybe a small value gate resistor is also a good idea too?

[@memylastname9972]

Glad it’s not just me that starts to fix some thing and end up fixing two other things I’m using before the finish ! Great to watch thanks

[@khronscave]

As a matter of course, thou shalt not replace fuses before investigating what made them blow in the first place. Otherwise, you risk blowing the new one, and possibly even causing (further) damage to the circuitry downstream.

[@imaginarypoint]

For those playing at home. Have the save PS. Same issue... but it was only the fuse that crapped the bed. Transistor was ok. Tested ok from min current to max current of channel1 using same 8500 electronic load. Thanks Dave!

[@tmurphy26a]

One of the best videos I have enjoyed from you. Glad to hear your opinion on the benefits between the BJT and the MOSFET. In school I spent most of my time playing with 2N3904, and very little time with the MOS. Thanks!

[@proluxelectronics7419]

Maybe you have transient spikes on your line voltage, Something upset the BK 8601. Big Thumbs Up.

[@bernardbernardin5187]

+Mike James Maybe the power plug was wiggling. I once killed the MOV and the auxiliary rail switcher IC of a non isolated SMPS that way.

[@proluxelectronics7419]

True, I have seen arcing primary side dry joints due to a loose live connection in plug.

[@JimGriffOne]

Thanks for the video, Dave. I was thinking about buying one of these and the information will help if the issue happens to me in the future. I will still definitely buy one of these power supplies as they are within my price range and do the job that I want.

[@loughkb]

If you revisit the relay video attempt, put a charged big-ass capacitor across the normally open contacts. Get a nice spark!

[@user-ws2iw3fv5n]

I have one of these in the A version. The current limiting is quite slow. I was using it to test some LEDs with the limit set to 10 mA and 5V. The first one I tested was fine, but then I blew 3 in a row before I figured that the current limit was just a suggestion for the initial few milliseconds. Unfortunately, Rigol was what I could afford. The control software over the network is also very bad, although they did update it and it is improved.

[@AldoBr549]

I believe this is the first video of yours I've watched where a repair was successful. Good job Dave! Love it. Also, I'm glad I'm not the only one that has equipment fail randomly and for no good reason.

[@GeorgeTsiros]

to me, it makes sense to have the 30 V channel paired with the 5 V channel on the same PCB instead of going for symmetry and putting both 30 V channelss on the same PCB. Spread the heat-load (or however you might call it).

[@loughkb]

I'm at the 15 minute point in the video, the close-up shot of the transistor. It looks like a poor solder joint on the rightmost pin.

[@ericastier1646]

Dave sounds like an old nice grandma in voice intonation, good integrity.

[@theradiomechanic9625]

Dave, Chinese built unit likely has some counterfeit Chinese transistors used in it.Therefore the pass transistor might not have met spec. Heat from dead short was just too much for the FET.Hand soldering is not such a surprise as the Heat sink has to go on after Wave Soldering and the transistor gets mounted on the heat sink then soldered.

[@AngDavies]

Have killed MOSFETs in a power supply before, seems the problem is less shorting, but shorting indecisively: on, off and then on again in short succession. My understanding being that the turn off causes a smallish back emf that causes the freewheel diode inside the transistor to conduct, but because it's a diode, and has a reverse recovery time, if the output is shorted again during that period, it's gonna conduct a lot of current till the diode recovers fully, which can be too late to save the transistor. So maybe the relay bounced/chattered? To protect it you'd probably need a freewheel diode that was simultaneously really fast, but with a lower voltage than the body diode, so it never gets conducting Schottky?

[@dickcheney6]

I wonder if the "response time" of the current sense + limiting circuitry has something to do with this, like maybe the FET is being overloaded just before it goes into CC, which is survivable as a one shot event, but causes a failure when done repeatedly

[@RickYorgason]

Gah, now you've got me going down the rabbit hole of trying to understand bleeder resistors and why that one doesn't affect the current sense. The fact that it's on the negative terminal doesn't really explain it for me.

[@RickYorgason]

Ah, I get it now. When you said it's on the other side, you meant it's *before* the sense resistor. And it's not in series with the load as it looks in the video, it's across the two rails (which you said, but which I didn't appreciate at first). Got it.

[@Smith8340]

You do a great job with these videos Dave. I love all your additional bits of info, as they can certainly help with repair of unrelated items. I learn so much from your efforts, and really appreciate you taking the time to pass on your knowledge.

[@erikthorsen328]

1975 while serving in the U.S.Navy overseas a USO band on tour came to our little military base. While playing a set the bass guitar amp blew up. It was an Acoustic 360 if I recall correctly. One of the guys who lived in the barracks across the parking lot from the club brought his bass amp over to the club so the band could continue that night. We hauled his amp out to the workshop and sure enough the output transistors were blown. Yes, the 2N3055 came to save the day! popped them in and were jammin'!

[@bohelsted7093]

2N3055 was the go-to transistor in many instrument amps then. And in some hifi gear too... The famous J. Linsley Hood class A amp, and the NAD 3020 (3055/2955). Their limit is really the Safe operating area, so above appr. 50 W output, the 2N3442 was a better choice. Kind of miss the days when an amp was 5-6 transistors, an output stage and a huge power supply...

[@erikthorsen328]

+Bo Helsted Yes I know exactly what you are talking about. My Pioneer SPEC 2 power amp is nothing but huge caps, a massive torridal transformer, TO-3 (6 per side) output transistors and massive heat sinks. 60+ pounds! I bought it while overseas in the PI in 1975 and it is still going strong. Probably be an item contested in my will when I pass on. And to think that we have Bob Carver and the Phase Linear 400 power amp that started it all.

[@pratwurschtgulasch6662]

i bought a power supply a few days ago and thanks to your reviews it was neither the siglent nor the rigol.

[@mausball]

Odds are the series pass transistor was hand soldered because it was bolted to the heatsink in a jig, and then the assy is bolted to the PCB, then the transistor is soldered in place. Very normal for that kind of thing that I've seen. And odds are they used no-clean flux.Not that that makes it ok....

[@Cisco8484]

Dang, I just bought one a month ago. I figured if it was good enough for Dave, it was good enough for me. Maybe I should have purchased a used HP on ebay.

[@darkriku12]

Eh, any component can fail at any time but pass all tests before shipping. It's why there's warranties!

[@tubical71]

That´s why i prefer the old, calssic, vintage, all analog power supplies as my standard lab supply. If they fail....i know why and it would be easily repairable.... If these fancy-pancy with nonsense bells and whizzles added USB/eth, digital, 7segment, all uC controlled rubbish fails...naah...too much hassle. If one of these fails due to a ground rail spike, i´ll bet the whole thing will fail and went imediately BER.

[@FrancisRodgers]

This is really cool. I am a noob to electronics and didn't understand half of what you were talking about, but I really learned a lot from the problem solving process you went through. Eventually, if I listened to enough electronics videos, I know I'll eventually understand all the jargon. Thanks Dave.

[@kieferonline]

I am also studying for a KZbin degree in electronics by watching these videos. This topic is so interesting!

[@BenjaminEsposti]

+Francis Rodgers You still need in-field experience - that in itself is the best kind of experience. Some colleges literally teach you the math, then send you out with a degree, without even getting you familiar with the difference (appearance-wise) between a resistor and a capacitor!

[@FrancisRodgers]

+Benjamin “Ozias” Esposti - As a software engineer, I understand and agree fully with what you are saying. This is why I will remain a lifetime member of KZbin University.

[@44R0Ndin]

If I was Rigol, I'd add a snubber network across source and drain, and then a low-pass filter on the gate pin right next to the output MOSFET to prevent self-oscillation (long traces can act as antennas). Mains power surges are unlikely in this specific case, as the supply is linear and uses a large (laminated iron) toroidal power transformer. This means that it's horrible at passing high frequency noise from the power input jack to anything that cares about it, including the digital logic.

[@btmiller14]

i think dave you need to get a local preist to do an exorcism of the lab just in case

[@EEVblog]

+Bruce Miller Yep. Need to summon the spirit of Bob Pease to do battle with the dark forces.

[@scottfirman]

check that power cord,if it blew the fuse in the other piece of equipment,mabe it caused a spike or short in your first event,also check your outlet,power strip or ground. the chances of having two problems with the same cord makes me suspect the cord.I have had that happen before,turned out the cord had an internal short in the plug,strange too,it wasnt molded in thats why it shorted,it allowed the wires to move and contact each other,but only occasionally.

[@pauljs75]

+scott firman That's what I'm thinking. If not the strip and gremlins are still showing up in more than one piece of equipment, I'd also look at what's going on with the mains coming from the wall. Could even be Dave's solar panel stuff switching, if not coming down the line from the power company.

[@BenjaminEsposti]

+scott firman My guess would be that the fuse is rated too low, or it isn't a time-delay fuse. The power supply in that thing will surely draw much more than half an amp, which may unexpectedly blow a weak fuse upon turn-on. (I've had this happen before :P) Some things have some serious inrush current. In fact, I thought I had a dead toroidial transformer, but it turned out that the thing had such a low impedance that the magnetizing current spike was enough to trip the circuitbreaker in our breaker panel!

[@smallenginedude71]

Blue smoke technology is what runs all solid state components. i just finished building a power supply today with four 2n3055 transistors. handles 20A. i am using a rewound MOT which gets nice and toasty as you would expect.

[@Pelnied]

At first I was worried because the video update at 15:33 says that the newer Power MOSFET was introduced after Nov 2015. Good thing I went to the forums because everywhere else says Nov 2013. I didn't want my DP832 that I got last Summer to be prone to transistor failure!

[@gl1500ctv]

In your best Dave accent: " Something must have blow-un! All flapping around in the breeze and Bob's your uncle!" This is priceless!

[@electronicsNmore]

Collapsing magnetic fields from relay coils/solenoids can be very destructive. I learned the hard way years ago. :-) Probably better off replacing the 5A fuse with a 5A polyswitch.

[@daveblane6442]

+electronicsNmore LOL. that would have affected the 12 volt source! NOT the 30 v.

[@ElectronicNoobBlog]

+electronicsNmore No, he was apply 30V 3A to contacts side - not coil side so no magnetic field - spikes happen. I would not go for polyfuses in that case, he have 5A fuse so it's way up 3A limited. Only scenario when this fuse will will go open is hardware fail - not done by user - so the best scenario is cut power. Poly will turn power again after getting cold. They are good for protecting output when You can accidentally short leads without current limiting.

[@electronicsNmore]

Dave Blane Well he mentions in the video "possibly" back EMF, talks about a blocking diode in the PSU, so clearly it was not back EMF if he did not connect the relay coil to the Rigol PSU. The PSU mosfet simply did not like the 30V/5A shorted output. Maybe he connected up the relay coil off camera to quickly test it, then later discovered the problem. Who knows.

[@BenjaminEsposti]

+Electronic Noob Blog Incorrect. The wire to the contacts is still an inductor, and it can create very fast, high frequency transients because of the low inductance.

[@minkorrh]

I read about this recently...it has something to do with the collapsing field and the voltage/current generated within the inductor and sends it backwards?

[@ericastier1646]

the relay has a bobin which can generate high voltage, and MOSFETS under excessive voltage can have current carriers punch through and short to the gate. ALso by exceeding the output supply voltage you forward biased the MOSFET internal source to substrate diode.

[@stickycricket2]

Did the same thing to a Mastech HY3005F-3 power supply, was shorting out channel 1 and it took a crap on me. I took it apart to see if I could find anything, not knowing anything about a series pass transistor (still learning) and I found nothing obviously wrong, when I powered it up again it worked fine.

[@krishnanaishadham4520]

I got a new Rigol DP 832 in 2021 and I am enjoying using it. Hopefully this problem that Dave mentioned will not happen in the newer units. One criticism that I have of Rigol is the lousy user guide (manual). They put everything under the sink in the guide without clear steps on how to quickly get to a particular feature and set it up. It would have been nice if they gave examples for setting pulsed waveforms. I need to pulse the current to a PCB heater circuit for a sensor application. If there is any KZbin video on this I would appreciate the info. Thanks.

[@LewisAvinash]

I had to change the same MOSFETs on pc SMPS many times... have quite a few on my cabinet.. Thanks for the video... Keep it u[

[@esnam6557]

Very nice video, thank you very much. You may need to order more CEP80N15 MOSFETs to due more failure in future!

[@BarriosGroupie]

The problem is possibly the speed of the short and whether the short protection circuit is fast enough to respond.

[@andymouse]

your right, but I think the point here is that it should be fast enough.

[@OsmosisHD]

Still weird, even more weird that the stand alone DC load suddenly blew it's fuse. Makes me think, what if it's not the output what's caused the issue with the legendary killer relay But the input, how stable is the AC outlet, maybe it spikes every now and then like crazy?

[@suppersreadysuppers1822]

Low Quality Mosfet ?

[@KieranHarkin]

I was loading down on a 10W resistor the other day glad mine didn't let the magic smoke out.

[@proyectosledar]

thanks Dave. I want that killer relay xD

[@Arek_R.]

Dave, I said you earlier that these chinese letters in rigol eq are colors of the wires, for assemly.

[@xx3868]

I love the 2n3055 15 amp transistor as it old fashioned but solid. I actually used one to make a spare ignition igniter for my old 30 year car to trigger the coil. Works perfectly and runs cool.

[@H-77]

Single most common failure in a linear supply (aside from filter caps after a certain number of years) is indeed the series pass transistor. Pretty much the first thing to check on a dead linear supply is a shorted series pass transistor.

[@MatthewSuffidy]

Bad day at the office. Dave was super lucky he did not blow his replacement board fuse when he was looking around for heat and whatnot. I guess he had another sort of non-functional board somewhere and he salvaged the mosfet?

[@thomasstran]

Single TO-220 device for a 90W power supply? Seems a bit wimpy to me.

[@jameswong7327]

I used two TO-247 MOSFET for my 30V5A/15V10A 150W-max bench power supply to get better thermal performance. With multi-tapping transformer, the max dropout power was < 50W.

[@Arek_R.]

I can dissipate 250W in such a package, though its back was soldered to piece of copper before going onto the heatsink.

[@Zadster]

If gear keeps failing, then I would be tempted to do some mains power quality monitoring. It sounds like you may be getting some surges propagating through gear causing failures.

[@EEVblog]

+Zadster Never had another item in the lab blow a mains fuse just doing nothing.

[@xmenken1]

Thanks, Dave just taught us how to fix a Rigol PS unit.

[@tkarlmann]

Quite typical to have hand soldering flux residue on heat-sink-mounted pass transistors. You didn't mention how you were controlling that relay or how fast. You may have cycled that relay fast enough to generate back EMF to blow the FET. ALL FET junctions have capacitance on them. You pick a correct frequency and you could pop the FET.

[@ElectricGears]

Well, it looks like Dave finally fell to the curse of the post video subscribe link. I don't really want KZbin to ban any URLs, but at lease it should make you click through a popup message saying they make you look desperate, along with a screen shot of the subscribe button (THAT IS DIRECTLY UNDER EVERY VIDEO) with a big red arrow pointing to it.

[@phillyphakename1255]

I fix power supplies for a living, and probably 70% of my repairs are shorted drive transistors and associated components. Another 20% are old bias caps that need to go into the landfill, solved by recapping. The last 10% is actually hard work, doing real circuit analysis, troubleshooting, etc. Do a diode/resustance check on the big transistors, and chances are you will have fixed the problem.

[@SaturnV2000]

Winnah winnah chicken dinnah! Good job! Yes, Murphy has crazy timing too. Fix one thing, something else goes bonkers . . .

[@raymundhofmann7661]

I guess the chinese power supply design engineer(s) forgot to protect the gate of the pass transistor properly. So when unexpectedly the voltage at its source changes/drops due to a short (and there might even be evil contact bouncing combined with inductances at the output), the Vgs could get too high. The output transistor might have a kind of zener diode protection for it's gate, but if the drive circuit can supply some current this may be useless. The Vgs is max +-20V for the SUP80N15, so when you do the short test at a lower output voltage, it might always survive, but at 30V it might get close to catastrophic failure, depending on the details of the driver circuit. Concerning SOA and avalance enerrgy, the MOSFET is clearly superior to a BJT for power supply circuits like this, but the maximumg Vgs... I stay with my old labsupply without this digital knob fiddling, display rubbish, long boot up time and potentially unreliable circuits.

[@RobbieHatley]

I recommend, never purposely short the output of any bench power supply, even though it's "current limited", because the current limiting doesn't kick-in in 0.000 picoseconds, more like 5 microseconds or something like that, so you're subjecting both the PS's output transistor, and the load, to a brief burst of high current (50A? 75? 100A?) for a few microseconds before the limiting kicks in and the current is reduced to whatever the limit is set to. The transistor may survive that, but each time you do that, it accumulates internal damage (highly-localized spot melting and re-freezing in the silicon), and eventually, when enough damage is accumulated, you'll let the smoke out. I've seen it happen. Preventing that is easy: just put a resistor in-line, enough to limit transient current to twice the rated max current of the PS. So if your PS is 30V@3A max, and you want to run 3A through relay contacts, I'd set it to 6V & 3A, and put a resistor in-line such that it limits current to 6A at 6V, which would be a 1Ω resistor. Select the power for 3A, not 6A, since the current limiting kicks in after the first few microseconds. So a 1Ω 10W would do fine. A 10¢ resistor can save a $100 PS. As for this video, I had to click the DIS-like button, because from beginning to end you failed to understand the cause and cure of the problem. And you STILL don't. Electronics lab power supplies are not arc welders! If you repeatedly short them, you WILL eventually blow the output transistor. And worse, you're spreading misinformation by deliberately demonstrating what NOT to do, at the end of the video. Most of your videos, I like; but not this one. Everyone watching THIS video is now dumber for having watched it.

[@mensaswede4028]

Meh, while it’s not “best practice” to short the output of even a current-limited a power supply, a well-designed power supply SHOULD be able to withstand it. This Rigol unit is not your Radio Shack basement bargain unit, and the fact that it failed this test says more about the Rigol’s design than it does about about the user. But I would agree that Dave could have at least mentioned in the video that this is not “best practice” so that “younger players” (as he calls them) would not be misled into regularly shorting PS output leads.

[@pentachronic]

Looks like there might be a need for a snubber acros the pass transistor too. This will stop high transients appearing when you power up.

[@bayareapianist]

I killed many power transistor to learn about this concept :)

[@RyanFinnie]

"It's not like I'd have one of these puppies lying in my junk bin, you'd have to be really into your pass transistors to have one of those." I currently have a tube of 50 lying on my kitchen counter. (OK, close enough, PSMN022-30PL 30V/30A. Technically would work as a replacement, but with no margin of error.)

[@trcostan]

I blew the whole lot of 2n3055's once as a young player. Charging a backup battery with a bench ps and connected the battery up backwards (someone had the colors backwards) magic smoke in quantity! Btw I love repair videos they are the most educating imo.

[@nickxia3209]

that is why there is always a diode used at the input terminal of relay to short the inductive voltage.

[@Paes64]

I think that was this particular transistor problem, not its control circuit. In circuit is fuse 5A but transistor should stand continuous drain current 55A in 100 °C and 76A in 25°C. Voltage at filter capacitors is 3x lower than 150V which transistor should stand. First, the transistor was damaged, and then the fuse blown, because there was no more current limit in the circuit. There is also a danger that if the transistor fails (breakdown), the full voltage from the capacitors (53V) will appear at the output. There is probably no protection in the system that disconnects the output voltage in such a situation? Or selfoscilation in control loop ...

[@educatedmanholecoverbyrich8890]

2N3055 are a lower base voltage and do tend to blow. I threw out all mine and used BUX20s for years with never a one blown, heck, but I'm an old school goose.

[@MaxKoschuh]

27:35 the MOSFET (and the heatsink with it's screws) was installed after the solder bath. Hence the hand soldering.

[@ThisDoesNotCompute]

I'm curious about the voltage rise during the test starting at 18:45. It seems to rise for a couple seconds, then pause, then rise some more, then pause, etc. I would have expected it to be smooth (then of course slow down as it reaches its maximum). Any idea what was going on there?

[@ahmedsherif2182]

This is just the meter refresh rate

[@Bubu567]

It's probably the sense/negative feedback causing the voltage to leak out onto the output when it's output transistor was blown. Why it isn't smooth, god knows. It could be the multimeter lagging as well.

[@BenjaminEsposti]

The reason why the soldered in fuse blew is because the pass transistor failed shorted for a split second, likely due to an internal arc.

[@pepe6666]

haha im just at the bit where his second supply failed. 'winner winner chicken dinner! oh what the hell??' such a quick turn around. i love this channel.

[@EcProjects]

Dave's got the fail button out O.o This is going to be interesting :D :D

[@GadgetAddict]

Did you use a diode when testing the relay? Otherwise it could have sent a very high voltage back into your power supply when it turned off.

[@BenjaminGoose]

+Gadget Addict I haven't watched the video yet but I'd be surprised if he didn't mention the back emf being a possible cause!

[@RWoody1995]

+Gadget Addict He talked about this and having not used one in the video but the PSU should have protection against it even a cheaper one would.

[@EEVblog]

+Gadget Addict Power supplies usually include reverse diode protection on the output. And the relay coil was on CH2, which was *not* the one that blew.

[@OneBiOzZ]

i had this problem on mine ... i replaced the transistor and still nothing so i stuck it up on ebay as broken ... should have checked the damn fuse! ... im not used to fuses of that style i might have just thought it was a resistor and glanced over it checking everything else it was a real nice unit and when i get the money again i will be happy to buy one again and just not abuse it again i think i was testing a design for a flyback DC/DC with way too high of a duty cycle when it popped

[@EEVblog]

+Alyx BioHaz So I'm not the only one, interesting!

[@OneBiOzZ]

not sure if it was overloading or a serious EMF spike ... the whole DC-DC converter was designed poorly on my end

[@stonent]

+Alyx BioHaz I didn't see how the fuses were laid out but I suppose when the pass transistor shorted it dumped the unregulated current back into the driving transistor. We saw it was pulling 7 amps so that then exceeded the rating of the fuse and the fuse popped. (Aww! He popped it.) Normal worst case scenario should be between .2 and .5 Watts through the driver transistor based on the resistance measured and a range of 30 to 53 volts (the unregulated input voltage) but the short kicked it up to 210 watts. (30V @7A). I'm surprised that it held up with that much heat being dissipated through it.

[@OneBiOzZ]

my best guess is that the relay somehow switched the current faster than the device could react to the change in current and popped the fuse causing an inductive spike on the fet popping it ... makes more sense in my failure but simply being shorted by a relay would not really explain it, you do have a nice theory hey dave, grab a few hundred of these and test them out! XP (added points for photonicinduction reference)

[@zynthos9]

Dave would you consider doing o video on VLSI concepts? What do SRAM and DRAM look like from a schematic view? How do they work?

[@joea3728]

Great video. Just a note: From what I understand the Chinese use anode and cathode instead of positive and negative. I have gotten several translated documents indicating this. so I believe those symbols might be for anode and cathode, Not positive and negative.

[@paulhamacher773]

I love those repair videos. One can learn so much from it.

[@7head7metal7]

I really like the endcard! Thanks to KZbin my "watch later"-list is automatically playing the videos, so when I forget to give my big thumbs up, I have to switch back to the video. So, much better now! Also, nice ethusiastic talk, gave me some good smile :D

Probably somebody baked that pass transistor when he was resoldering and even resoldered id before mounting on the heatsink. Finally, he was so exhausted he even didn`t bother to clean it up ('why clean if it is hidden'). Who knows, just a guess. But it`s always sad to see flawed or patched parts in product, which should be brand new...

[@1337NoMad1337]

SPOILER, Dave! 21:05 You tell us the pass Transistor has failed before ever having measured it. Unless correcting mistakes in your footage, please use YT annotations, so we playing along at home can turn them off.

[@aaronwilliams1249]

I bet that transistor was soldered on after the fact because the big heatsink was probably installed later and they attached the transistor to the heatsink to make sure it's positioned properly before soldering it.

[@duggiewest8181]

Love the troubleshooting , thanks Dave.

[@turboslag]

Could simply have been a rogue Mosfet at the extreme lower limit of MTBF, it happens. More surprising is the use of only one pass element in a 3 amp supply! I have a number of old school linear PSU's of various ratings and typically, even the 1 amp units have 2 pass elements, a Farnell 5 amp unit has four 2N3055's!! Although a single 2N3055 can handle 15 amps.

[@daveblane6442]

+turboslag -- CHEAPER!

[@turboslag]

Dave Blane Sad but probably true!

[@tubical71]

+turboslag 2N3055 are known to have issues with the second breakdown effect...that´s why they use them in parallel...and of course to stay *always* within SOA.... These TO220 packages can only dissipate 50Watts best case....but not 90Watts. Read *any* datasheet according this....you´ll find it.... I doubt the rigol-staff read any of them....

[@BenjaminEsposti]

+TubiCal Yep, and the datasheets can trick you as well! They say a transistor in a TO-220 can dissipate say 150W or so, but _this is if you have an infinite heatsink, which stays at 25C no matter what_ XD ... it's very unrealistic, but what it does show, is the effect of the junction to case thermal resistance :)

[@ProfessorFartsalot]

Just watched IanScottJohnson repair a Farnell LT30-2 which had a blown 2N3055, maybe not 'Bullet Proof' as you say, Dave but a beefy as transistor either way

[@GruntUltra]

OMG - the man has tools to fix tools. RESPECT!

[@fimbles1015]

Cheers for all the amazing videos :)

[@Lissica1]

Iam working now in electronic development for over 10 years and we have not one device from Rigol in our lab! Maybe for a good reason:)

[@102819921]

I learned from working on car circuits, a blown fuse is never the problem itself... if only...

[@Graham_Langley]

+max margherio Indeed - correctly rated fuses blow for a reason. That said the internal fuse on the microwave here seemed to pop for no apparent reason but it's identically-rated replacement is still OK years later.

[@chewmanfoo]

Dave - that was amazing! Two questions: 1.) when you replaced that soldered fuse, why didn't you replace it with replaceable fuse? (I'm a noob - I'm not questioning your judgement, I'm trying to learn) 2.) more basic question: would you say, if I'm not capable of fixing my power supply were it to fail as yours did, I shouldn't own this power supply?

[@BenjaminEsposti]

Also, the "mystery power" might have been current flowing through a capacitor on the output that was only rated for 35V. (Dunno if there is one, but it's something to check)

[@niceguy60]

Like I said 7 years ago if your going to use your power supply to trip mechanical relays at least only do so by applying and removing the lead instead, do not deactivate the relay by simply hitting the power switch on the power supply as that sends a massive voltage spike back through the output of the supply which is what happened here. Or you can simply buy mechanical relays with flywheels built into them.