Reverse Voltage & Output Signal Protection (REVISED) (ECE Design Fundamentals, Georgia Tech course)

[@OldCrowCS80]

My preferred method of reverse supply voltage protection is usings FETs. You just need to know how much current the circuit is expected to use (I use 300mA parts in most cases). P-channel JFET for positive rail: drain is wired supply input, gate is wired to supply common, source is your circuit's positive supply rail. No voltage drop when properly conducting the supply, and when supply is reversed (negative and positive rails swapped), the FET simply does not turn on. Use N-channel part in same manner for a negative supply rail.

[@mrlurkidy6288]

I heard people call the parallel "protection" diode the warranty diode. If that's blown then your warranty is void

[@Lantertronics]

That's hilarious!

[@dryan8377]

@@Lantertronics I was a radio tech for a long time. All the cb radios I repaired were designed with a like 1n2002 or such diode on the positive input, so no matter what you did, applying reverse power had no effect whatsoever.

[@chaosopher23]

I find that little circuits such as stomp boxes do well with a small bridge rectifier before a single-cap filter. If I don't care about the voltage going in, such as if I plug a laptop supply in, I'll put a regulator after that. While I do lose the voltage drop and the regulator's losses, it's a lot cheaper to stuff a few parts in that have inherent protection. "AC/DC up to 36 volts non-polarized" is a future label on the portable shortwave radio I'm building.

[@CircularMirror7]

Great video. I was wondering how reverse voltage was dealt with in pedals. Looking forward to that next video. 😀

[@Lantertronics]

Thanks!

[@ahbushnell1]

If you put the feedback on the opamp on the other side of the 1kohm resistor it will correct for the error caused by the resistor.

[@Lantertronics]

I've particularly seen people do that with the 741 -- maybe something about the particular way it handles short circuit protection?

[@ahbushnell1]

@@Lantertronics I just looked up a general TI opamp ~10MHz and it has unlimited short circuit protection. I bet your correct that the 741 did not have that.

[@ahbushnell1]

@@Lantertronics The current TI 741 has short circuit protection. People do things based on tradition. I still use my Burr Brown opamp books copy right 1971. :)

[@djohnson2499]

I think the idea with that really simple polarity protection is that a player will plug in their pedal, attempt to use it, and then when it does not work unplug it and investigate the issue. And for something that short term it's fine. I have definitely opened up older Boss pedals and seen a scorched diode though.

[@TranscendentBen]

The Moog Werkstatt has a 12V zener right across its 12V power input jack! I wonder about the wisdom of that as well.

[@egorreshetnikov3864]

6:22 yes. got ds-1 recenly: prot. diode cracked in half and opamp is gone

[@Abossow77]

Great content as always! If i lived in the us/had similar classes in my uni, i would probably have taken a completely different path in my career hahaha

[@Loscha]

Those diodes do get quite hot. I was repairing a non Boss pedal that someone had monkeyed with, and they had replaced that reverse "protection" 1n4004 diode in backwards. I ended up with a 1n4001 diode shaped burn on my finger for about 2 weeks. I was powering that pedal with a bench supply, not a 9v battery, so there was oodles of current running through it. I reversed the diode, and the pedal started working correctly again. I charged the client for the full half hour.

[@Lantertronics]

A couple folks noticed that back in the 70s people were likely using whatever supply Boss provided, and it wasn't until later that using beefier power supplies that could power a whole pedalboard became commonplace.

[@Mogwai06]

How in the shit i never foUnd that DIY strat site when ive been making my own pedals and wiring and rewiring my strats for a few years now, ill never know. but hell yeah, thanks, brother!!

[@martingerken7094]

The TL07x have short circuit protection; so I'm not shure if the 1k resistor is needed... it will disturb the signal a bit, but 1k in relation to the usual 100k input resitance in modular synth is little

[@Lantertronics]

I'm nervous about trusting the short circuit protection in chips. In any case, I've ever seen a commercially produced module that has an op amp output brought straight to an output jack.

[@JariSuominen]

@@Lantertronics I was just fixing a Serge module and found an opamp output accidentally grounded by precious tech. And it had been like that for years. Chip was running hot, but to my big surprise, once the short was sorted out this humble mc4558 was still working fine! I have seen opamps straight out on EMS gear, at least Synthi 100. I think EDP Spider should at least according to the schematics be straight out. Pretty sure I have seen it somewhere else too. Still prefer to have output resistor though.

[@waynegram8907]

The internal resistance of a battery with either the parallel or series protection diode will produce harmonics and non linearity? Because when using batteries compared to using a DC wallwart or transformer, it sounds very sharp and edging isn't a DC wallwart compared to using batteries with guitar effects, any reasons why?

[@Lantertronics]

I conjecture internal resistance of the battery isn't going to necessarily provide more or less non-ideal behavior than the AC supply. There may be differences in terms of the particular voltages a particular wall wart happens to give relative to what a battery gives, and that will effect things. I've heard of guitarists use batteries that are deliberately run down a bit to give a particular effect. It would be fun to just hook a pedal in question up to a benchtop power supply and start the voltage at 9 and then dial it back to see what happens. This is all just conjecture, though.

[@waynegram8907]

@@Lantertronics Check this video out because the carbon batteries have higher internal resistance so it current limits and ducks the voltage at uS in time. When using an external Power supply it makes the same guitar pedal sound very sharp and edge. Its weird how DC voltage and DC current can make the same pedal sound edge and shape compared to batteries to more dynamic with your picking attack kzbin.info/www/bejne/f323ZqysoMZ8mac

[@JariSuominen]

I have seen one DOD pedal after reverse power accident. On that model the trace between the DC socket and the diode was super thin and had burned open. It seemed fairly obvious that it was designed to work as a fuse in such situations. Which I guess is a slight improvement to the circuit. Still dreadful design.

[@Rob_65]

Both parallel and series diode protection are no good. Most likely the idea behind the parallel diode is that only small adapters are used to power the equipment. A series diode might look good but if you plug in a supply with correct polarity but a too high voltage, there is still a high chance of smoke ... With correct power, the series diode will (as you mentioned) influence the actual voltage on the circuit. A better way is to use a MOSFET instead. Only with the correct polarity the gate will be opened and full power is available to the circuit behind it. A second MOSFET with a zener diode and a PTC fuse can be used to protect the circuit against overvoltage (to a certain limit). The output protection you show with the 1K resistor is working but depending on the load (a 10K potmeter on the input of the next circuit) the output will never reach the full level and this is even worse when this is the output of an oscillator that is being used to drive multiple modules (using a passive mult). If R14 is placed inside the loop (so between the output of the opamp and the resistor to the negative input), the output still has a low impedance while still being able to limit the current in case of a short circuit. But this does depend also on part of the circuit not shown: what happens when a +10V CV signal is fed to the output of a VCO with a 10Vpp (so -5V - +5V) VCO. It is possible that part of the circuit that is not shown cannot handle the 10V signal ... With the resistor where is it shown here, the low impedance of the opamp circuit most likely prevents this.

[@PeterCocteau]

I'm not an electronic master (only a beginner) and my english is not fluent so i may have missed something...but what about a parallel diode with a resistor? resistor may limit current if you plug jack badly. No?

[@johnparnell9488]

Current flowing through the resistor develops a voltage across it. Not good!

[@possible-realities]

The parallel diode tries to short out the wrong polarity voltage (which as noted may be a really bad idea if whatever provides that voltage has a low output impedance). A resistor in series with the parallel diode will protect the diode, but it stop the diode from shorting out the voltage, so it's almost like you didn't put a diode there in the first place - it diode and resistor will just draw some extra power from the power supply, while the rest of the circuit still takes damage from the negative polarity.

[@ScottyBrockway]

I repair a lot of pedals, this circuit is bad. They used a zener in the older pedals. Other companies use a 470 ohm resistor in series with the diode to keep it from overheating, but that doesn't work for long either. Old MXR pedals used a 1n270 diode in series with the input voltage like you suggest with a schottkey, but the diode can also break down with too much input voltage.

[@Lantertronics]

I heard someone refer to it as a "warranty diode" -- as in if Roland opens it up and sees it busted, they can say you hooked it to the wrong thing and violated your warranty. Which, intentional or not, it really awful design as you note!

[@ScottyBrockway]

@@Lantertronics Warranty diode, that make a lot of sense actually hahaha. Oh wow...

[@AnalogDude_]

Hey professor. don't know if you have seen this, but here goes: Hackaday Reading Silicon: How to Reverse Engineer Integrated Circuits. a must see video for people that use semi conductors. 9:31, that 1K is standard for many, sometime 180R is used as in the case for Mutable Instruments Kinks, Links, but you forgot to mention the opamp's have a short circuit protection,says datasheet, if you watch the above video, you'll discover how its work as i found out recently. hmm, someone said i own you an apology, that i might have mistaken the operators on a formula. But how was it? P = I² x R or P = I² / R? P = R² x I or P = R² / I? P = V² / R? I forgot to make written notes when we last discussed this. 😊

[@rillloudmother]

I killed my boss blues driver with reverse voltage. As soon as I plugged it in there was a puff of smoke and that was the end of the pedal.

[@Lantertronics]

Oh no! Very sorry to hear that. (Did you open it up and take a look? It's likely fixable).

[@rillloudmother]

@@Lantertronics I did open it, but with the schematic, my meager electronics skills I was unable to identify which components had fried. Every once in a while I get it out and orbs around to no avail.