Ebay / Aliexpress Variable PSU Part 2 - Flames , Test and Conclude

It Went UP In SMOKE - not quite as planned Power Supply part 2.
Test and Conclusion.
Schematic at the end of the video.
Please Like and subscribe to the channel

[@djisydneyaustralia]

7:35 did the max voltage trim affect the minimum?

[@mikepanchaud1]

Thanks for a very thorough review and test! I've been researching this very board and others have mentioned the 'fake' op amps and you've certainly proved that. I'll probably go for a lower voltage transformer as you suggest. Also I think a multiturn pot for the voltage would be good. I agree it's agood design, but not suitable for a 3 amp output just because that's the max rating of the input diodes!! Still it's a very cheap option even after getting proper op amps and transformer. Do you think it will function OK with a 12 volt transformer ? I may go for 15 volts anyway to give abit more flexibility to give maybe 18 volts DC out. I've subscribed to your channel, thanks.

[@Joftec]

Hi, Thanks for your great comments. It will work with both 12 and 15 volt transformers, of course depending on what maximum output voltage you want. I don’t know your level of knowledge, so sorry if I am stating the obvious, if you are using a standard iron core transformer EI type for example, they have terrible load regulation. By that I mean the that the output voltage of the transformer will drop depending on the load, and most importantly the power rating. For example if you have a 24v 72VA rated transformer, ie 24v output at 3A , the open circuit voltage could be over 30v dropping to 24v as the load increases. If you have a 200VA transformer for example, at 3A the output voltage will still be way over 24V. So in summary check the transformer voltage unloaded and loaded to make sure it fits your requirements.

[@mikepanchaud1]

@@Joftec Good advice there. It's a shame they cut costs by using useless opamps, very off putting for beginners. I'll get the op amps from Rapid electronics at just 49 pence each! They do a large range of toroidal transformers too.

[@sebastian19745]

I got one of this kit and I built it. While I was using it with a 9V, 12V or 15V transformer, it worked very well. But when I powered from an 24V transformer (12V+12V) it suddenly died. I traced the fault to those opamps (TL081), two of them were faulty. Maybe theirs max voltage was lower than specified. However, I ordered 3 more and when I tested them, each behaved differently. I made an oscillator on a breadboard to test them, and all oscillated but the waveform was different for each one. I then bought 3 more, from a local shop and I got 3 ICs each one different but two of then behaved identical in the oscillator test circuit. So, I used those two and another one known good IC. I tested every component on the board and each one tested good. However, when I installed the ICs, the PSU did not worked, even with an 9V transformer. It went into current limit (the LED was on) and the output voltage stayed at 0.3V. I found that the negative voltage generated went to -0.6V when IC3 (the one that went in flames in your circuit) was connected and that IC was warm. With another IC, the V- was the intended -5V but the output voltage was 0V. Now I am planning to remake the kit with my own parts, each one selected and tested, just to see what is wrong with it. My guess is that these opamps are not originals, maybe clones, and theirs Vcc need to be lower than the original datasheet specify.

[@Joftec]

Hi, I think there are two things you need to watch out for here. If you watched the first part of the video the op amps supplied in the kit were unmarked, they could have been anything. I used genuine TI TL081 CP. Also the absolute maximum input voltage is 40 VDC, the circuit uses the 5v1 zener diode to offset the - side therefore the total input voltage is whatever the output is (smoothed DC from the rectifier) plus 5.1V. Therefore at 24vac @ 1.414 = 33.8V + 5.1 = 39V, very very close to the max input voltage. If you are using an EI type transformer, the output will for sure be higher than 24V unloaded . I also grounded the inputs of the op amp prior to switching on, as they are jfet, just to make sure they were totally off.

[@sebastian19745]

@@Joftec You are right about the transformer. It is an 220V to 2x12V E+I.But, the mains voltage is 230-240V here in Europe. So that was the cause of the failure, I know. But what drives me mad is that with the new opamps that I bought already are not working as intended. Right now I am waiting new ICs from Digikey, and I hope that will solve the problem. And I plan to use it with an 2x9V transformer that will be something around 27V input. Thanks for your reply.

[@Joftec]

Hi, measure the voltage unloaded. But if it’s a 220v primary and you are at 230-240 and an EI transformer I would guess your output is way over spec. Also remember these are fet input devices, therefore the inputs can be turned and possibly stay on via static electricity. Ground the inputs before power up, use static sensitive precautions also.

[@mikebond6328]

Common problem.

[@mikehibbett3301]

Where was the smoke? can you link. Didn't see it in the first post.

[@Joftec]

Hi, it’s at the start, about 1 minute in

[@harritronics360]

Can it output 2 volt 4amps?

[@Joftec]

No I don’t think it can in standard form due to the output transistor .