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Need help with bench PS

Zeebit

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Today, I was finally able to put together the bench power supply I've been building. But yes, it failed. I'll explain what happened below but here is the schematic of the circuit. Its based on the circuit on the LM317 datasheet with some modifications.

bench_ps.jpg



When I turned it on for the first time, nothing started to smoke so I was a bit relieved. :crackup: Both voltage and current adjustment pots were zeroed at this time. I switched on the output and the minimum voltage was about 1.8V. Its supposed to be 1.25 but the voltage pot did not go all the way to zero ohms. I cranked it all the way up and it went to about 24V.

The voltage was doing ok so I thought I'd test the current next.

I got my multimeter set to amps and connected the leads to the output. The current adjustment pot(its 5K) was at minimum (it also did not go all the way down to zero) at this time. I gave it a small turn and my multimeter and the panel meter displayed around 3.5A. I was surprised so I quickly turned the power supply off because it wasn't supposed to do that.


I zeroed both pots and turned it on again. This time, the voltage was stuck at about (negative) -1 volt. I probed the output pin of the LM317 and sure enough it was producing that voltage. I also noticed that the fuse for the main rectifier section was blown. I used a 4 amp rated fuse there.

I replaced the fuse and the LM317. The voltage is now ok. It goes from 1.8V to 24V. The problem is that a slight turn of the current pot from minimum caused the CC led to light up even if nothing is connected to the output. I tried to measure the output current and again, it was at around 3.5A.

I'm not sure what is happening here. No components have released the magic smoke yet.

I would like to ask help from the electronic geeks here in finding out what is wrong.

BTW, big thank to djQUAN for helping me on this project. :)
 
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djQUAN

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Check your connections on the LM317. I see a lot of mis-connected parts around there especially the out and adj pins of the 317. The out pin should be connected to the PS output. What is R4 doing there?

your CC and CV pot names are also interchanged.
 

Zeebit

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Didn't see all those errors. I'm sleepy now and I made that schematic in a hurry.

The out pin of the 317 is actually connected to output in the pcb I made and R4 is 680 ohms.

Edit: there are also bypass caps on the op amp that I forgot to include. Gotta sleep now:tired:
 
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Zeebit

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I've edited the schematic in the OP. Please tell me where I should start in troubleshooting the circuit.

Here is what I know so far:
*The rectifier section and the two regulators are still working fine
*All electrolytic caps and the tantalum are in the correct polarity
*All the diodes are also in the correct polarity
*The pass transistor is still ok
*The op amp pins are not shorted
*There doesn't seem to be shorts on the pcb
*The magic smoke has not yet been released :p
 

Zeebit

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Ah, dammit. The net beside it got too close. Its actually not connected. Thanks for the heads-up. Time to edit again.

*Done
 
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Since U2 is only active during current limit, I'd say that's where your problem is. Probably chose a wrong resistor, or miswired it.

First rule of troubleshooting: check voltages. Important ones in this case would be supply rails, output voltage, inputs and output of U2, and the three pins of U1
 

djQUAN

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D6 is supposed to point towards the PS output. Check the original schematic here: CC/CV Regulated PS

R9 is also too high in value. with 200K you won't get much current flowing through the LEDs.

FP, pin5 is not connected in the original circuit.
 

Zeebit

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Yes, another error on the schematic. In the pcb I made, the cathode of D6 is connected to out and the anode to pin 8 on U1 just like in the original schematic.

I'll check the voltages after I get home and revise the schematic.
 
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Zeebit

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I've corrected the schematic in the OP and changed R9 down to 2.2K.

All voltages seem fine. The main rectifier puts out 26.5V. The two voltage regulators also do their job. U1 also seems fine. The 1.25V reference is still there.

The problem was that a slight turn of the current adjust pot would trigger CC mode. I've found out that the output was shorted to ground. It was reading at about 1K ohm. I've removed the output caps and its still shorted.

What should I look into next? :can:
 

djQUAN

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Can you post a picture of your entire setup? How you built the PS, how it's assembled in the case, and how it's wired to your test equipment.
 
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What are the voltages (to as many decimal places as you can read) of pins 2, 3, and 6 of U2?
It would be helpful to read these both in CV and CC. Put a load on the output to help limit the current if necessary.
 
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Zeebit

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I'll post pics when I find some time later or maybe tonight. Expect it to be messy.
 

Zeebit

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No time to take pics of the setup right now. I think its R12 and R13 that is causing the low resistance between out and gnd, no?
 
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You connected your multimeter set to amps directly to the output? With no load in between?

Wouldn't that be a dead short?
 

djQUAN

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You connected your multimeter set to amps directly to the output? With no load in between?

Wouldn't that be a dead short?

Yes, my supply works just fine driving constant current mode into a dead short.
 




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