Electronics > Beginners
LM317AHVT and Heatsink
mike_mike:
Thank you for your help.
I observed that sometimes, at random moments of time the voltage is decreasing without any reason from 26.1V to about 24.3V. This happens randomly, and it happens with load and without load at the output of the power supply.
So I tested with the scope - again - and I attached some of the screenshots.
I found that the voltage is sometimes coming back to the initial value if I move the PCB. I checked the connections on the PCB and they are good.
The screenshots were took when the power supply had 26.1V at the output. I did not managed to check with the scope when the voltage was 24.3V.
Why the voltage drops suddenly ?
Are the screenshots good ?
And sorry for bothering you with my noob questions
Please help me. It is about 2 months since I started this project and I still have problems with it and I know that it is a simple project.
mike_mike:
I found that sometimes the voltage at the output of the power supply drops suddenly from about 19V to about 17V without any reason. This happens is both situation - with load and without load.
Sometimes, if I move repetitive the PCB, then the voltage comes back to 19V.
What should I do to solve this problem ?
Please have a look at the above message.
Zero999:
There's no reason why that would happen, other than overheating/load. Are you sure about the connections? Try moving and shaking the PCB with the power on and no load connected.
mike_mike:
I checked the circuit and the LM317 and the power transistors are not overheating I can hold relaxed the hand on the case of those components.
The behavior is the same with and without load if I shake the PCB, sometimes the voltage drops and sometimes it comes back to the initial value.
I checked the connections and they seems to be good.
I used a 1.5A load at 19.87V for the last tests.
I checked the voltages while the output was 19.87V and while the output dropped to 17.87V and I found that in both cases, the input voltage in LM317 and the voltage on the filter capacitors was the same. The problem is on the output of the power supply, because there the voltage dropped. I also tried with different LM317AHV and the results were the same.
floobydust:
It's a bit puzzling because it is an intermittent problem.
I would usually suspect the parts on the ADJ pin like a noisy potentiometer or leaky C5 (rated for >50V?).
The LM317 might have gotten damaged if you shorted C-E (input-output) earlier.
Careful because a damaged LM317 likes to smoke and burn the potentiometer- the ADJ pin can stay at high voltage even though the pot is towards zero ohms and then it smokes. Try jumper across the pot and see if you get 1.25V out.
A pass-transistor TIP36C could also get damaged with a C-E short. I would test them out of circuit.
If this isn't it this stuff, then worst-case the circuit is oscillating.
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