Electronics > Beginners
LM317AHVT and Heatsink
mike_mike:
I replaced the potentiometer and now it seems to work good.
I reused the old potentiometer in another power supply, made using the same schematic and the problem did not appeared.
I checked with the scope on the ouptut for oscillations please have a look at reply #45. Are the results good ?
I measured the voltages while the voltage at the output was low (about 17.87V) and I found that the voltage on the 100R resistor was 1.25V while the voltage on the potentiometer was lower in comparation with the moment when the voltage at the output was good (19.87V).
How can I check to see what component is causing those problems ?
floobydust:
In electronics, you don't want to fix a problem unless you really know first what went wrong. So you go after it.
My math, for 19.87V out RV2 was 1,483 ohms, and 17.87V is 1,324 ohms, a drop of 159 ohms (with R6=100R).
But the potentiomenter works fine in another build. So is it the pot? It's not making sense...
If the pot has a noisy spot, you can test it with a multimeter. Or connect it backwards (reverse rotation) so the wiper is somewhere else to get the same ohms but not on the noisy spot.
Try to get it (PSU) to misbehave, go back to the old potentiometer or use a 1.5k resistor in place of RV2, connect a scope to the output and look for oscillations to rule them out.
At these currents you will need to pay attention to wiring and single-point grounding. The end of R6 sampling output voltage should be as close as possible to the output terminals/binding posts. "regulator to minimize line drops which effectively appear in series with the reference, thereby degrading regulation."
The J2 output ground should connect to the input big filter caps' ground after J1, as well as the pot RV2. Not at the end of a long trace/wire that runs to all the little parts. All this is to get the best regulation and stability possible- you already got 10mV at 6.17A which is very good so your layout must be fine, but a pic helps. It is possible to wire a PSU like this so that it starts to oscillate under heavy load.
Those scope measurements look fine. You are basically looking at noise at 2mV/div. Is the raw DC from a mains transformer and rectifier, or is it from a SMPS?
mike_mike:
--- Quote from: floobydust on December 01, 2018, 09:55:09 pm ---Those scope measurements look fine. You are basically looking at noise at 2mV/div. Is the raw DC from a mains transformer and rectifier, or is it from a SMPS?
--- End quote ---
I used a 50Hz transformer, with 30V secondary and 10A current and a 35A rectifier.
I checked again, with another PCB and new components and now, the voltage does not drop. And I used the same potentiometer which I used when the voltage dropped.
Zero999:
Going from what you've said, I suspect it was the solder joint to the potentiometer. It will probably work if you put the original potentiometer back. As I said before, it's likely it was a connection problem.
mike_mike:
--- Quote from: Hero999 on December 02, 2018, 01:26:39 pm ---Going from what you've said, I suspect it was the solder joint to the potentiometer. It will probably work if you put the original potentiometer back. As I said before, it's likely it was a connection problem.
--- End quote ---
Before I change the PCB, I tested with the wires of the potentiometer soldered on the PCB, and the result was that the voltage still dropped from about 19.87V to about 17.87V.
In this case, does it still was a connection problem ?
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version