Power supply filter cap overvoltage but not damaged...? - Page 1
Forum section: electronics / projects

author: spicytofu
author_rank: Contributor
timestamp: September 02, 2021, 10:35:47 pm
content: Help me out here. The power supply filter cap is rated for 35V, but the rail voltage is 56V. This is how it came from the factory, BK Precision 1744. I have two power supplies that show the same problem. How is this cap still alive?!? I am really curious. Nothing is wrong with unit, I was just calibrating it.BTW, there is no pre-regulator and the transformer taps go straight to the cap. My 34401 and Tek scope show no ripple, just a steady 56V on a 35V cap!

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 02, 2021, 10:57:38 pm
content: Could it be an intentional mislabel to snag those trying to copy the product?

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 03, 2021, 10:36:17 am
content: I don't see where second wire (black one) goes. Do you really measure voltage across a cap, or?And what are cap dimensions (we have to compare it to a datasheet)?

author: spicytofu
author_rank: Contributor
timestamp: September 08, 2021, 07:11:52 pm
content: Yes you can see that the black wire connects to the output. Since its is a floating supply, the negative output and the cap are on the same node. Also, I didn't want two clips close together and risk a short. As far as marking goes, you see 35V on the cap and 56V on TWO of my 34401A DMM. I trust my DMM more than the cap.Also, this a 35V supply and a 56V rectified voltage make sense as there will be Vdrops and losses in the wires. It is a 10A supply. Still puzzles me....

author: srb1954
author_rank: Super Contributor
timestamp: September 08, 2021, 11:06:17 pm
content: Quote from: spicytofu on September 02, 2021, 10:35:47 pmHave you checked that the transformer primary taps are set correctly for your local supply voltage? If your supply voltage is more than 115V then you should select the appropriate higher voltage primary taps to cut the maximum voltage on the smoothing capacitor a little.Even so, this seems a dodgy design. The capacitor might not fail immediately on the higher than specified voltage as electrolytic capacitors are typically specified for short term surge voltages of 20% greater than their nominal rated voltage. However, prolonged operation at higher than rated voltage will shorten the capacitor's life and it might eventually fail catastrophically.Another dodgy aspect of the circuit design is the capacitance value - 22000uF seems a little small for a PSU rated at 10A continuous operation. Operating the PSU at full load current will create a significant ripple current through the capacitor, which will heat it up and also shorten its life. If you have a true RMS clamp meter it might pay measure the ripple current flowing into the capacitor and to verify that the measured ripple current is within the permitted ripple current rating for the cap.One benefit of the smallish capacitance is that the DC voltage across the cap will drop significantly under full load slightly easing the over-voltage problem.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 09, 2021, 01:40:49 am
content: Perhaps check the actual capacitance as well as its physical dimensions? My guess is that it's not really a 35V capacitor, just a trap so that those trying to copy the product will use 35V capacitors and have lots of failures. Or they managed to get a batch of mislabeled capacitors for cheap.

author: bdunham7
author_rank: Super Contributor
timestamp: September 09, 2021, 02:04:11 am
content: Quote from: spicytofu on September 08, 2021, 07:11:52 pmI actually can't see that and that cap appears to be a bit....unusual? Perhaps a careful power-off connection direct to the cap terminals followed by another test? Or just check the voltage on the other terminal relative to wherever your black DMM leads are now?I can't find a schematic for this, perhaps you could contact BK Precision and see if they can send you one. BK Precision stuff generally isn't complete junk and there has to be an explanation.

author: spicytofu
author_rank: Contributor
timestamp: September 13, 2021, 01:26:50 pm
content: Quote from: srb1954 on September 08, 2021, 11:06:17 pmThis power supply has been in service since late 2010s I believe. It was given to me. I have tested the voltage and current, I get rated 35V at 10A so this power supply looks great. I am just confused why the transformer taps are 56V (its a multi-tap, but the highest one is 56V) and the cap is 35V and this thing did not explode. I havent test the leakage current on it, I would imagine it to be high.

author: spicytofu
author_rank: Contributor
timestamp: September 13, 2021, 01:37:51 pm
content: Quote from: bdunham7 on September 09, 2021, 02:04:11 amSorry my fault for assuming it was easily seen, I had to angle it to show the DMM reading. But yes, the output negative is the same as the capacitor negative. I cannot check the positive terminal since that will be after the regulator. This PS is the basic line and it seems to be of really cheap quality. I never used BK equipment, but on their website, the nice programmable PS has a MUCH better user manual than this model.At full 10 A load, Vp-Vp is 2.7V ripple at the cap. Voltage drops down to 41V and with ripple, we can expect 38V minimum at the regulator. Ripple seems high IMO. Agreed, there has to be some explanation of this, BK touts this as a "precision" PS. I have not tested the line and load regulation on this, I probably wont since I would use this at the least demanding power source. I have better PS to use if I need a clean source.

author: tunk
author_rank: Super Contributor
timestamp: September 13, 2021, 01:44:38 pm
content: I think a rule of thumb is to use ~2000uF/A for a filter capacitor and thatthis will give you ~4Vpp ripple. So 2.7V is a bit lower than expected.