Can this be a 100V capacitor? - Page 1
Forum section: electronics / repair

author: bdunham7
author_rank: Super Contributor
timestamp: December 14, 2020, 11:32:18 pm
content: I have a three-channel linear power supply, BK 1652, that is supposedly capable of having each channel floated up to 100V from ground. I can't recall ever having done so, but I did have a 'blow up your oscilloscope' moment recently (no harm done) and I discovered that the 'B' channel had 170 ohms or so to ground--not isolated. I took it apart and found that all three channels had their negative connection tied to the ground terminal via a 1uF MLCC just marked '105'. One of them had gone bad and read about 170 ohms.It looks very small (see photo) and is well under 2mm thick. I was looking at replacements and wondering if I should replace them all and upgrade the voltage and maybe downgrade the capacitance (I can't see a good reason for that much and a previous version of this unit doesn't have these capacitors) when I realized that none of the 1uF 100V replacements were as small as this one. Is it likely that these are actually 100V rated given their size? I don't have a schematic or parts list available, unfortunately.

author: TheMG
author_rank: Frequent Contributor
timestamp: December 15, 2020, 12:27:25 am
content: There are a few approx that size:https://www.digikey.ca/en/products/filter/ceramic-capacitors/60?s=N4IgjCBcpgLFoDGUBmBDANgZwKYBoQB7KAbRADYBOcgJkpAF0CAHAFyhAGVWAnASwB2AcxABfAlQQhkkdNnxFSIWGDAAOSgHZGLdpC69BI8SACsABjVSZc3AWKQylWDXM6QbDt37CxBV7D00NKomHaKjuAArQBi7p76AKoCfKwA8igAsjhoWACuPDh%2BIAC0NNZQvHkKDmSm8XogYqKiQADefinitely no more than 100V though.Poor component choice nevertheless, capacitor chosen should have been rated for way more than 100V, never a good idea to have a capacitor (or any component for that matter) run right up to its maximum rating. You also should have some margin to allow for brief transients.I'd replace them with Y-rated capacitors 500V or higher rating. I don't know why you would need 1uF though, I assume the primary purpose is to bypass external common-mode RFI to chassis ground? Could go with a much smaller capacitor in that case.

author: floobydust
author_rank: Super Contributor
timestamp: December 15, 2020, 12:32:45 am
content: That looks too small for 1uF 100V, and generally a terrible idea. They might have just put in whatever was lying around as a bodge for a common-mode noise problem but it's a linear PSU? There should be two caps, one for each channel?1652 (digital) and 1651a (analog meters) might be similar to 1651.BAMAhas the B&K 1651 schematic but no sign of Y-caps. I wish that site was indexed by search engines, they completely miss it.I let my bench PSU's (output) float noisy with hum etc. OR direct earth-ground OR 1MEG/22nF Y-cap 275VAC if I need to float with less CM noise. It depends on the power transformer's winding capacitance. B&K might have bungled the electrostatic shield.

author: bdunham7
author_rank: Super Contributor
timestamp: December 15, 2020, 01:05:52 am
content: Quote from: floobydust on December 15, 2020, 12:32:45 amI have the 1651A manual, looks similar to the 1651. BK sent it to me when I asked for schematics for the 1652 and they said they use it to repair the 1652, since the only difference from the 1651A is that they replace the analog meters with these nice but laughable 3-digit LED meters run by an old school 40-pin Intersil IC. However, that doesn't match up to reality because my 1652 has input ranging relays and I've stared at those schematics until I'm blind, but I don't see the relays.The caps are definitely 1uF and the PSU actually works just fine for the most part. Whoever originally designed it decades ago did a good job. No turn-on spikes, takes any sort of abuse, outputs are well regulated and quiet. No idea why they are there. Cutting them off would be easy, replacing them is a bit harder.

author: floobydust
author_rank: Super Contributor
timestamp: December 15, 2020, 01:47:41 am
content: They're 50X bigger than need be and can effect circuits you're working on. I would put in 22nF 275VAC Y-caps at the banana posts.