Linear PSU trips RCD - Page 1
Forum section: electronics / projects

author: daxliniere
author_rank: Regular Contributor
timestamp: January 12, 2025, 03:15:21 pm
content: Hey everyone,I have a linear PSU for a 1980s Soundcraft mixing console, model CPS-450, which has begun tripping the RCD at power on more and more frequently. Having done a bit of research, some other posts have suggested the Y-2 caps being dead.I'm not in front of the unit right now, but the schematic I found doesn't show any such caps.https://medias.audiofanzine.com/files/soundcraft-psu-cps450-schema-478529.pdfI'll put it on the bench tomorrow - any ideas on what to look for?Thanks in advance,Dax.

author: nvmR
author_rank: Regular Contributor
timestamp: January 12, 2025, 03:45:15 pm
content: The xfmr might be leaking (insulation gone bad), or the rcd my be going bad.I would start by getting a clamp multimeter and measuring the ground ac and dc current. You're looking for the 10's of mA

author: coppice
author_rank: Super Contributor
timestamp: January 12, 2025, 04:31:28 pm
content: If its from the 1980s, has it been cleaned lately? Dirt combined with a humid day has been known to be something which trips RCDs. This kind of thing can leak some current, the current warms things up, which dries things out, and now everything looks good, until things get moist again. As a first step, check the impedance between the live, neutral and earth pins at the plug.

author: bdunham7
author_rank: Super Contributor
timestamp: January 12, 2025, 04:33:54 pm
content: I assume this is a small transformer, not a huge one that would likely trip an imperfectly balanced RCD. If my presumption is incorrect, try it on a different RCD.Moving on, the first thing to check is the resistance from the LINE plug terminal to the GND terminal with the power switch on (and uplugged, obviously..) with a meter that can read at least 10M if not a lot more. Make sure the DUT is on an insulated bench with nothing touching it. Then, if you have an LCR meter, measure the same terminals for capacitance.

author: daxliniere
author_rank: Regular Contributor
timestamp: January 13, 2025, 08:13:33 pm
content: Hey everyone,Thanks for your replies and sharing your knowledge.The unit is clean inside and out, I reflowed all solder joints about 6 months ago. I didn't re-cap it, but none of the caps look damaged in any way and it ispossiblethat it was re-capped some time before I got it in 2018.It was morning when it was turned on, which means the room would have been a bit cold (English winters...), but humidity not too high.This afternoon when I arrived it turned on without tripping the RCD, so..Resistances tested with Fluke 15B+ are:L-N: 2R8L-E: >40MRN-E: >40MRIncidentally, I got to the studio today and saw it is in fact a CPS-650. Same thing, but some higher spec'd parts (and a regulator for the 48v phantom rail).(https://elektrotanya.com/soundcraft_cps-650b_console_power_supply_user_and_technical_manual.pdf/download.html#dl)

author: daxliniere
author_rank: Regular Contributor
timestamp: January 19, 2025, 01:12:12 am
content: Anyone have any thoughts on this? Can we categorically say that it's the main caps? Or the regulators themselves?

author: bdunham7
author_rank: Super Contributor
timestamp: January 19, 2025, 01:50:36 am
content: Has it tripped an RCD lately? I asked how big the transformer and unit is--how many watts or amps is it rated for on the label? 2R8 DC resistance implies the possibility of inrush current exceeding 100A, that might trip a slightly imbalanced RCD that works perfectly otherwise. It's not likely regulators or capacitors. If trying a different RCD is out of the question, then modifying the unit for less inrush might help. The easiest way would be an appropriately sized PTC in line with the transformer.

author: daxliniere
author_rank: Regular Contributor
timestamp: January 19, 2025, 01:56:19 am
content: Thanks for your reply @bdunham7.I haven't tested if it's tripping the RCD again since the last message.It's model is CPS-650, so I believe capable of delivering up to 650 watts, though the connected console is only 24 channels with a handful of TL072 per channel, so probably nowhere near 650w.How would I select the correct NTC for this job? (I'm assuming you meant NTC not PTC?)

author: bdunham7
author_rank: Super Contributor
timestamp: January 19, 2025, 05:49:15 am
content: Quote from: daxliniere on January 19, 2025, 01:56:19 amI looked at the manual and it says 600W maximum input power. There's a 6.3A input fuse and unless I missed it, that appears to be used for all input voltages from 100 to 240V. So it shouldn't really be a surge monster. Is the RCD that trips a combination unit or stand-alone RCD? And it is set for 240V, right?Yes, I mean NTC. You pick one that has a high enough current rating but not too high. Maybe a 4A version like the Ametherm MS22 50004-B. But given the info you've provided I'm not convinced your issue is actually caused by a surge. Can you check for leakage to ground using actual 240VAC power? It's bit tricky and dangerous--you have to put a resistor, say 10k, in series with the ground lead and then measure the voltage across it. I have a device that allows me to do that without making the case or chassis live.

author: Someone
author_rank: Super Contributor
timestamp: January 19, 2025, 06:11:27 am
content: Quote from: bdunham7 on January 19, 2025, 05:49:15 amOP is in the UK, insulation/leakage measurements are routine and readily available:https://www.patinfo.co.uk/insulation-resistance

author: bdunham7
author_rank: Super Contributor
timestamp: January 19, 2025, 03:17:06 pm
content: Quote from: Someone on January 19, 2025, 06:11:27 amIs that PAT testing done with HV DC or with 240V AC? I have both, HiPot and AC, and I was referring to AC leakage since that includes capacitive coupling.

author: Someone
author_rank: Super Contributor
timestamp: January 20, 2025, 05:12:20 am
content: Quote from: bdunham7 on January 19, 2025, 03:17:06 pmAs I recall both are acceptable methods for some tests, so need to read the specific instruments instructions/documentation. Live testing while devices are powered and operating is sometimes needed (relays to engage isolations etc).