Should this capacitor be safety rated? - Page 1
Forum section: electronics / beginners

author: jpyeron
author_rank: Regular Contributor
timestamp: June 09, 2024, 02:49:47 pm
content: I have a cheap (bare) cassis mount switch mode supply. The chassis will not close because the post bridge rectifier cap is in the way of mounting.So I decided the best course of action was to replace the cap in a 90 degree mount orientation.I thought this capacitor should be X type rated, but I cannot seem to find that value with such a rating, and close values are out of stock at digikey.Am I tilting at windmills?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 09, 2024, 03:24:51 pm
content: X approval requirement goes away after the FWB. Which is itself a transient-limited device, for better or worse: either the FWB survives, or it dies and takes out the fuse. (Granted, real FWBs measure ballpark ~1400V breakdown, not exactly tiny compared to commercial-level differential surge.)A cap of that value will have significant impact on the surge itself, though not enough to fully protect what's around it; voltage rise on the capacitance is part of it, but ESR will be the dominant effect. A MOV at the input may be desirable, or a few ohms in the CMC if you can afford it.Tim

author: CaptDon
author_rank: Super Contributor
timestamp: June 09, 2024, 03:34:06 pm
content: That circuit is the nearly standard front end of TV's, DVD Players, Lower cost home HiFi gear etc. Most of them have plain ordinary electrolytic capacitors rated at 400VDC. The only obvious difference, and maybe you omitted it is a line fuse. I run a particular series of powerful audio amps and they have basically the same input circuit, however the capacitors are 10,000ufd each and with the power switch in the off position they precharge the capacitors through a resistor. The operator is warned not to plug the units in to the mains with the switch in the on position as the inrush current would be huge. There is also a warning not to switch the units to the ON position until the capacitors have had 20 seconds or more to charge. Often the circuit you have shown makes a distinctive 'POP' as you plug the unit into the mains. They often do not have any charge current limiting for the capacitor which is a brutal design flaw.

author: jpyeron
author_rank: Regular Contributor
timestamp: June 09, 2024, 08:37:33 pm
content: Thanks, makes sense "diodes limiting...".I did not detail the pre FWB.Side note, turns out the wrong board was sent, so less work, more waiting.Here are the pre pics, for the curious

author: jpyeron
author_rank: Regular Contributor
timestamp: June 09, 2024, 08:38:09 pm
content: 3rd pic

author: Zero999
author_rank: Super Contributor
timestamp: June 10, 2024, 08:10:17 am
content: There's clearly a 1A fuse.Why are you replacing the capacitor? I looks fine. Have you tesed the ESR (Effective, Series, Resistance) or value and found it to be bad? Just use an ordinary aluminium electrolytic capacitor. Don't cheap out to much and stick try to get from a propper distributor.The black circular, ceramic coated device, labelled NTC (Negative Temperature Coefficient), is a in-rush limiter. It starts out with a high resistors, which limits the charge current to the capacitor, thus protecting the dioes and fuse, then its resistance drops, as it warms up.

author: jpyeron
author_rank: Regular Contributor
timestamp: June 10, 2024, 02:17:15 pm
content: "The chassis will not close because the post bridge rectifier cap is in the way of mounting."

author: Zero999
author_rank: Super Contributor
timestamp: June 10, 2024, 06:33:17 pm
content: The trouble is, you'll have difficulty finding something smaller, with the same characteristics: ESR, voltage, capacitance.If you're never going to run it off 230VAC, then you could reduce the voltage rating, keeping the value the same.