Under rated cap in a pinch

[Sowhat]

I believe most everyone uses a computer for their reference manuals these days. Actually I have two, one at the shop and one at home. They are both the same and use the same old Viewsonic 2021m monitors. These things are over 10 years old and both have had to get their SMPS's recapped over the years. One of my monitors started acting up and you could see the SMPS trying to fire up again and again but no cigar. Once the ESR get's too high on the secondary filters it don't want to work anymore. I opened it up and replaced the secondary filters and I thought that was that. Still it did not want to fire up. It would run for about 10 minutes and then go into death spasms clicking on and off repeatedly. I almost burned myself when I noticed the primary filter, a 150uF 400 volt electrolytic was hotter than the blazes… well, there's my problem. I popped that out and it had an ESR over 100 and a capacitance in the nanofarads, it was a little bulgy too. These things generally don't go bad and are not usually included in cap kits for these monitors. I didn't have one in the shop but I did have a new 100uF 250 volt cap so that would have to do for right now. I installed that and the monitor came back to life just fine. The cap did not get over 100 degrees F and seemed happy to take on the primary filter job. A 400 volt cap is severe overkill on U.S. mains of 120 volts but that primary filter cap takes a slamming so a 400 volt one just gives you that much more overhead and life. 100uF versus 150uF don't really make much difference to the monitors SMPS and it's happy to run with either. I wonder how long this one will last being under rated as it is.

[Mick Bailey]

Many SMPS share the same circuit for multiple voltages and maybe that's the reason for the higher voltage caps - one size fits all. On 240v supply you'd get a working voltage of around 340v. On older supplies the voltage change consists of installing/removing links on the PCB. The voltage rating is probably fine for your supply.

[darkfenriz]

Provided it is a simple flyback converter (I'm 99% sure it is) and not a full boost-PFC (that'd be overkill IMHO) - yes, good deal and well acceptable for US mains

[Mick Bailey]

Good point - PFC is the caveat. Most of those supplies produce the same high-voltage DC regardless of input voltage. Look for the additional inductor if it's passive, or a dedicated control chip if active PFC. Thankfully there are plenty of older SMPS that don't use this - especially if they're low power consumption.