Cap voltage values

[Sparky]

Something I've noticed from repairing a number of LCD monitors is they commonly use 25V caps on the secondary side (and seen a couple 35V even). However, in every case the secondaries were only 12V and 5V, or sometimes 13.5V and 5.5V rated (or some strange value there). Testing with a DMM shows that to be true

I stuck some 16V caps that I had in one to replace some 25V caps on the 12V line and naturally it worked fine... But I can't help but wonder why they used 25V. Is there a good reason behind it, or were they just trying to give a little bit of a buffer since they knew they were using junk caps in the first place?

I'm guessing the latter as that seems to make the most sense, but figured I'd double check with the pros here before I assumed too much and had trouble down the line with the 16V caps.

[Logistics]

Re: Cap voltage values

I'd also like to know about this as I'm about to replace some caps in a SyncMaster 203B, namely the 25V jobbers which have leaked, with 16V caps because the output says it is 13.5V. I can only imagine needing 25V for huge spikes, but I can't imagine that actually happening in this monitor.

[jetadm123]

Re: Cap voltage values

It's probably because the 12V side that powers the inverter section is unregulated. It's approx. 12-14V under load, but can jump up to 16-19V when the load is taken off. Therefore, you probably don't want to use a 16V cap in place of a 25V one. It will work for a while, but will probably fail quickly.

[mariushm]

Re: Cap voltage values

That's pretty much it.

Most inverters are fed from 12v but the regulation is not great, when the monitor goes in standby the voltage can go up to 14-15v which is too close to the 16v limit.

Another explanation could be the higher resistance to heat. Capacitors have different life time rating depending on the diameter and the height of the capacitors. 25v or 35v rated caps are usually 12.5mm in diameter, and these have more hours @ 105c than thinner capacitors.

Though some manufacturers don't seem to really care or design the monitors with parts barely good enough to go over the warranty period or they're too nationalists (Samsung using Samwha because they're South Korean)

[Logistics]

Re: Cap voltage values

Well, I replaced the two CapXon 820uF 25V jobbers which occupied the same trace with a single 1800uF 16V Rubycon which I had from a motherboard job or something. There was a 330 I replaced with a 330 poly, and anyway the monitor came right up after replacing the caps. Now that I know it works I will give it a full recap with appropriate values.