Replacing capacitors in a power supply circuit

Question

Recently, my computer monitor has stopped turning on, I think this could be down to a few faulty capacitors... I think. I tested the output of the power circuit and the outputs, 12v and 5v, were working, when I added some load (using a 4.4Ω resistor) the 5v line died.

I removed all the capacitors from the PCB and tested them with a multimeter set to measure resistance. I can watch the capacitors charge; they all look fine though (I'm not sure if there's a better way to test them than this though?).

I still thought that it could be a problem with a few capacitors as they looked like they had slight bulges in the tops, so I found a capacitor from an old computer PSU that matched the spec (1000µf 25v) and tried my monitor with this... success. However this morning, the monitor has stopped working again, so I'm going to just replace all of the capacitors and see if that resolves the issue.

I've just started looking for capacitors online to replace all of the ones on the power supply board, however there are so many different types. I'm looking on uk.rs-online.com, but say I've searched for 1000µf 25v electrolytic capacitors, loads of different ones are returned at varying prices, does it matter which one I choose? Some of them are labelled 25v dc, could you explain what the difference between a capacitor labelled 25v and one labelled 25v dc are? Are there other things I need to take into consideration when buying these capacitors other than voltage and capacitance?

Here is the power supply board that is faulty:

A close up of a couple of the capacitors:

Answer 1

If you want to test capacitors fully, you will need what is called an ESR meter(Equivalent series resistance)

Electrolytics have an ESR which increases through general use (age and heat are main factors).

SMPS (Switch-mode power supplies) are pretty sensitive to ESR. The ripple voltage on the output is calculated as \$V_{ripple} = I\times ESR\$. This means that as the ESR of your caps increases so does the amount of voltage ripple. I can't say with certainty the problems cause by ripple voltage so I've included an extract from Wikipedia.

Effects of ripple

1. Ripple is undesirable in many electronic applications for a variety of reasons: The ripple frequency and its harmonics are within the audio band and will therefore be audible on equipment such as radio receivers, equipment for playing recordings and professional studio equipment.

2. The ripple frequency is within television video bandwidth. Analogue TV receivers will exhibit a pattern of moving wavy lines if too much ripple is present.

3. The presence of ripple can reduce the resolution of electronic test and measurement instruments. On an oscilloscope it will manifest itself as a visible pattern on screen.

4. Within digital circuits, it reduces the threshold, as does any form of supply rail noise, at which logic circuits give incorrect outputs and data is corrupted.

5. High-amplitude ripple currents shorten the life of electrolytic capacitors.

Now to answer your actual question. As long as the capacitance and voltage rating match that of your current capacitors then that's all you need. Personally I'd recommend Panasonic capacitors, every time I change an aluminium electrolytic I always change it for a Panasonic capacitor.

The backlight of your monitor shouldn't make any difference to the capacitors you need on your power supply.

Answer 2

Ripple voltage may be an indicator, but it is ripple current that destroys electrolytic capacitors. More precisely, it is the \$I^2R_{esr}\$ power dissipation that causes a capacitor's internal temperature to rise, and it is the effects of this temperature that are destructive.

Electrolytic capacitors intended for use in switch-mode power supply (SMPS) roles are rated for maximum RMS ripple current. This provides an equivalent \$I\$ (current) figure that when driven through the \$R_{esr}\$ of the capacitor will keep the power dissipation (and therefore heat build-up) within acceptable limits (often a 10°C internal temperature rise above ambient).

The maximum ripple current figure is related both to the capacitor's ESR and to its physical size. A bigger capacitor presents more area with which to dissipate heat to the surrounding air and can therefore tolerate more internal power dissipation without a destructive temperature rise.

Consequently, the capacity (\$\mu F\$) and voltage rating are not enough by themselves to identify a suitable replacement in these cases. You would generally definitely not want to use a capacitor significantly smaller in size, for example.

Capacitors of this type are listed (in Mouser at least) as "low impedance" or "low ESR", and definitely not "general purpose". If the datasheet does not specify a maximum RMS ripple current, look for one that does.

Nichicon, United Chemi-con, and Panasonic are all good names. This is a typical datasheet, in this case for a Nichicon UPM series component. Manufacturers market different families of these capacitors as a series, often with a few-letter series name (such as UPM in this case) that is printed on the plastic sleeve. A family will have a range of voltages and capacitance values all sharing a general application role.

You may be able to find a datasheet for the original capacitor using its series identifier and capacity and voltage. If so, this may give you some information as to its ripple current and help identify a suitable replacement that will last indefinitely rather than a few hours :)

For example, here is a datasheet for the SAMXON EKM series.