How to measure Ripple current in th eoutput side of AC/DC

[Oslec]

Hi,
I'm doing a lifetime calculation on all the electrolytic capacitor of a UPS. One of the requirement is the ripple current. Can help me to figure out how can I measure the actual ripple current present ia a capacitor ?

 

[fxguthri]

Hi - how ya doing?

The best way would be to use a clamp-on oscilloscope type current probe. You can then not only measure the current but also see spikes and waveshapes that the capacitior is putting up with.

If the electrolytic is mounted on a PCB, then it's a bit of a problem, as you will need heavy wire to extend the leads of the cap to raise it up enough to get the current probe around one wire. A heavy wire is needed so as to not appreciable affect the ESR of the circuit.

If the cap has screw terminals, with wire leads to a PCB or other wiring, all you need to do is place the current probe around one wire.

If you don't have a clamp-on curent probe, then you might be able to rent one, which will be a lot less expensive then buying one. It's also important to make sure that its in reasonable calibration if you need accurate measurements. If you have never used one of these probes, take great care of the clamp-on probe head, as they are very delicate and very expensive. I suggest a padded protector for it when its not in use. Most definitely - don't drop it!

There are old-time charts for figuring out a lot of things having to do with electrolytic filtering but much has changed since the advent of switchers. If you are involved with a switcher, it is very important to make sure that the circuit design is such as to assure that the ripple current ratings are not exceeded. If you do exceed them, then the cap will overheat and may vent (explode), which can lead to other major circuit failures in short order.

One 'lazy' way of checking if the cap is being stressed is to take its temperature. This should be done in the 'worst case' scenario - the power supply is all enclosed, mounted in place where it belongs, and the product operating at its maximum rated environmental temperature. If the cap is too stressed, it will show up in it own temperature readings. You should also keep in mind that ripple currents will vary depending on many factors, such as load and ambient temp.

Finally - pay close attention to how charging AND discharging currents flow into and out of the electrolytic. Stray currents caused by poor layout can cause strange, seemingly unsolveable, problems - especially if high gain current or voltage sensing circuits are nearby.

I hope that this has helped. Good luck.

Frank

 

[gezcooke]

Oslec,

If you're concerned with how long the UPS is going to work reliably then ripple current in the battery is the thing to watch. If this exceed more that 0.1CA then expect the battery to be on it's way out well before any electrolytics have died! I once measured over 125A of ripple on a 24AH battery! It was u/s in less than 2 months but the user only finds this out after a blackout!