×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

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

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

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

(OP)
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 ?

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

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

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

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!

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources