Electronics > Projects, Designs, and Technical Stuff

lowish frequency EMC filtering - lowest ESR or highest Capacitance

(1/6) > >>

Simon:
I am looking to design a filter. The frequencies of concern are hundreds of kHz to a few MHz. The item clearly does not have much bulk capacitance. I have a 35mohm ESR 1500µF capacitor that is sizable and 2x 68µF 35mohm capacitors to give lower ESR even without a lot of capacitance.

Problem is this is becoming quite bulky. I'd like to drop something. What do I go for, leave the big capacitance with pretty low ESR or the keeping 2x 68µF caps for half the ESR.

Basically is 1500µF over the top and not helping but less than 35mOhms a better bet. This is a 5A fan motor.

Kleinstein:
Filtering needs more than just a low ESR capacitor. The usual filter needs at least also an resistor or inductor. The more practical filter is something like a T or Pi topology, so 2 caps and an inductor or the other way round.

For the capacitor one may get better of combining 2 caps. An electrolytic cap for the lower frequencies and a relatively small ceramic one (with low ESR) for the higher ones. 1500 µF is not that much for 5 A current when it comes to the low frequency end (a few kHz).
The inductor may be as bulky as the caps   :(

Simon:
I am indeed putting in pi filters, it's a case of I have to have inductors, they are the big chunky things so one for common mode and one for differential. But the capacitors have more options to play with as like you say i can use several.

I have a number of ceramics, they are not bulky items. I am looking first to the bulky items which are the large and medium caps. Basically if 1500µF is not going to help sat behind 0.035 ohms ESR then I can take that out and put a smaller one in. In the area of concern i have 2x 68µF and 1x 1500µF all 0.035R ESR so maybe I should just make that 3x 68µF and save some room while keeping the ESR.

Conrad Hoffman:
I've had good results using Oscon capacitors for noise/EMC compared to regular aluminum electrolytics. You have to bench test as data sheets don't reveal everything.

Zero999:
One thing which is often forgotten is filters also often need resistors, as well as inductors, otherwise they can resonate at certain frequencies, making things worse, rather than better. Often discrete resistors aren't required, because the inductors act as resistors, at problem frequencies, but it's something to watch out for.

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod