Capacitor nightmares, too large to be practical.

Thread Starter

SheldonNyce

Joined Aug 6, 2019
26
Hi guys. I am designing an EMI filter for a light dimmer circuit, to filter the incoming 230Vac (50Hz) source. The damping capacitor, according to calculations should be 23.5 uF, so lets say 25uF for ease of use. Polypropylene film capacitors rated to 230Vac are HUGE!!! ~55mm or greater in height! How does one get around this? It's connected between the live and neutral terminal of the source, so unfortunately an electrolytic won't be able to be used. Any ideas? I've searched the Polypropylene film and Polyester Film capacitors, and they are ALL to large to be practically be implemented in the circuit, at that capacitance, at that voltage. Is there perhaps some mystical type of capacitors that I have missed, that could provide the right capacitance with high ESR in a small package, that can handle 230Vac? Any advice would be greatly appreciated.
 
Last edited:

shortbus

Joined Sep 30, 2009
6,979
Hi guys. I am designing an EMI filter for a light dimmer circuit, to filter the incoming 230Vac (50Hz) source. The damping capacitor, according to calculations should be 23.5 uF, so lets say 25uF for ease of use.
Since they don't seem to be made in the value you need, is it possible you did your math wrong?? If you are talking across the line caps they have to be of a certain size for the required electrical "stand off" to suit the voltage used, that is one condition that isn't able to be down sized.
 

Thread Starter

SheldonNyce

Joined Aug 6, 2019
26
Would it be safe to assume that I cannot use an electrolytic capacitor in this instance?
 

Janis59

Joined Aug 21, 2017
767
My coin - if the unit will work on 50 Hz, then avctual noise will happen with at lest 100 Hz (1st harmonic) but exist a zillions or publications argued that actual spectra emitted comes out up to the 23rd harmonic, Your case 23*0,1 kHz=2.3 kHz. Not all capacitor will give a good habits at high-power of such frequency. I mean the REEACTIVE POWER will warm them up until the blow. For example- if Your aim is to take some 25 Amps from the 3F network at 380 Volts, the reactive loss factor in prost caps are order of magnitude tan(delta)=0,03. As the tan(delta) for small angles less than 5 degrees is equal to power factor cos(fī) then thermal flux will happen N=V*i*cos(fi)=V*i*tan(delta)= 535 Volt* 25A*3*0,03=1,2 kW of pure thermal flux. It will devastate all around the crater of blow.
My choice would be to alter the coil to diminish the cap.
 

Thread Starter

SheldonNyce

Joined Aug 6, 2019
26

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cmartinez

Joined Jan 17, 2007
6,464
There are two possible techniques I can think of for using electrolytic capacitors that could help you solve this conundrum. I'll leave it to the more knowledgeable members of this form to explain how recommendable it is that you actually use them. I'm not exactly an expert, after all.

First, take a look at this page.

It explains these two arrangements that could let you use electrolytic caps on an AC circuit:

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LowQCab

Joined Nov 6, 2012
16
As per usual............
The question must be asked.
What is the exact description of the problem you are trying to solve ????
You may be able to simply install an Isolation Transformer and be done.
It may also be possible that you could rethink the power conditioning section of the circuit you are trying to protect.
There are plenty of active MOSFET protection schemes on the internet.
Or are you trying to keep noise out of your mains wiring so that it won't be re-broadcast into the surrounding area ???
If so, maybe there are more efficient ways of suppressing the noise generated by your device.

Please be specific.
.
.
 
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