Capacitors for PWM circuit (20-40kHz)

[Jim]

Hi all,

Im after a bit of advise regarding the different types capacitors,

I need to "trick" a PWM circuit that monitors feedback from the solenoid it controls, in order to give the solenid less current. [long story for its purpose, the PWM circuit cannot be modified because it has been potted)

My intentions are to use a resistor and capacitor in parallel with the solenoid, and a resistor in series with the solenoid. (I have it currently working the way I want it.)
The unit that controls the solenoid will go into a fault mode if the parallel resistance is too little (<20 ohm), or if the capacitor is omitted, and also will
go into fault mode if a parallel resistor and capacitor is added, without the series resistor (3 ohm).

I have got it all working the way I want, basically I need to find out what type of 1uf capacitor will be suitable for the parallel connection
So far I think Metallised Polypropylene or Metallised Polyester might be the way to go.
The frequency range of the PWM is 20-40kHz, and the current dissipated by the parallel resistor and capacitor would around 10 watts maximum.

Any suggestions on a suitable type capacitor for long term usage of this circuit?

--- + supply-----------------------[20 OHM]\
[O] |
[L] |
[E] |
[N]Rs=4.5OHM -- 1.0uF
[O] --
|
[D] |
---pwm--out-------[3 OHM]------------------/
(low side switching)

Thanks
Jim

 

[John Popelish]

"Jim
(snip)
I have got it all working the way I want, basically I need to find
out what type of 1uf capacitor will be suitable for the parallel
connection
So far I think Metallised Polypropylene or Metallised Polyester
might be the way to go.
The frequency range of the PWM is 20-40kHz, and the current
dissipated by the parallel resistor and capacitor would around 10
watts maximum.

Any suggestions on a suitable type capacitor for long term usage of
this circuit?

--- + supply-----------------------[20 OHM]\
[O] |
[L] |
[E] |
[N]Rs=4.5OHM -- 1.0uF
[O] --
|
[D] |
---pwm--out-------[3 OHM]------------------/
(low side switching)

Either should work, at room temperature. The polypropylene ones will
probably handle a warmer ambient a bit better. And cost more and be
larger. Pick a unit that has a DC voltage rating about twice the peak
voltage the PWM applies, to compensate for the high AC content of the
voltage and internal temperature rise from ripple current.

--
John Popelish