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Capacitors for PWM circuit (20-40kHz)

Discussion in 'Electronic Basics' started by Jim, Nov 7, 2004.

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  1. Jim

    Jim Guest

    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)

  2. "Jim

    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.
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