PWM Load control … Smoothed current, how?

Discussion in 'General Electronics Chat' started by Dyslexicbloke, Jun 5, 2011.

  1. Dyslexicbloke

    Thread Starter Well-Known Member

    Sep 4, 2010
    Hi folks,
    I need to create a dump load in order to test the power output curve of a water turbine.

    The unit has a theoretical maximum output of around 500W and is coupled to a three phase PM alternator which should produce about 24V at the target rpm.

    I have a huge 0.9Ohm WW power resistor which would draw over 26A (600W+) that I think will be ideal if I use PWM to control its current.

    Now I understand about average power and I have some reclaimed FET’s that will handle the required current and switch the load at a few hundred Hz but I am unsure what my DC clamp will make of the current waveform.

    Do I need some sort of choke/inductor in the circuit to smooth out the current waveform and if so how do I go about working out what?

    I don’t know much about ‘magnetics’, I get that a series inductor would essentially store and release current, smoothing the square waveform. I am also assuming I would need a bypass diode, or another FET, to maintain the current path when the driver FET is off, allowing the inductor to discharge.
    What concerns me is that I am probably oversimplifying … Am I not in buck converter territory here? Which as I understand it is far from as simple as it seems and regarded as a specific field of electronics design.

    There are probably many questions I should be asking but the only one I have thought of thus far is what happens when the PWM signal approaches 100%, wouldn’t the inductor simply saturate and become useless?
    Is that, saturation I mean, why the PWM chip has two outputs, each doing 0-50% duty and 180Deg apart … do I actually need two distinct driver circuits, FET, choke and cap, feeding the load?

    Any and all advice welcome,