Hello PICList.

I am trying to make a high current, variable load. What i have is an
approx 15V rectified 50Hz AC source and i would like to:

1. Be able to sink currents from zero to 30A in 10 discrete steps of
3A each (ie: 0, 3A, 6A, 9A, ... 27A, 30A).
2. Do the above by clipping the source rather than increasing or
decreasing the load (ie: somewhat digital control)
3. Preferably do it without any micro, etc.

So far i've tried connecting the source through a FET and load
resistor, then feeding the current sense voltage from the resistor to
the inverting input on an opamp, and setting an adjustable value on
the noninverting input. What i'm finding is that the opamp output
swings too quickly - as soon as the threshold (clip) voltage is
reached the output goes low very quick and i get more of a PWM looking
waveform than a clipped waveform. I've tried decreasing the gain of
the opamp to "slow it down" but this hasn't helped.

My experiments work OK with DC but as soon as i apply the rectified
50Hz i get the symptoms above.

Any advice would be greatly appreciated.

Rgds, Kris.

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