Low pass filter ( RC ? ) either in op amp feedback ( integrator )
or from FET load resistor to op amp or combination of both.


K S wrote:
> 
> 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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