K S wrote:
> 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.

The problem is that your control scheme is barely stable, so it reacts badly
to transients.  Decreasing the gain isn't necessarily going to increase
stability.  A brute force method is to slow down the opamp, probably with
use of a compensation cap, although that may reduce the bandwidth too much
to be useful.  Without careful analysis (and probably requiring parameters
not available) the active feedback part with the gain needs to be the
slowest thing in the system.  I'm guessing that the opamp driving the FET
gate capacitance is slower than the opamp itself, hence the instability.

However, rethinking the circuit topology is in order.  Why are you using a
FET in the first place?  Bipolar transistors operate on current.  The
control circuit for a bipolar will have to do less "work" to maintain
constant current thru the collector.  In other words, bipolars are
inherently more stable when used as current sinks.

You're also going to have a problem of power dissipation.  You are talking
about sinking up to 450 watts.  That's a lot more than a single transistor
even with heat sink and forced air cooling can handle.  You will need a bank
of transistors on heat sinks with fans.  Swithing in load resistors also
comes to mind.

This is NOT a trivial project.  Frankly if you have to ask here about it,
you need to get someone else to do this.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist