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'[EE] Using a high current MOSFET asavariableresist'
On 16/04/2011 00:46, V G wrote:
> But I don't understand why the high side sensing is difficult. Isn't it the
> same idea? Just measure the voltage drop across the resistor? I'll just
> sample some nice diff amp from LT or something.
> With the low side sensing, I can just use a good opamp?
How accurate do you want this to be?
I would probably just use a 2N3055 and Fet with low side sense resistor, plus a basic opamp with pot.
For displaying the current value you could use one of the dedicated display modules with the intelligence built in, or a small PIC and 2/3 digits.
I know you said you have little time, but I would really advise spending ~15 mins watching the EEVBlog video - it's pretty much exactly what you are looking for (just lower current IIRC), and roughly what is being proposed by folk here. In any case, those 15 mins may actually save much more trying to figure all this out.
On 16/04/2011 01:07, Bob Blick wrote:
>> > But I don't understand why the high side sensing is difficult. Isn't it
>> > the
>> > same idea? Just measure the voltage drop across the resistor? I'll just
>> > sample some nice diff amp from LT or something.
On "high" side you need the difference of two voltages, traditionally one may be at supply rail level. In this application you could use a separate +9V or +12V rail to power op amp.
Traditionally there are common mode issues with high side sensing. Often the "norton" type amp LM3900 has been used (e.g. supply current sense of regulators, Transmitters and lamps at the battery / supply side. The LM3900 has essentially unlimited positive common mode above its own supply rail as it current driven.
> Operating dual supply it would be no problem but single supply you will
> need a good single supply diff amp or buffer a divider to feed a regular
> diff amp reference. So choosing your diff amp will be an exercise in
> datasheet comprehension.
>> > With the low side sensing, I can just use a good opamp?
> Even an LM324 would work on the low side. It just needs to have inputs
> and outputs valid to ground.
On the "low" side you have a voltage WRT to ground proportional to the current. A differential amp and common mode ranges are not an issue. If there is an ADC it may even read the voltage directly
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