Re: gEDA-user: powermeter power supply update

[Joerg <joergsch@xxxxxxxxxxxxxxxxxxxxx>]

DJ Delorie wrote:
>> 4.5 watts? That must be one hell of a flashlight.
> 
> It's a big 6v rechargable one.  The flashlight doesn't work, but the
> bulb does.
> 

We got one of those big yellow Lowes deals. You can make a street sign 
half a mile away light up with it. $10 including charger, 8 years old, 
used daily, gel cell in there, still works, amazing.


>>> One of these days I'm going to make a digitally-controlled load.  It's
>>> just a monster FET with some control circuitry, right?
>> Basically, yes. But in order to dial in resistances you must find two 
>> that are nearly identical (pairs are rare in the power class)
> 
> I saw an appnote about one FET that had a different temperature
> coefficient, so could be used paired (for heavier loads), because if
> one overheats it *reduced* its conductivity - they ended up balancing.
> But I suspect that's not what you're talking about - you're talking
> about a current mirror, yes?
> 

No, a real servo. But for that they must remain at identical die 
temperatures. Not easy at large loads.


>> and servo it with an opamp.
> 
> I knew that part.
> 
>> Or just command currents, then it's down to one big FET.
> 
> One FET, one current sense resistor, and an opamp, plus however you
> derive the control setting for it.
> 

Yep. You can also set load resistance that way if you also sense the 
voltage and divide by the sensed current. Either a reasonably fast uC or 
an "analog computer chip" from Analog Devices (will be a bit pricey).

-- 
Regards, Joerg

http://www.analogconsultants.com/



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