On Friday, Apr 16, 2004, at 11:50 US/Pacific, Rick Regan wrote:

> you turn the "current knob" to the desired setting, but what is really
> happening inside?  Isn't it just adjusting the voltage (and perhaps
> resistance?) to get to the desired current?  If that is true, then
> current is not really being manipulated - it is just a by-product of
> the V/R combo.

No.  Like I said before, if your power supply contains bipolar
transistors, then it's all BASED on current, since bipolar transistors
are current-controlling devices.

There's this big transistor on the back, probably on a heat sink.  The
power supply allows the base current of that transistor to rise
(increasing the collector current as well, but the beta factor) until
it senses that a reference voltage has reached a correct value.  In CV
mode, the reference voltage is just the output voltage  In CC mode, the
reference voltage is the voltage across some current-sensing resistor.
Your linear regulators (7805, etc) work the same way.  The internal
components are controlling current, rather than voltage directly.

A transistor is a fine example of a device that permits current to be
controlled directly.  Bipolar transistors are current-controlled, FETs
are voltage controlled, but in each case it's the current that is being
manipulated.  (Come to think of it, tubes work this way too...)

BillW

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads