Al Shinn <alshinn <at> mindspring.com> writes:

>=20
> If a FET is connected from the collector to base of a bjt(like a=20
> Darlington but with a FET for the "top" transistor), can the bjt=20
> transistor achieve saturation specs? Apropos of the OP's desire for a=20
> variable load for a battery tester.


I have been thinking lately more about the variable load and OP's desire to
drive the battery under test to 0V.=20
Darlington transistors have been suggested, but rejected due to high satura=
tion
V.=20
The summary of specs IIRC is as follows:
1. Discharge battery under test to as close to 0V as possible;
2. Constant current;
3. Accurate current measurement.

If one to use a Darlington or IGBT and connect Emitter to ground, and drive=
 base
current from a separate supply, one can draw battery down to 0.3V?  If we u=
se
MOSFET, then that limit could be close to 0V. =20

The only issue is measuring current to provide appropriate feedback.  High-=
side
sensors were rejected due to high cost/ low accuracy.  How about Hall-effec=
t
sensor, something like MLX91205?  link:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=3DMLX91205=
KDC-HB-
ND

This arrangement seems to satisfy OP requirements.

Thanks,


Sergey Dryga
http://beaglerobotics.com



--=20
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