Re: Reversed Biased MOSFETS & BJT's?

"RogerN" <regor@[EMAIL PROTECTED]
> wrote in message 
news:O-ydncbRR-7ynW_VnZ2dnUVZ_o7inZ2d@[EMAIL PROTECTED]
> I'm working on a battery charger / discharger design.  There is always
the 
> possibility that someone will connect a battery reverse polarity.  I
don't 
> want this to destroy the charger.  I remember the stuff about correctly 
> biasing transistors but how will a BJT or MOSTES act if you have
Collector 
> Emitter or Drain Source reverse biased?  I'm trying to avoid diodes in
the 
> discharge circuit unless the are required as I want to be able to
dicharge 
> to 1V while measuring current and voltage.
>
> I tried this on simulation and it seems to work (not conduct) but I
don't 
> know if the simulation is realistic with reverse bias.  I can also get a

> 1A diode to work at 500A in simulation.
>
> Thanks!
>
> RogerN
>
>

It may be worth avoiding the issue altogether - on most of the commercial 
battery chargers I have, a relay click can be heard which is the reverse 
polarity protection. Although most of the chargers contain a 
microcontroller, the protection could be implemented much more simply in 
your charger with a comparator driving a relay to connect the charger 
circuitry to the output leads.

This would have the same shortcoming as the commercial units that if the 
battery is discharged dead flat there is no voltage for the comparator to 
sense as "correct polarity" I keep a spare current limiting PSU on hand to

get the battery started on these rare occasions.