In article <O-ydncbRR-7ynW_VnZ2dnUVZ_o7inZ2d@[EMAIL PROTECTED]
>,
RogerN <regor@[EMAIL PROTECTED]
> wrote:
>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?
Bipolar transistors will often "work" when connected with emitter and
collector reversed (although their speed, gain, and current-carrying
capacity will likely be quite a bit different than in their normal
orientation).
In your situation, whether the BJT will conduct will depend on what
the base voltage is, compared to the "emitter" (i.e. what should be
the collector) - if the base is biased to a voltage which will allow
current to flow through this junction, then the collector/emitter path
will start to conduct as well.
Most power MOSFETs have a "substrate diode" between source and drain.
This diode is reverse-biased during normal operation, but will be
forward-biased (and will start to conduct) if you reverse the source and
drain connections. If your charger/discharger puts the MOSFET across
the battery, with a current limiting resistor, then it's likely that
reversing this battery connection will (in effect) put the battery
into "discharge" mode regardless of what the control logic is doing to
the gate.
> 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.
.... until it turns into a NED (noise-emitting diode) for a brief moment.
You might need to use a Schottky diode to isolate the battery from the
charge-discharge circuit, and run a second connection to the battery
(bypassing the diode) to do the voltage sensing.
--
Dave Platt <dplatt@[EMAIL PROTECTED]
> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
|
