Reverse polarity protection
Reverse polarity protection
(OP)
Well basicly what im looking for is a way on protecting a circut from a reverse battery condition.
I have considered several common solutions such as:
-Diode (.7 Voltage drop for Silicon Diodes is to high,germanium
voltage drop still not ideal)
-Fuse (only works once)
What i am looking for is a way to protect a microprocessor,H-brige,support circutry from reverse polerity.
The ideal solution would be to have something that will ither shut of power to the circut or even better to internally reverse the the polerity to still power the circut.
Also it cant have much of a voltage drop preferable no voltage drop
Anyone have any ideas,
thanks for your time
I have considered several common solutions such as:
-Diode (.7 Voltage drop for Silicon Diodes is to high,germanium
voltage drop still not ideal)
-Fuse (only works once)
What i am looking for is a way to protect a microprocessor,H-brige,support circutry from reverse polerity.
The ideal solution would be to have something that will ither shut of power to the circut or even better to internally reverse the the polerity to still power the circut.
Also it cant have much of a voltage drop preferable no voltage drop
Anyone have any ideas,
thanks for your time





RE: Reverse polarity protection
yours
RE: Reverse polarity protection
I once saw a design idea that used a MOSFET for polarity protection, but cannot now find where it was.
Instead, try a diode that activates a relay when the polarity is correct. That way you only have only the miniscule drop of the relay contacts.
RE: Reverse polarity protection
The Relay option sounds pretty good exept id like to keep this relativly small, but maybe il find a source of really small relays, il have a look around
Any one else have any idias, are there maybe some ICs specificly for this task?
RE: Reverse polarity protection
Easier yet is you use a P-FET pulled down to ground with about a 1M resistor. Hooked up backwards gives no current. Forward very little drop.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Reverse polarity protection
If you flipped the PMOS so that the source feeds the output, it might work as described, but the turn on will be soft.
You could try a Shottky diode:
http:
TTFN
RE: Reverse polarity protection
The parasitic diode is used to power the circuit until the mosfet is enhanced.
h
RE: Reverse polarity protection
TTFN
RE: Reverse polarity protection
(polyswitches have voltage and Ipeak requirements, they are not ideal devices but they might suit your app.)
RE: Reverse polarity protection
RE: Reverse polarity protection
im leaning towards the MOSFET solution(great current capability, low voltage drop)
RE: Reverse polarity protection
RE: Reverse polarity protection
RE: Reverse polarity protection
Another choice could be a Schottky diode (lower voltage drop than a normal rectifier).
RE: Reverse polarity protection
I'm not sure how to insert a picture here, so try this...
D S G
PFET 1: battery+ Load+ battery-
PFET 2: battery- Load- battery+
NFET 1: battery- Load- battery+
NFET 2: battery+ Load+ battery-
PFET 1 & NFET 1 are on at the same time when the battery is coonnected with polarity as indicated.
PFET 2 and NFET 2 are on when the battery is conencted opposite.
The load always sees the correct polarity.
Again, mind the maximum gate voltage. If the battery voltage is too low, the circuit will still provide full wave rectification through the substrate diodes.
You can also use this with an AC input, again, minding the voltage & current ratings.
Jim
RE: Reverse polarity protection
D S G
PFET 1: battery+ Load+ battery-
PFET 2: battery- Load+ battery+
NFET 1: battery- Load- battery+
NFET 2: battery+ Load- battery-
Sorry about that,
Jim
RE: Reverse polarity protection
Picture posting:
FAQ238-1161
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Reverse polarity protection
Thanks itsmoked.
The above is a starting point. You may want to make additions like gate resistors depending on your application.
The source voltage can be either polarity, the load will always see positive on the top end.
Jim