How to protect circuits from reverse current flow?

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I never said I was qualified :p Just hopeful! I think more efficient speakers would be better used instead

Won't the diode drop knock off a bit of the voltage? I need a 9v supply and was hoping to use a 9v input for ease of availability for when I'm not at home (EVERYONE seems to have 9v plugs, not so much 12v though)
 
I've read in a datasheet before about using a P channel mosfet as a "lossless" diode. forgot where though. I think it was connecting drain on + supply, source on load +, and gate on ground/common.

wiring supply backwards doesn't make the mosfet conduct and internal body diode is reverse biased so load is safe.

someone correct me if I remember wrong though.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Use a bridge rectifier if you can stand the voltloss, that way it doesn't matter which way round you connect... the bridge output is always correct.

For zero volt loss use a relay with diode feeding the coil... only connects when polarity is OK. Use two relays and make it as the bridge... works either way round.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
I've read in a datasheet before about using a P channel mosfet as a "lossless" diode. forgot where though. I think it was connecting drain on + supply, source on load +, and gate on ground/common.

wiring supply backwards doesn't make the mosfet conduct and internal body diode is reverse biased so load is safe.

someone correct me if I remember wrong though.

You are correct... it can be done... but it's a bit of an "abuse" of the FET.

You fit the FET backwards, drain and source reversed, so the internal diode works correctly for polarity protection, and the low on resistance of the FET "shorts out" the diode in normal use giving very low volt drop. The gate is tied to ground via a resistor.

It works for a limited range of supplies but it's bad practice IMO.
 

paulb

Member
2001-06-01 4:53 pm
Calgary
I've read in a datasheet before about using a P channel mosfet as a "lossless" diode. forgot where though. I think it was connecting drain on + supply, source on load +, and gate on ground/common.

wiring supply backwards doesn't make the mosfet conduct and internal body diode is reverse biased so load is safe.

someone correct me if I remember wrong though.
Yes. Google for "p-channel mosfet reverse polarity protection", here's the first result I got:
Advanced Power Switching and Polarity Protection for Effects
 
Nice! Does this circuit also allow an external power supply to be connected? It seems to suggest this from the inclusion of a 'power jack' connected to ground and the battery.
Also, while we're here, how would the zener diodes be connected exactly? What specification as well, I've seen a range of characteristics being talked about as desirable
 
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