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Old 30th August 2010, 08:39 PM   #1
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Default How to protect circuits from reverse current flow?

Okay I've gt a DC power connection lined up but does anyone have add-ons that can prevent a current reverse of what is desired possibly ruining something? I thought I had something with a diode in but I can't quite find it, if it exists
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Old 30th August 2010, 08:46 PM   #2
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Yes, a simple forward biased diode is probably what you saw and are looking for. You probably something using a 1N400x for that service.

Given up on the 500W SE OTL tube amp you said you were qualified to design and build?....just teasing
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Old 31st August 2010, 01:32 PM   #3
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I never said I was qualified 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)
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Old 31st August 2010, 02:10 PM   #4
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Search for "Low forward voltage Schottky diode".

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Old 31st August 2010, 02:34 PM   #5
djQUAN is offline djQUAN  Philippines
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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.
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Old 31st August 2010, 04:59 PM   #6
Mooly is offline Mooly  United Kingdom
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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.
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Old 31st August 2010, 05:15 PM   #7
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by djQUAN View Post
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.
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Old 31st August 2010, 05:42 PM   #8
paulb is offline paulb  Canada
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Quote:
Originally Posted by djQUAN View Post
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
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Old 31st August 2010, 06:52 PM   #9
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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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Old 31st August 2010, 07:50 PM   #10
pjp is offline pjp  India
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Personally I prefer a diode in reverse across the supply rails, coupled with a fuse.

A diode or a MOSFET in series with your device does not protect sufficiently.
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