How to protect circuits from reverse current flow?

Test Electrix · 30th August 2010, 08:39 PM

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

leadbelly · 30th August 2010, 08:46 PM

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

Test Electrix · 31st August 2010, 01:32 PM

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)

stormsonic · 31st August 2010, 02:10 PM

Search for "Low forward voltage Schottky diode".

CLICK

djQUAN · 31st August 2010, 02:34 PM

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 · 31st August 2010, 04:59 PM

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 · 31st August 2010, 05:15 PM

Quote:

Originally Posted by djQUAN

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 · 31st August 2010, 05:42 PM

Quote:

Originally Posted by djQUAN

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

Test Electrix · 31st August 2010, 06:52 PM

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

pjp · 31st August 2010, 07:50 PM

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.

30th August 2010, 08:39 PM	#1
Test Electrix diyAudio Member Join Date: Mar 2009	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

30th August 2010, 08:46 PM	#2
leadbelly diyAudio Member Join Date: Dec 2002 Location: Calgary, Alberta	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 __________________ Aerodynamics are for people who can't build engines. Enzo Ferrari

31st August 2010, 04:59 PM	#6
Mooly diyAudio Member Join Date: Sep 2007	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. __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*

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31st August 2010, 01:32 PM	#3
Test Electrix diyAudio Member Join Date: Mar 2009	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)

31st August 2010, 02:10 PM	#4
stormsonic diyAudio Member Join Date: Aug 2008	Search for "Low forward voltage Schottky diode". CLICK

31st August 2010, 02:34 PM	#5
djQUAN diyAudio Member Join Date: May 2004 Location: manila	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.

31st August 2010, 06:52 PM	#9
Test Electrix diyAudio Member Join Date: Mar 2009	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

31st August 2010, 07:50 PM	#10
pjp diyAudio Member Join Date: Nov 2006 Location: under the rainclouds	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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