So i wan't to protect my circuit from being connected with reversed polarity, I have had a hunt around on the internet but have only found bits and bobs, mainly focused on protecting from a voltage spike rather than reverse polarity.

I ruled out using a diode in series due to the voltage drop associated with it as well as cost.

I have read about the Crowbar Circuit with a diode in parallel and fuse in series but no idea how to select the specifications of the diode and fuse appropriate to my circuit?

Its a 24V battery and the circuit draws approx 1A.

Thanks for your time in advance !
Rob

 

Any other solution will cost more than a diode. However you can use a schottky diode which has a lower Vdrop than a regular diode. You can also use 2 diode with a relay for an almost no Vdrop solution http://www.chris.org/Modifications/reverse-polarity-protection.html

 

The fuse is in series and the diode is connected to ground (anode to ground for a plus supply) after the fuse. Then if you apply a reverse voltage the diode conducts and blows the fuse.

The fuse has to carry the load current and should probably have about a 1.5-2A rating. (Does the circuit draw any surge current?)

The diode should have about a 4-5A rating. No heat sink is required.

 

You could use an N-ch power MOSFET and a couple of Zener diodes back-to-back with a resistor to provide the reverse polarity protection. The voltage drop across the MOSFET will be minimal, if you select one that has a low Rds(on).

The Zener diodes are necessary, because most MOSFETs will be permanently damaged if you exceed ±20v from the source terminal to the gate terminal. The MOSFETs are used to ensure the Vgs stays within that limit.

If the battery is connected improperly (reverse voltage) then the MOSFET will remain off.

See the attached.

 

You could use an N-ch power MOSFET and a couple of Zener diodes back-to-back with a resistor to provide the reverse polarity protection. The voltage drop across the MOSFET will be minimal, if you select one that has a low Rds(on).

This is an active rectifier app

 

If you need to put the MOSFET in the positive line instead of the common return you could use a P-MOSFET.

 

Try this....its easy and it works ......

 

Carl's assertion about the P-ch MOSFET is correct.

I showed the circuit using an N-ch power MOSFET. Unless there is some reason that you need to switch the positive supply (like the ground is permanently connected to a common frame or the like) then I suggest that you will be better off using the N-channel solution to switch the ground side of the load; as you have a far wider selection of N-ch MOSFETs as P-ch MOSFETs.

 

Try this....its easy and it works ......

... and you wind up with voltage drops across two diodes instead of just one diode.

 

at 24v the little tiny bit you lose shouldnt make a difference really

 

at 24v the little tiny bit you lose shouldnt make a difference really

With standard silicon rectifiers you would lose about 1.5V. I wouldn't call that a "tiny bit".

 

With standard silicon rectifiers you would lose about 1.5V. I wouldn't call that a "tiny bit".

Yeah, just a tad over 6%.

Hey, mind if I cut your salary by the same? After all, it's just a "tiny bite."

 

The fuse is in series and the diode is connected to ground (anode to ground for a plus supply) after the fuse. Then if you apply a reverse voltage the diode conducts and blows the fuse.

The fuse has to carry the load current and should probably have about a 1.5-2A rating. (Does the circuit draw any surge current?)

The diode should have about a 4-5A rating. No heat sink is required.

Say one uses this solution, could one use a polyswitch (self resetable fuse) instead of a fuse?

 

Say one uses this solution, could one use a polyswitch (self resetable fuse) instead of a fuse?

You could, but the speed of the fuse and the maximum available supply current is a factor. If it's too slow the diode will tend to blow before the fuse opens.

 

Yeah, just a tad over 6%.

Hey, mind if I cut your salary by the same? After all, it's just a "tiny bite."

hahaha well it wouldnt make much of a difference here but then you Americans are generally well rich moneywise over there....