Diode selection

fluxanode

Joined Mar 23, 2014
24
Can someone suggest a Diode for reverse polarity protection? My voltage is 24VDC and Max current is 3.5 amps. I need to protect my circuit from being connected to the power input backwards.

crutschow

Joined Mar 14, 2008
33,318
Can someone suggest a Diode for reverse polarity protection? My voltage is 24VDC and Max current is 3.5 amps. I need to protect my circuit from being connected to the power input backwards.
To avoid the voltage drop and power dissipation of a power diode, you can use a MOSFET for that purpose.
It only has a drop equal to its on-resistance (which can be in the low milliohm region) times the current.

Circuit with simulation is shown below:
The circuit works because a MOSFET conducts equally well in either direction when biased on, as occurs when +24V is applied to the input.
But if a negative voltage is applied to the input, the MOSFET stays turned off and blocks any current.

So the output voltage (green trace) basically follows the input voltage (red trace) until the input goes negative, at which point the output stays at 0V.

The yellow trace shows the input to output voltage drop for the MOSFET model I used, which stays below 100mV until the MOSFET turn-on threshold voltage is reached at about 4V, at which point the MOSFET starts turning off and the substrate diode starts conducting with the drop increasing to about 0.6V until the voltage input voltage goes below ≈0.5V, at which point the output goes to and stays at 0V.

You can use just about any P-MOSFET with a voltage rating of at least 40V, and an on-resistance of no more than about 30 milliohms, which will give a voltage drop of only about 100mV with 3.5A circuit current.
That also gives a power dissipation in the MOSFET of only about 0.35W.

Last edited:

LowQCab

Joined Nov 6, 2012
3,555
If your Circuit will still operate satisfactorily with a Power-Supply Voltage-Loss of ~0.35-Volts,
and You don't mind that the Diode will have to dissipate ~1.25-Watts of Heat,
then use a Schottkey-Diode.
This p/n will work ......... SBR12A45SD1-TDICT-ND ~\$0.90 cents.
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fluxanode

Joined Mar 23, 2014
24
Thank you

crutschow

Joined Mar 14, 2008
33,318
Thank you
You're welcome.
So have you decided whether to use the diode or the MOSFET?

fluxanode

Joined Mar 23, 2014
24
Diode

KL7AJ72

Joined Apr 15, 2021
22
Can someone suggest a Diode for reverse polarity protection? My voltage is 24VDC and Max current is 3.5 amps. I need to protect my circuit from being connected to the power input backwards.
They also now make Schottkey power diodes....I never encountered any until I picked up a whole pile of surplus ones. What will they think of next?