Adding protection diode in the PCB

[engr_joni_ee]

I forget to add protection diode in the PCB for someone who mistake reverse the power supply cables. The current consumption in the PCB is around 2.6 A at 12 V. I am wondering if I add 1N5402 in reverse bias, would it serve the purpose ? i.e., in normal operation if the power supply connection is correct the diode will be reverse bias and if the power supply connection is wrong the the diode will be forward bias to protect the rest of the electronics.

 

[betwixt]

Yes, that is exactly what to do but you also need a fuse or over-current protection in the supply because if it is connected backwards the current drawn will be very high.

Brian.

 

[engr_joni_ee]

Hi, what about if I don't have the fuse in house. How to protect the diode then, if power supply connected with reverse polarity then the diode will be forward bias and will run high current. Any idea other then fuse how to protect diode ?

 

[KlausST]

Hi,

I`d use a 3A schottky diode in series. Like SB340.

Benefit:
* No short circuit current
* You don´t need a current limiter
* one part solution
* No part needs to be replaced after a wrong connection.

Drawback:
* about 1W of power dissipation
* about 0.4V of voltage drop

Klaus

 

[FvM]

Safety regulations require some kind of short circuit and overcurrent protection for the power supply itself. The diode should be chosen with sufficient current capability according to the power supply.

You should consider however, that the diode foward voltage drop occuring during reverse supply connection can be still high enough to destroy ICs connected directly parallel to the diode. A series diode or a MOSFET as reverse power disconnect (ideal diode) is the better alternative anyway.

 

[engr_joni_ee]

Hi, yes I see that the forward voltage for SB340 is 0.5V if I add in series.

I am still wondering about the other solution with a diode in reverse bias if there exist a fuse that we don't replace when the power connector cables are reversed. Something like a circuit breaker with a knob ? then we don't need to replace the fuse.

 

[KlausST]

Hi,

fuses may be slow, especially those ones with a knob, or polyfuses.

You deed to calculate the max expectable current during short circuit condition and you need to calculate the time for the fuse to trip.
Then you need to see if the diode can withstand the current for this time.

Klaus

 

[crutschow]

A series P-MOSFET can be used to protect against reverse input voltage.
The forward drop can be very low if you use a low on-resistance device, and requires no fuse or other circuitry.

Ltspice simulation below:
The P-MOSFET may appear to be connected in reverse, but that's so it can block with reverse bias (negative input).
It works because a MOSFET conducts equally well in either direction when biased on.

As can be seen, for a input voltage going from +12V to -12V (yellow trace), the output (red trace) goes from +12V to 0V, while the output current (green trace) goes from 3A to 0A, showing the block of the negative voltage.

When the normal voltage is applied, the MOSFET substrate diode initially carries the current until the voltage becomes high enough to turn on the MOSFET (that's the little kink in the output current and voltage near 0V).
Then the voltage drop is determined by the MOSFET on-resistance.
Pick a MOSFET with a low enough on-resistance to give you the desired minimum voltage drop.
The device I used for the simulation gives a drop of about 100mv @ 3A.