Hello All

I am building a simple motor controller and was thinking about reverse voltage protection.

Is it possible to provide reverse voltage protection to my board by simply placing a diode on the Vcc and Vcc2 input lines? (with Vcc being my 5+ logic voltage and Vcc2 running between 3-32volts). The motor driver should run a maximum of 2A on Vcc2 and minimal on Vcc.

If so, which diodes would I use? I have the following in stock:-
1N4001 Rectifier diode 1A 50v
1N4002 Rectifier diode 1A 100v
1N4148 Signal diode
1N914 Signal diode 75v
SB160 Schottky 60v 1A
N5821 Schottky 30v 3A (seem very chunky so probably overkill)

It would be usefull if you could tell me what I should take into account when selecting the diode. What to look for on the relevant datasheets etc. I imagine a fast switching diode is unneccesary so a Schottky diode would be unneccesary.

Thanks

 

For VCC, I would use a 1N4002, cathode to load side of 1/2A to 1 A fuse, anode to ground. 5V ckts. using 5V do not like 4.3V so a series diode not advisable. For VCC2 A 3A diode in series should be OK, one 3A or 3 parallel 1N4002's.

 

Hi Bernard

So do I understand that for Vcc (the 5volt side) you are suggesting using a diode to cause a short circuit between VCC & GND if reverse voltage is applied?

Cheers

 

For VCC, I would use a 1N4002, cathode to load side of 1/2A to 1 A fuse, anode to ground. 5V ckts. using 5V do not like 4.3V so a series diode not advisable. For VCC2 A 3A diode in series should be OK, one 3A or 3 parallel 1N4002's.

Never put diodes in parallel unless they are thermally coupled. The nonlinear behaviour of them will lead to one taking all the current which would cause it to overheat.

 

Diodes are great for reverse voltage protection.. But as stated above, diodes have an associated voltage drop. If you can live with that then go ahead.. If you can't sometimes you need to look for other methods.. Even something as simple as a "keyed" connector can be used that prevents it being installed in backwards. Of course there are many times when that doesn't work either.

 

How does that work with a fuse before the diode?
When the reverse voltage is applied, the circuit will be protected for maybe a few milliseconds prior to the fuse opening up. As soon as the fuse opens, the diode is no longer in the circuit and you have no more protection.

If you chose to use this method, make sure the diode can handle the current output of the 5v supply, which is not the current your circuit consumes. This is probably not ideal.

If you can go back before the 5v source and put in a series diode ahead of the 5v regulator, you'd be better off.

Be careful putting diodes in parallel, current hogging will occur if their voltage drops are different.

When fuse blows there is no more VCC on ckt., diode is still in ckt. Diode only has to be rated for fuse capacity & can carry considerlby more than rated current on a one shot basis .

 

At first, I misunderstood where the diode was, About 47μS after I hit the submit button, it became very clear very quickly that I was on the wrong road.

 

I would suggest using a bridge rectifier module for reverse polarity protection. I use them.

 

I would suggest using a bridge rectifier module for reverse polarity protection. I use them.

On a 5V ckt.?

 

You may be confused about how to use a diode for reverse polarity protection. There are two ways to do this ... seried diode, or parallel diode.

(1) Series diode: Just place the diode in series with the input voltage. When the input is the correct polarity, current will flow. When votlage is reverset, current will be blocked. Make sure the diode can handle the maximum input current and check that it won't get too hot. Remember, there will be a voltage drop across the diode, this reduces the input voltage to your circuit.

(2) Parallel diode. Place a reverse biased diode from your input to ground AND place an externally accessible fuse in series with the input, before the diode.Tthe diode has no effect when the input voltage polarity is correct. But, the diode acts like a short when reverse voltage is applied and blows the fuse. If possible, choose a diode that can handle the fuse current, but remember that the diode will only conduct current for a short period of time and therefore will not heat up and fail, even if it is under-rated (be sure to test this). Use this scheme when your circuit can't tolerate the voltage drop from a series diode.

 

You can also use a MOSFET for reverse polarity protection.

This page describes a few example circuits.