# Is it possible to protect a circuit from reverse voltage by adding a diode between two poles?

I have a circuit with 50 serirs connected LEDs. When I apply reverse voltage to the circuit, the LEDs breaks down.

Is it possible to protect the circuit from reverse voltage by adding a diode between the two poles as shown below? I run this circuit with a driver.

• If a reverse supply connected, the diode will make supply short circuit this time. Maybe using a zener diode could be help. Dec 2, 2022 at 14:48
• How can +15 volts drive 50 LEDs (2 volts each nominally)? What is the output current limit of your drive voltage (worst case and placed in reverse)? Are you using current limiting power supplies or is there a current limiting resistor somewhere? Please add the data sheet for the LED type used. Dec 2, 2022 at 15:17
• Check the reverse voltage rating of the LEDs, it is higher then the forward voltage. When in series as yours are these voltages also add. Since you did not state the current of the LEDs I am assuming they are maybe 20mA so a simple diode in the 1N series at least 400V would be OK and would not get warm. You can add diode voltage to the sum of the reverse voltages to calculate how high a voltage the circuit should survive.
– Gil
Dec 2, 2022 at 18:48

Short answer: Yes, but it depends

You can use a diode as reverse polarity protection in two configurations:

• In Series
• In Parallel

simulate this circuit – Schematic created using CircuitLab

In the series case, the voltage on the load will be reduced by the voltage drop across the diode (0.5V-0.9V normally for silicon diodes) and the diode needs to be able to take the current trough the load. In case of reverse polarity there will be (mostly) no current flow because the diode blocks the current. The maximum reverse voltage of the diode must be higher than the voltage of the source.

In the parallel case the diode has no influence in the "normal" case since it's blocking (modern diodes have leaage currents in the nanoamps range). But in the reverse case it will limit the voltage across the load to -0.5V - -0.9V. But it must be able to handle all the current, the source can deliver (which can be 100s of ams for battery packs for example). To protect the circuit continuously we normally have to either install a fuse that blows when the current gets too high or limit the current otherwise (for example by including a resistor in series to the input).

simulate this circuit

In your post you mention a driver. I suppose you mean a constant current LED driver, which is a circuit that adapts its output voltage in order to deliver a preset current to the LED string.

If this is the case, an "antiparallel" rectifier diode could work as long as some conditions are met.

1. The diode must be able to handle the current put out by the driver. E.g. if the driver outputs 350mA, you must use a diode that can tolerate such a current with a reasonable safety margin (otherwise it could overheat). As a ballpark figure, I'd go for double the current from the driver and then approximate to the next higher standard rated current. E.g. 350mA x 2 = 700mA, so I'd use a 1A diode (jellybean 1N4007, for example).

2. The driver must have a compliance voltage range that includes the typical forward voltage of the diode, because to regulate its output current it will have to reduce its voltage to about 0.7V. If the driver cannot do that, it could output an higher current, possibly failing or overloading the diode. In this case you could put N equal diodes in series until their total forward voltage (about N x 0.7V) is in the compliance range of the driver.

BTW, in case you don't know, the compliance range of a practical constant-current source is the range of voltages the source can generate. The maximum voltage limits how many LEDs can be driven at the rated current, the minimum voltage limits how few LEDs can be put in the string.

Is it possible to protect the circuit from reverse voltage by adding a diode between the two poles as shown below?

A parallel diode will divert the current but the diode will be in a short condition, and one of two things will happen. If the power supply has short circuit protection then it might not come up.

The other thing that can happen is the diode would probably heat up or overheat because it would be taking all the current from the power supply.

A better thing would be to use the short circuit protection circuits that use a p mosfet in series with the high side that shut off current completely if a reverse polarity is on the input

Another simpler circuit is to put a diode in series with the high side. But this has a voltage drop and the diode will also heat up. Which is undesirable and if the ratings are not followed the diode in series could fail