Reverse polarity LED problem

Hi all,

I have a challenge for you, a problem for me. I have a circuit that uses a LED membrane, the LED's cathodes are all connected together and to ground on one pin. The anodes are all open for input. Each LED serves as a particular indicator. One of the LEDs was intended to be used as a reverse polarity indicator.

The challenge is how can I activate that one LED when there is reverse polarity?

I have been trying different ways with a bridge rectifier, mosfets, and transistors but to no avail.

Basically, I need it to light up with a voltage on the cathode or make it so that the cathode is somehow diverted to ground during a reverse polarity connection.

Please help me with your ideas! Thanks!

You can't wire a LED to light up when the voltage across it is reversed. You need to provide a fixed polarity, the same as for all the other LEDs, and use an external circuit to detect the reversal and provide current to that LED in exactly the same way as the other LEDs.

Brian.

Right. You pretty much just restated my problem.

Why can't you connect the led inverted, directly to the input voltage (using a resistor) so that it lights when the input voltage is reversed?
Is there a reason for the led to be controlled by another device?

Alex

The LEDs are a part of a membrane user display which has an 8-pin FFC connection to the board; 7 pins to 7 LED anodes and 1 pin that connects all of their cathodes to ground. So all the LEDs share the same common ground pin, I only have access to each individual anode and the "common" cathode. During a reverse polarity condition all of them could be lit up, but the request is to have only the one specified LED to light up during this condition (because of the user display the LEDs are contained in). Since they are all connected to the same ground pin I need to isolate that single LED from the rest. I was trying to make it so that in either condition they have a path to ground by using an NPN and a PNP transistor, in which one conducts a path to ground during reverse polarity and the other during normal operation, respectively. I have a circuit that almost does this... I can post if you want to see, but I was hoping to see some other ideas.

Do you think you could use something like


The only problem is that even in normal operation the ground would be connected through the diode, I don't know if the voltage drop will be a problem in your circuit.

Alex

I tried to help but you are not providing enough information.

When you say "reverse polarity" are you referring to the supply to your whole circuit or are you monitoring the voltage between two points?

What I was trying to point out was that because the diodes share a common connection, the polarity to light them has to be the same for all diodes. It is not possible to make one light with the polarity of voltage to the LED array reversed, however, if a constant correct polarity is always available, a monitoring circuit CAN detect the reversal and still provide the correct polarity to light the LED.

Brian.

Betwixt: Sorry, I didn't mean to deny you from helping me. That's the last thing I wanted to do. Right, I am referring to the supply to the whole board. I am looking to monitor the supply voltage polarity for a reverse connection then provide the correct polarity to the LEDs so that I may light one of them up to indicate the reverse connection.

It looks like Alex's post may work for me. I am going to look it over more and try it out.

Thanks guys!