LED FAILURE HELP

[b_p4tel]

hi

I have a basically a 12v supply with 14 Green LEDS in pairs with a resistor in-between each LED. Each pair is in parallel.

I need to add a further ciruit which has a RED LED that can detect when a green LED fails (open circuit or short circuit) the RED LED will light up.

Is there an easy way to do this?

[awright]

Put the resistor supplying each pair of green LEDs between the power supply and the two LEDs. This will not affect the green led functioning. The voltage at the junction of each LED pair and the resistor will be about 4.2 volts. If any LED fails open, the voltage at the low end of the resistor will go high, the exact voltage depending upon any other load on that resistor.

Connect the anodes of seven identical diodes (type non-critical) to each resistor/LED junction and connect all seven cathodes together. Connect a few forward-biased common silicon diodes in series from the common cathodes of the seven diodes to the anode of a red LED and ground the cathode of the red LED.

The voltage drop across the multiple diodes in series with the red LED will prevent current from flowing through the red LED as long as the green LEDs are all conducting. If a green LED opens, the voltage at the low end of the resistor on that leg will rise, forward biasing its diode and causing current to flow through the multiple diodes and through the red LED. The red LED voltage drop will be about 2.2 volts so if we assume about 0.7 volts drop across the multiple series diodes, four diodes in series should allow the red LED to turn on with 5 volts across the diode/LED string. The red LED will be driven by slightly less current than normally flows through the each green LED pair.

Do LEDs fail shorted often enough to warrant monitoring for it? If you must, then you will have to get a little fancier with your monitoring circuit. You might be able to pull the same trick using a second red diode connected to the 12 volt supply rail and using a Zener diode in series with that LED. This gets more critical because you will only have a 2.1 volt drop in voltage (from 4.2 volts to 2.1 volts) if one green LED fails short. Might work with a 7 volt Zener in series with this second red LED (or a lower voltage Zener trimmed up with one or more series diodes). You will also need seven more diodes cinnected to the resistor/green LED junctions with the opposite polarity from the first seven diodes.

If that doesn't work reliably due to temperature coefficients or sloppy diode turn-on, you might have to go with a more elaborate comparator circuit.

awright