# Why LED reversed bias voltage cannot be measured?

As I have just found out, typical LED does not behave similarly to standard diode when measuring reversed bias voltage. On typical diode, you can measure reverse bias voltage about .3 to .6V, but not on LED. I have read you can measure it using constant current source, but why? What is the physical principle behind this?

• Those sound like forward bias voltages to me... Commented Dec 5, 2014 at 11:22

LEDs behave electrically like other types of diodes, just with different parameters. They can be modeled reasonably well by the Shockley diode equation with a series resistor.

Regular diodes tend to have a reverse breakdown in the 50V-1000V range, LEDs it's on the lower end of that range. Usually we don't use LEDs reverse biased except to stand off a few volts in multiplexed circuits, so the specs tend to be very loose (maybe a 5V spec on an LED that can withstand 50V or 100V of reverse bias).

Regular diodes have a forward voltage of 0.6V or so at currents of mA, with LEDs it's more like 1V-3.5V (the lower values for IR LEDs, the higher for UV).

The ideality factor n tends to be around 2 for regular diodes, and it can be quite different for LEDs.

Sorry, but you're wrong.

The forward and reverse bias voltages and currents can be measured on an LED as easily as they can on any other diode, and all that's needed is a voltmeter, an ammeter and a voltage source.

In 2, the voltmeter is a balanced bridge which draws no current.

• Or perhaps "the voltmeter must be a balanced bridge so that no current is drawn"? Commented Dec 5, 2014 at 11:47
• Perhaps, but for the utmost in accuracy - and confusion- "In 2, 'V' is a potentiometer" works best Commented Dec 5, 2014 at 12:06

I assume you are having trouble measuring the forward voltage with your DMM Diode function. Some DMM's have enough compliance to measure LED's and some don't. So you could try another DMM. If not follow EM-fields idea and just use your power supply on the bench. Put some series resistance (maybe 1k ohm) before the LED to limit the current... and as Sphero said you will be measuring the forward voltage at only one current.

The reverse voltage of an LED is another story.. as Spehro said numbers in the 50 - 100 V range are common. (Again a series resistor will be needed so as to not damage the LED.) You can do some fun things with a reverse biased LED. (Link on request.)

• I am interested in that link about reverse biased LED applications :-) Commented Dec 7, 2014 at 3:09
• @LorenzoDonati, Well first I found that this only works (at low voltage.) with the GaP red LED's from here. purdyelectronics.com/pdf/LED-SFC.pdf AND113R or AND114R (the clear 113 is better.) Reverse bias to ~25V and it's a sinlge photon detector. (with kinda terrible efficiency.) See here, teachspin.com/newsletters/TeachSpin_MAY13FINALFOR%20WEB.pdf Commented Dec 7, 2014 at 16:26