Testing LEDs

[VicS]

Any idea what to expect if a LED is "tested" with a simple multimeter on an ohms range or diode test range

The only one I have shows infinite resistance on all ohms ranges on my cheap Maplin meter regardless of polarity.

I thought it might behave like an ordinary diode on the diode test range but no response either way round.

It lights OK when connected to 3 volts DC but does not light on 1.5 volts

I guess what I have observed is normal

Needless to say someone has asked me !

.

 

[William_H]

Testing LED

yes you are correct on all counts. An LED (note here we are talking about the actual diode not replacement bulb set up for 12v) is a diode. However unlike a silicon diode with a forward drop of .7 volt an LED might have a lot more voltage drop. The 1 watt LED diodes seem to have a drop of 3.3 volts. Smaller LEDs seem to have lower volt drop. A 5050 LED has 3 diodes usually wired in series and so needs about 9v to get any current at all. The diode test on your multimeter probably does not go to such high voltages.

A diode can be seen a bit like a non return valve with spring holding a ball in place. In the reverse direction of course you get no fluid flow. In the forward direction you get no flow until the pressure is high enough to push past the spring pressure to move the ball of its seat. Small but sometimes significant.

Using you multimeter on ohms range again there is a fairly small voltage trying to push current through a diode if the voltage is not enough you get nothing while for a resistor you get some current even with low voltage. So high reading resistance for LED both ways is normal.
This current to voltage graph can be described as non linear. It also explains why when the voltage is enough The current rises dramatically which 9is why you need current limiting control of LED current on a 12v supply. The series resistor forces the LED to look more like a resistor with more linear current to voltage or can be described as setting the LED current.
This also explains why a LED cluster running on 12v with series resistor will dramatically lose brightness and current when 12v supply drops in voltage until typically at 9 volts or so there is no current or light.
good luck olewill