testing led torch

[andrew]

I knocked my 96 led rolson torch to the floor, when it was switched off, and broke it. Investigation showed the leds were simply soldered to a circuit board with a maze of alternating concentric rings with 4.5V fed to the centre via a low value resistor ( blue, grey, silver but I measure 3Ohms). One end of the resistor has become detached from the board.

Measuring the forward voltage of the leds I get open circuit both ways. Is this because I broke them all or my dmm doesn't provide enough forward voltage?

AJH

[Bob Minchin]

The latter

[Graham.]

Using a voltage range and getting O.C doesn't make sense. If you were using the resistance range of a DMM then there won't be enough voltage to make them conduct in either direction. That's why your DMM has a diode test range. Have you tried that?

[andrew]

That's what I was referring to, its output is given in mV but if the forward voltage the dmm outputs isn't above the forward voltage of the led then no current can flow. Ordinary diodes are in the range 0.4V-0.7V if my memory from 45 years back is right or new materials have changed that. Leds seem to need 3V to drive them.

AJH

[cj]

Another reason I like my trust analogue avo ,but blue grey should give 68 not 3 ohms however there are normally 3 colours to give the resistance. On a different note I always test leds with a AA or AAA battery and a fly lead as the 1.5v will not normally hurt the led even if reversed for a short time. But as AJ has posted things may have changed in 45yrs. Leds are certainly a lot cheeper and I believe you can get BLUE ones now!!!! CJ

[Tabby]

The latter. Dont make the mistake of switching the dmm to a high resistance range that runs on 9v, 9v reverse bias will kill LEDs instantly.

NT

[John Rumm]

These are presumably white leds? They (and blue leds) have higher forward voltages than your traditional red/orange/green leds.

[BartC]

I doubt LEDs can get 'broken' by dropping (if they look anything like conventional LEDs). Perhaps the solder joints are damaged or PCB is cracked.

What happens when you operate it whilst holding the resistor in place with a finger? (This sounds like a current-limiting resistor designed to operate when all 96 LEDs are connected in parallel.)

With the meter, measure the voltage across one LED, or just trace the voltage from the battery, to the tracks.