[Book]

Hey all!

Today I took apart my laser pointer, and after quite some hassle, I was able to get the very little PCB with the laser diode on it. Normally this operated with 3 x 1,5 Volts (yielding 4,5 volts) batteries, and I happened to have 3 AAA batteries, so I set them up in the same way, and powered the board, which lit as it should. While holding the switch to (on) I used my multimeter, and measured 2,10 Volts between the terminals of the diode. That sounded logical, since I've seen many such operating voltages for diodes in the internet. However, I then thought, what do I need the small PCB anyway, since I could power the diode directly with 2,10 volts or more.

When I gave it about 2,30 volts, it lit but it was hardly visible. I raised the voltage, and still it wasn't as powerful as with the PCB, and then I gave it the full 4,50 volts and it lit with about the same power as with the board.

The question is, how's it possible since when operating normally, with the PCB, the operating voltage of the diode to be 2,10 volts (measured exactly), but when I give it this voltage myself, it is underpowered (and needs 4,5 volts to light just as much)?

[JohnWill]

The circuit board is to limit current to the diode. I suspect the battery life will be MUCH longer with the circuit in place. Did you measure the battery current draw with and without the LED circuit in the mix?

[Book]

Yeah, I did some tests and I can assume that the only reason why it didn't work is because I forgot to take into account the internal resistance of my batteries.

But since I measured 2,10 volts between the terminals of the diode when it operated normally, then whenever I apply the same voltage (not forgetting the resistance of the source), then the same current will be drawn and it will light just as much.

However, there does seem to be a problem with the diode now. At first (before testing it a lot), the diode operated at 2,10Volts and it was drawing 44,8mA of current. That means is has 46,875Ohms of resistance. But now, I use 3,15Volts on it (with the help of my PC PSU) and it's drawing 250mA! That gives 12,6Ohms of resistance, and it's not even as powerful as it was in the first place (appears quite dimmer)! I do understand that I may have damaged the diode, and that's fine with me, but how about lowering its resistance?

[Frank4d]

The voltage vs. current curve for diodes is exponential, which explains your calculated resistance. Diodes have an "on" voltage (in this case about 2.1 volts) below which current through the diode is near zero, and it produces no light. For voltages at or above 2.1 volts, current increases exponentially. If you exceed the maximum current rating the diode may become damaged or burn out.

[Book]

Ah, I didn't know that! But it must have undergone some damage, because even with 250mA it doesn't light too much (as it did before). I think I can't draw much more than that because my PSU shuts down after a certain point.

[JohnWill]

If it doesn't come to full brilliance at around 30-40MA, you probably killed it.

[Book]

I just tested a diode that I got out of an old DVD drive (not sure if it was a reader or a writer, or even if it was DVD or CD drive, anyway). First thing I noticed was that this diode was looking exactly as the laser pointer one, but had its positive and negative terminals reversed. So I stuck two wires on it (using a liquid metal, which heat the diode quite a lot for a few seconds..), connected a variable resistance and an ampere-meter in series, and then connected it to the PSU at 3,3 Volts. By changing the resistance I was able to get over 100mA which lit the diode to being hardly visible (red, and since it emits monochromatic light, I guess it doesn't emit any infra red or whatever).

Could the heat have damaged the diode to the point where it needs so much amps in order to light it? If not, then what's the fault?

[JohnWill]

Running current over the maximum specification for the LED will indeed destroy them. The failure modes are varied, so predicting the exact type of failure is impossible. I know of very few of those diodes that can handle more than 30-40MA.

[Book]

Can high power diodes like those in DVD drives operate at such a high power with only the same amount of current?

[JohnWill]

No, there are LEDs that can dissipate amps of power, but the standard LED types are much more limited. However, any LED will normally require some sort of current limiting on the P/S in order to avoid damage. For indicators, it's typically just a resistor. For high-power applications, it's more likely an active constant current source.