660nm and 405nm laser diode output comparisons

[James Lehman]

I don't know how diodes work.

I've heard a few things that might be true.

Is it accurate to think of a diode as a one-way, heavy, spring-loaded door? It takes some pressure to open it and hold it open, but once that door is open, it's a dead short to any voltage above the junction. Is this somewhat correct?

I've played around a lot with LEDs. I figure out what's going on by putting 5 or 6 volts in series with about a 1K ohm resister and then through the diode. I measure the voltage across the diode. I subtract that voltage from my DC supply voltage and do Ohms Law to figure out how much resistance I need to make the current flow at about 20 mA.

I guess I don't understand how a DC power supply can be either voltage or current regulated without being both at the same time all the time.

James.

[ajohnson30]

You are essentially right. A forward-biased diode (one that's reached it's threshold of forward current, thus opening your spring door) can be thought of as having little or no resistance. It does, however, have a fairly fixed voltage drop, which can be useful in other situations.

http://www.antonine-education.co.uk/..._4__diodes.htm

Has some good info.

As to current vs voltage regulation, the LM317 regulator is popular for this. It has an input, and output, and an adjust connection that's always 1.25 volts less than the output. Placing a resistor R1 between the output and the adjust, with a second resistor R2 from adjust to ground, you can easily figure out what values of resistors to use to get a given output, since you know the voltage drop across R1 is 1.25v.

See http://www.reuk.co.uk/Using-The-LM31...te-Voltage.htm for a good example of this.

Current regulation also takes advantage of the fact that it's adjust connection is 1.25v less than it's output connection. If you stick 1 resistor, R1, between the adjust and the output, and connect the load to the adjust, you now have a fixed current going to your load because the voltage is ALWAYS 1.25v across R1, so the current I = 1.24v/R1.

See http://www.reuk.co.uk/Using-The-LM31...D-Lighting.htm for that example.

[heroic]

However without the voltages on the chart the current measurement is totally useless and has no meaning whatsoever. Ohms law still applies.

Ohm's law doesn't apply to non-Ohmic conductors like semiconductor junctions.

Is it accurate to think of a diode as a one-way, heavy, spring-loaded door? It takes some pressure to open it and hold it open, but once that door is open, it's a dead short to any voltage above the junction. Is this somewhat correct?

It's more like a floor with a step in it. Electrons in one type of silicon have a higher potential energy than in the other type of silicon, so it's easy for them to roll from the higher floor to the lower floor, but they can't make it over the step rolling in the other direction. Of course if you roll them really really fast, they can bounce over the step- that's how Zener diodes work.

Of course this is an oversimplification, but it's a pretty good model all the same :-)

[jackhao]

hi,drlava, for the 405nm diode laser used for GGW-H20L, I wonder how much maximum output power it can reach? Thank you.

jack.hao

[thesk8nmidget]

there are some people running the H20L diode at 200mw over at laser pointer forums.

im currently waiting for my 3 diodes and should be doing some more testing on the H20L diode

[jackhao]

hi, Josh, thanks for your information, waiting for your test results of 405nm LD output power for H20L.

jack.hao