My 'Daedal Circuit' Confusions

[chimo]

OK, So I still don't know what I can do to stop this problem from happenning to my next diode.
Right Now, I don't know if it's sill gonna do the same thing.

Read about and understand the functioning of the circuit you are trying to implement.

Think about what you are doing to the energized circuit before you do it. Set it up at a safe current and work your way to the dangerous current levels - not the other way.

Experiment with different components using a dummy load that is close to what the laser diode would represent in the circuit.

Anyone hwo has messed with electronics has likely fried a component or two (or three... ). Use each failure as a learning experience to ensure you don't repeat the same mistake twice.

 

[Benm]

Next time, make sure to connect the diode to the driver circuit before you power it up. As long as you never (re)connect the LD to the driver while the driver is powered, the capacitor will not pose any danger.

If you need to measure diode current, solder a 1 ohm resistor between driver and LD, and measure the voltage across that. With a constant current source the resistor will have no effect on performance whatsoever.

 

[Daedal]

Could another problem have been when I tried to measure the currentof the LD, I removed the + lead which was connected to the capacitor, when I reattachedf it, could it have killed it then?

Well... Not you know exactly how you killed it! :-?

When you detached the laser diode from the capacitor, you gave the capacitor the chance to charge up to its full potential. At this point the capacitor is the only thing taking current in... Once you reconnected it, you all of a sudden let loose the full charge of the capacitor at the maximum rating that your capacitor is rated at (the V amount on your capacitor). I say it many times... solder the capacitor to the LD because if they detach and then reconnect, even if for a split-second, the LD would be dead.

This is a learning experience, and you can never get better if you don't try

GL;
DDL

 

[Daedal]

Next time, make sure to connect the diode to the driver circuit before you power it up. As long as you never (re)connect the LD to the driver while the driver is powered, the capacitor will not pose any danger.

If you need to measure diode current, solder a 1 ohm resistor between driver and LD, and measure the voltage across that. With a constant current source the resistor will have no effect on performance whatsoever.

That works as an excellent current sense resistor too. Get something with a rather high rating though. Look for 1W or more.

The current flowing through that specific resistor should be equal to the voltage across it...

--DDL

 

[Benm]

No need to with LDs from dvd burners... a 0.25W 1 ohms resistor would suffice up to 500 mA.

Once you reconnected it, you all of a sudden let loose the full charge of the capacitor at the maximum rating that your capacitor is rated at (the V amount on your capacitor).

The rating on the capacitor doesnt matter, but it does charge up to supply voltage. So if you run the current source from a 9V wall adapter, chances are it will charge up to (almost) 9V, but never beyond.

 

[Daedal]

Thanx Ben... My bad...

As about the resistor... I'm suggesting a high rating only to minimize the heat generated by it...

--DDL

 

[Gazoo]

After rereading through this now I think the way Comidt was measuring current very possibly killed the diode, or it still could have been a combination of too much current and reconnecting the meter between the capacitor and diode. I wonder how much voltage the capacitor was charged up to with no load connected across it. Either way, the capacitor does need to be soldered across the diode. Not necessarily right at the diode, this can be done in the circuit unless breadboarding. I take my current readings between the output and the capacitor.

Since I am using a rheostat, I always have it turned all the way down when I apply power, and my meter is already connected to the circuit. Then I turn the rheostat up until I reach my desired current. When I am finished, I turn the rheostat down, then disconnect power. After I disconnect power, my diode stays lit for about a half a second as the capacitor discharges, but when I have my rheostat turned all the way down there is only 50ma's going into the diode.

I really like the other way of measuring current with a 1 ohm resistor and may give it a try. The only thing I would add is it would be best to use a resistor with a low tolerance of 1% for the most accuracy.

 

[Benm]

Dont worry about resistor tolerances. I had a discussion about that with someone once, and he ended up measuring a 100 pcs bag of 100k resistors rated 5% tolerance. All 100 were within 1% of indicated value.

 

[Gazoo]

Dont worry about resistor tolerances. I had a discussion about that with someone once, and he ended up measuring a 100 pcs bag of 100k resistors rated 5% tolerance. All 100 were within 1% of indicated value.

Well it just so happens I have a 5 watt 1 ohm resistor I bought at Rat Shack years ago. I did a comparison measuring the voltage and reading it as current across the resistor, then measuring current with my meter and the readings were very close. They might have been closer if I didn't have to use those damn test leads. I am going to use the resistor method to measure current on my TEC diode from now on. Thanks for the tip..