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Old 04-16-2008, 11:19 PM #1
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Default current measuring problem

As you probably recall, I made a homemade driver from recommended parts. It's an LM317T with a rheostat and just in case resistor then a capacitor and one direcitonal current diode thingy then a LD. So along with the capacitor and diode, I also added in parallel two wires with aligator clamps and I clamped them onto the probes to my multimeter. So I measured the voltage while the LD was on and turned the resistance on the rheostats up and down a bit and the voltage chaned a little and that was all neat and stuff except really I hooked it up that way to measure current cuz I wanted a digital readout of how much current was going to the LD. And yes, I did change the terminal thingy over on the multimeter so it could measure current. But I must not be doing it quite right cuz the LD wouldn't turn on when I was measuring current. I even turned it way up to 40 ma, the recommended operating current for the LD, and it still wouldn't turn on. I thought you measure current with the terminals at + to + and - to - in parallel but that must not be right. Either that or something stranger is going on. Any insights?


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Old 04-16-2008, 11:42 PM #2
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Default Re: current measuring problem

Connecting the DMM like that is 'shorting out' the DMM. The LD will not get current. You will get the correct current measurement on your DMM, but this could fry the DMM.

Here is a good guide by rog8811 on testing the DDL circuit:
http://www.laserpointerforums.com/fo...num=1197651171
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Old 04-17-2008, 12:39 AM #3
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Default Re: current measuring problem

I think you're metering the current wrong. No need to tap into the circuit where the diode is, anywhere will do.

I break the circuit right next to the battery. Take the negative lead from the battery, hook that up to the negative lead of the DMM. Then hook the positive lead of the DMM to the rest of the circuit (after the break). Make sure you've got the DMM set to measure current.

See this picture of me metering up my bluray for the first time. I tapped into the circuit right off the negative lead of the battery.. then adjusted the pot up while keeping an eye on the multimeter

{Edit} If you're using a DMM, make certain you've got the polarity correct - try swapping the probes.


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Old 04-20-2008, 10:31 PM #4
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Default Re: current measuring problem

To measure the current you need to put the meter in series with the load (between the circuit output and the diode.

Unfortunatelly this is not very safe, as it means you are disconnecting the diode from the circuit to put the meter in, and a cap on the circuit can charge up and later fry your diode.

Because of this, it is much better to measure the current between the battery and the circuit, but this will be a few mA off, since the circuit also consumes a bit of current.


The best way to do it is to permanently put a 1 Ohm 1% resistor between the circuit output and the diode. After that you can simply measure the voltage across this resistor in the mV range, and the voltage drop on the resistor in mV is the same as the current through it in mA.

This resistor doesn't affect the operation of the circuit much. In fact, nothing you need to worry about. The diode will still be getting exactly what it needs.


What you did was shorting the circuit output, so the entire current was flowing through the DMM and the diode could not light up. You still should have been able to see changes in the current when you turned the pot, if you did the rest correctly.
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