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Old 07-14-2010, 01:04 PM #1
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Default Current metering on a constant current driver.

Good morning,
New guy here and this is my first post.

I am in the process of building a test platform for my laser experiments. I have a 1.5 amp constant current driver on order (Meridith Instruments) and plan to add digital metering for output voltage as well as output current. I will be removing the board mounted pot and installing a 10 turn pot externally to adjust the current.
In the paperwork that came with another driver I built, the manufacturer said to never try to measure the output current on a CC driver. They say it will destroy the laser diode. I am trying to figure out how metering the current of the LD could damage it. I am an engineer by trade and measure current all the time.Is there something about a LD that is different than other electronic devices such that you can't measure its current?
Sorry for the dumb question.

George


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Old 07-14-2010, 01:23 PM #2
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Default Re: Current metering on a constant current driver.

It all depend on the type of current regulated driver...
If it is a Linear Driver like the LM317 Current regulated circuit
that members use here then I can't see a problem slowly
turning up the current to the LD to it's rated Max Specs..

On the other hand...If you are using a Boost Current driver...
that may perhaps be a different story depending on the Boost
Circuit being used..

It is difficult to be sure of the quoted info for the Driver without
seeing the circuit of said driver..


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Old 07-14-2010, 01:45 PM #3
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Default Re: Current metering on a constant current driver.

Well.. i think it should be possible without a problem:

Insert a 0.1 ohm resistor between the driver and the diode. That should not significantly change the load for the driver, but it does allow you to measure current by looking at the voltage over this resistor (multiply by 10).

Normally i'd recommend a 1 ohm resistor, but when going beyond 0.5 amps those get rather hot (0.25w is dissipated at that point), and also drop half a volt which could be too much.

I think the reason they say you should not measure the current is that people turn on the driver, then poke with the multimeter leads to complete the circuit (which it will when set to measure current). This sudden change in load could damage the current source, but more likely the laser diode if the current source has output capacitors.
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Old 07-14-2010, 02:29 PM #4
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Question Re: Current metering on a constant current driver.

What about using a 0,1 ohm 1Wprecision resistor, and an op-amp set for 10X gain, for read the voltage at the resistor leads ? ..... commonly op-amps have an input impedance of megaohms, so must not influence the circuit in any way (and the 10x gain let you read 1mV=1mA also with a 0,1 ohm resistor)
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Old 07-14-2010, 02:44 PM #5
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Default Re: Current metering on a constant current driver.

Quote:
Originally Posted by Benm View Post
Well.. i think it should be possible without a problem:
I think the reason they say you should not measure the current is that people turn on the driver, then poke with the multimeter leads to complete the circuit (which it will when set to measure current). This sudden change in load could damage the current source, but more likely the laser diode if the current source has output capacitors.
That's probably it. Of course, nobody I know would ever insert an ammeter into a circuit like that. Actually, I can't even remember the last time I had a meter with a working mA setting. The fuse usually gets blown about the second time I ever use it in that mode, and I never get around to replacing it.
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Old 07-14-2010, 05:12 PM #6
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Default Re: Current metering on a constant current driver.

Quote:
Originally Posted by Benm View Post

I think the reason they say you should not measure the current is that people turn on the driver, then poke with the multimeter leads to complete the circuit (which it will when set to measure current). This sudden change in load could damage the current source, but more likely the laser diode if the current source has output capacitors.
I suspect that is probably what they are worried about. That point seems obvious to me but maybe not to everyone. Ha Ha
The digital ammeter I have specifies a 0.1 ohm 5 watt resistor for the shunt. This meter is good up to 2 amps this way. Actually this same meter is really a voltmeter. Measuring across the shunt makes it function as an ammeter. Am using two identical units for my setup.

Anyhoo, thanx for all the input guys. I am really looking forward to building this setup and experimenting with different LD's.

George
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Old 07-14-2010, 05:27 PM #7
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Default Re: Current metering on a constant current driver.

mA ranges on most cheap multimeters don't work very well anyway. The problem is that the connection is made through the rotating button, and that consists of springs/sliders running over some PCB tracks. As stuff wears out this causes the meter to have a high resistance, although the mesurement is still good.

For measuring current i nearly always use a shunt and go for voltage readings. The 10/20A range i sometimes use though, for checking battery current draw and such.... just dont forget to put the wire back before say, doublechecking mains voltage
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Old 07-16-2010, 02:04 PM #8
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Default Re: Current metering on a constant current driver.

My MI driver arrived and it already has test points for measuring current. That takes care of the metering question.
Now I just have to put on my magnigying glasses to see where to solder the parts. And my large hands don't help the situation either...ha ha



Later

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Old 07-16-2010, 02:44 PM #9
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Default Re: Current metering on a constant current driver.

I've gotten out of the habit of changing the current setting on "live" lasers because those pots can sometimes be finicky and cause a momentary disconnect/reconnect. Just be careful!
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Old 07-16-2010, 03:07 PM #10
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Default Re: Current metering on a constant current driver.

Good point... I plan to mount this driver in some type of enclosure and replace the pot on the driver board with a good 10 turn pot mounted external from the driver. Of course, my best bet would be to build some of the loads described in other threads. My LD's would probably last longer that way. Come to think of it, I have a whole box of 1N4001 diodes stashed away somewhere...

Later

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Old 07-19-2010, 11:40 PM #11
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Default Re: Current metering on a constant current driver.

Good thread.
just to be clear the reason its unsafe to turn a pot on a live laser is because those pots arent made for that however a quality pot like that found on fml's fcd is?

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Old 07-20-2010, 12:29 AM #12
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Default Re: Current metering on a constant current driver.

Any pot will suffer from 'bouncing' - momentarily disconnects while you adjust it. This is completely normal and not indicative of failure.

The problem is: what does the driver do when the wiper gets disconnected from the track? On a good current source it would assume you ment 'zero' and just drop output when it skids off the track.

Often, the wiper of the pot is just connected to the input of an opamp, and the addition of one single 1 cent resistor can ensure it defaults to ground/zero rather than to 'random' when the wiper gets off the track. <-- people designing/selling drivers, please read that
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Old 07-20-2010, 08:51 AM #13
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Wink Re: Current metering on a constant current driver.

Quote:
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Any pot will suffer from 'bouncing' - momentarily disconnects while you adjust it. This is completely normal and not indicative of failure.

The problem is: what does the driver do when the wiper gets disconnected from the track? On a good current source it would assume you ment 'zero' and just drop output when it skids off the track.

Often, the wiper of the pot is just connected to the input of an opamp, and the addition of one single 1 cent resistor can ensure it defaults to ground/zero rather than to 'random' when the wiper gets off the track. <-- people designing/selling drivers, please read that
This is one of the main reasons for which, usually, the cursor is connected to one of the side pins, in these types of circuits ..... in this way, it can go to the maximum resistance of the trimmer, when it disconnect, but never go in open circuit (that for some types of circuits is the worst thing)
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Old 07-20-2010, 09:18 AM #14
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Default Re: Current metering on a constant current driver.

I always use a 1.0 ohm resistor between the laser diode and the driver to set the LD current dead on. I have found there can be a good bit of difference between the dummy load and the actual LD current. I use this practice on both linear and boost type drivers.
Since I am only setting the driver and not running it for any real length of time the wattage going through the resistor realy dosn't matter, haven't burned one up yet
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Old 07-20-2010, 10:59 AM #15
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Default Re: Current metering on a constant current driver.

Speaking of dummy loads, you could maybe simulate your circuit with a dummy load. If you measure the current/voltage live with the dummy load while moving the pot, you could check for holes or any irregularity? With a quality meter or scope of course.
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Old 07-20-2010, 11:08 AM #16
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Default Re: Current metering on a constant current driver.

With a scope its very obvious to see that potmeters bounce - connect a 10k pot between ground and 5 volt, and look at the slider signal on a scope. You'll see it momentarily disconnects when you adjust it.

How a driver handels that is a different story though - as long as it goes to zero or at least lower when the slide bounces, i think its fine.
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