What value sense resistor on laser?

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What value sense resistor on laser?

Postby BenJackson » Mon Aug 01, 2011 11:36 pm

Anyone know what the digital ammeters are using for a sense resistor between the tube and the laser PSU? My intuition says it can be fairly large considering the voltage drop across the tube is on the order of 10kV+ but a laser tube is not exactly a linear device...
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Re: What value sense resistor on laser?

Postby r691175002 » Tue Aug 02, 2011 2:40 am

I used a 1ohm 5W resistor that came with a meter light-object sells.
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Re: What value sense resistor on laser?

Postby trwalters001 » Tue Aug 02, 2011 12:15 pm

Another idea (which I'm using) is to get a digital multimeter from Harbor Freight for $5 and use it for your current meter.

Just connect it in series with the laser-tube ground lead. Works like a charm.

AND - you have a voltmeter and ohmmeter too.

Take a look: http://www.harborfreight.com/7-function-multimeter-98025.html

If something ever happens to it, throw it away and get another. It costs more than $5 to go to McDonald's...

Tim
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Re: What value sense resistor on laser?

Postby naPS » Tue Aug 02, 2011 2:51 pm

Is that thing rated for the voltages it's going to see? Typically they are only rated to a few hundred volts, AC or DC.
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Re: What value sense resistor on laser?

Postby BenJackson » Tue Aug 02, 2011 5:49 pm

naPS wrote:Is that thing rated for the voltages it's going to see? Typically they are only rated to a few hundred volts, AC or DC.

The ground lead is at ground potential. If you put a series sense resistor in line with it the potential on the laser side of the resistor will be E=IR so for a 1R sense resistor the voltage will be 0.020A * 1R = 0.020V or 20mV. The voltmeter has some similar arrangement internally when in "amps" mode.

This is why I was curious how these were wired up: You could put a 250R resistor in there and have a voltmeter read 0..1.0V where 1.0 is 100% power. The power dissipated in the resistor would be I^2 R or 0.1W.
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Re: What value sense resistor on laser?

Postby trwalters001 » Tue Aug 02, 2011 7:20 pm

The only time you need a sense resistor is if you were going to measure VOLTAGE and convert the reading to current using I=E/R.

The DMM measures current directly and doesn't need the resistor.

Since one lead of the DMM is connected to ground, and the DMM looks like an (almost) short circuit, it sees only minimal voltage. I've been using this setup for months with no problems at all.
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Re: What value sense resistor on laser?

Postby BenJackson » Tue Aug 02, 2011 8:03 pm

trwalters001 wrote:The DMM measures current directly and doesn't need the resistor.


It has a resistor internally and measures current using I=E/R.
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