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Old 03-06-2015, 06:16 PM #1
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Lightbulb Display current - LM317

Hey,

I'm pretty new at electronics. And i didn't found anything similar.

Can i connect AMP meter (for example this one Amp Current Meter | eBay) between laser diode and LM317 for permanent use ?

Like this:


I don't know if these AMP meters are causing some side effects, which can destroy diode.

What do you think ?


Sorry for by bad english and thanks for help



Last edited by Seeder98; 03-06-2015 at 06:16 PM.
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Old 03-06-2015, 06:53 PM #2
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Default Re: Display current - LM317

Hi,
in theory you can measure the anywhere between the driver and laser diode. But with the mentioned current meter you might get into trouble. The current meter shunt resistor (used to measure the voltage drop of an internal resistor R=U/I) is connected to the GND of the supply voltage of the meter. This would be a short circuit. With some luck nothing will happen to the diode (just stays dark)... On a bad day the diode is no more.

The best way to measure the diode current is to put a resistor with low resistance (0.1 to 1Ohm) in series with the cathode and GND of your driver. Now, you can measure the voltage drop accross the resistor with any simple voltage meter and calculate it directly into current.

R=U/I

With a 1 Ohm resistor each mA of current will cause a 1mV voltage drop.

That's it :-)

For higher current diodes use a 0.1 Ohm or lower resistor. If not the voltage drop is too high and too much energy is dropped into the resistor -> hot... Not good. Also take care on global warming ;-)

-mo-

P.s you might also use the resistor next to the lm317 (it's nothing else as the current "meter" of the lm317)... but same as above, the supply of the meter shouldn't be connected to the input of the meter. If not...this diode 100% in laser heaven.
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Old 03-06-2015, 07:46 PM #3
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Default Re: Display current - LM317

Quote:
Originally Posted by mojo_1234 View Post
Hi,
in theory you can measure the anywhere between the driver and laser diode. But with the mentioned current meter you might get into trouble. The current meter shunt resistor (used to measure the voltage drop of an internal resistor R=U/I) is connected to the GND of the supply voltage of the meter. This would be a short circuit. With some luck nothing will happen to the diode (just stays dark)... On a bad day the diode is no more.

The best way to measure the diode current is to put a resistor with low resistance (0.1 to 1Ohm) in series with the cathode and GND of your driver. Now, you can measure the voltage drop accross the resistor with any simple voltage meter and calculate it directly into current.

R=U/I

With a 1 Ohm resistor each mA of current will cause a 1mV voltage drop.

That's it :-)

For higher current diodes use a 0.1 Ohm or lower resistor. If not the voltage drop is too high and too much energy is dropped into the resistor -> hot... Not good. Also take care on global warming ;-)

-mo-

P.s you might also use the resistor next to the lm317 (it's nothing else as the current "meter" of the lm317)... but same as above, the supply of the meter shouldn't be connected to the input of the meter. If not...this diode 100% in laser heaven.
Hey ! thanks a lot for reply !

The point is i want some notification about current strange of the laser. So, display was my first idea.

If i'll put 1 Ohm resistor between Cathode and GND (1V will be 1A - right ? ). Can i just use this meter Voltage meter | Ebay to measure voltage ? I'm just asking because i have no idea if these meters are working as normal DMM.

Last edited by Seeder98; 03-06-2015 at 08:22 PM.
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Old 03-06-2015, 08:41 PM #4
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Default Re: Display current - LM317

You can move the ammeter to the other side of the diode. This way power ground and shunt ground are common. Some of those cheap chinese meters have their own internal regulator so they can operate up to 30V power input. I don't see a reg first glance on that one. You should have a separate 5V regulator if you want to use that meter.
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