Can't get green LED meter to work

photonic

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Mar 29, 2004
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Can\'t get green LED meter to work

I bought a 100uA panel meter and hooked it up to an LED (yes I tried both polarities) and while shining a 5mW (or so) green laser at it I can't get any reading. The needle moves very very slightly but it doesn't read anything significant (<1uA) /ubbthreads/images/graemlins/confused.gif

Again this is an analog panel meter with 1K internal resistance according to the datasheet that came with it. Maybe the green LED laser power meter only works with digital meters due to their lower impedance?
 
Re: Can\'t get green LED meter to work

dunno I tried it with my fluke on a yellow led and got readings...

(havent bought/found a green one yet..)

I know i got one somewhere...(looking?)
 
Re: Can\'t get green LED meter to work

Are you using a true green LED? I don't believe that a clear LED that outputs green light will work, it has to be green all the way through. .
 
Re: Can\'t get green LED meter to work

You need to use a digital meter, an analog will not work because the impedence of the meter is much too low, the digital meters have input impedences of 1 meg ohm or more.
analog meters load the led too much giving no or little indication.
 
Re: Can\'t get green LED meter to work

A regular analog VOM should work as long as it has a low enough scale. Some meters require the user to plug the probes into a different socket on the front panel for current measurements. I use a 20kOhms/volt analog VOM and I can get reasonable readings on it when using LEDs "backwards" as photovoltaics.
 
Re: Can\'t get green LED meter to work

Ok sounds like the concurrence here is an analog panel meter won't work as it's got too low an internal resistance (by the way I was using a true green LED with green lens though I don't see what the plastic around the LED die has to do with it unless it helps filter out other colors to provide a more accurate reading). The DMM I checked didn't register the small amount of current coming from the LED. I guess I'll try a Fluke DMM (got one at work) and see if that works.
 
Re: Can\'t get green LED meter to work

I have both analog and digital meters and they both work fine with my green LED. Digital amp meter must also work on low impedence in order not to affect the testing circuit too much(ideally should be zero ohm). In another word, we are measuring the current of the green LED just as we measure any other photo diode.

Try to see if you panel meter is working. For instance, you can series a 20k resistor and measure a dry cell battery. Otherwise, try another LED.
 
Re: Can\'t get green LED meter to work

bg2vo I think the meter should ideally present infinite resistance to the LED, not zero. That way current through the meter won't load the LED and swamp the reader. I did try to apply a small current limited voltage to the panel meter (I put my DMM across it on the resistance setting). The meter did deflect.
 
Re: Can\'t get green LED meter to work

[ QUOTE ]
photonic said:
(by the way I was using a true green LED with green lens though I don't see what the plastic around the LED die has to do with it unless it helps filter out other colors to provide a more accurate reading).

[/ QUOTE ]
Just FYI you need to use a diffused lens yellow-green (GaAlP) LED, not a true-green InGaN one.
 
Re: Can\'t get green LED meter to work

Ummmm, yeah, what Craig said /ubbthreads/images/graemlins/wink.gif
 
Re: Can\'t get green LED meter to work

photonic, ideally speaking, the light CURRENT of a photodiode is proportional to the light, not the VOLTAGE. If you look at the responsivity specification of a photodiode, the unit usually is A/W(or A/lx). This means that the input is power(in Watt) and the output is current(in Amp). The LED in our LED meter acts just as a photodiode and the best way to measure the current is not let the voltage going much higher than necessary(<0.2V, say). If we present infinite resistance to the LED, we actually measure the voltage of the LED, which is not proportional to the power. There is an article Photodiode Characteristics and Applications explains this well.
 
Re: Can\'t get green LED meter to work

LED Museum guy: Sorry yea I said 'true green' colloquially but I believe it is the correct LED. This was an older one with a 5mm body and diffused green case. Does the age of this LED (I think it's >10years old) mean it can't be a InGaN LED?
 
Re: Can\'t get green LED meter to work

bg2vo: Fair enough, then why doesn't the panel meter work? Is it presenting too high a resistance to the LED? Current operated devices always bothered me. The problem is I don't know how moving coil meters work, I assumed there was a small shunt resistance inside it and the meter measured the voltage drop across it.
Finding out that the panel meter has a 1K resistance kind of broke my brain...
 
Re: Can\'t get green LED meter to work

[ QUOTE ]
photonic said:
LED Museum guy: Sorry yea I said 'true green' colloquially but I believe it is the correct LED. This was an older one with a 5mm body and diffused green case. Does the age of this LED (I think it's >10years old) mean it can't be a InGaN LED?

[/ QUOTE ]
As long as the LED is a GaAlP yellow-green, you'll be alright. If you got it ten years or more ago, I think it's a pretty safe bet it's the right one.
 
Re: Can\'t get green LED meter to work

Hi photonic, try this (it works for me):
Contact the negative input of your panel meter with negative pole of a battery, wet your two fingers and touch the the two positives at the same time. Got a reading(I got 2-3uA)? If yes, then the LED is bad otherwise the meter is broken.
 
Re: Can\'t get green LED meter to work

I dont think I can trust this led meter thing at all. I tried 20 different green led's. About half gave no reading at all. The lime ones did give readings. But using the 2.8 number that is usually quoted, no two gave me results that were similar. A stock leadlight showed from .7mw to 10.7mw. And the readings fluctuate wildly even when then laser and led are perfectly still. I would never be able to watch the stability of a laser like this. And I cant check how linear this is by hitting it with 2 lasers to see if it adds up.

Instead I use a solar cell from Radio Shack. You can hit it from a distance and the reading is stable. At 532 I use milliamps x 2.6 giving 3.6mw for the same stock Leadlight at the same time I was getting .7mw-10.7mw with leds. I think I trust this within reason up to about 40 mw.

If somebody can think of why photovoltaic's are not good for this please say
 
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