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Multimeter reading of LED

tedstruk

Jan 7, 2012
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The measurement is of forward voltage drop, right? and the setting on the multimeter is the diode setting, it doesn't read the diode backwards right? OK so when you test an LED and it says 1730 and you are supposed to be figuring out the proper resistor to use so you don't burn out the expensive LED array... what reading is the meter giving me? is this 1730 reading in millivolts?
 

Terry01

Jul 5, 2017
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Yes,I think your correct. 1.730 volts sounds correct for an LED. I also think you should wait for one of the more knowledgeable guys to confirm it. They'll tell you exactly what you need too so you don't burn your LED out. I'm still learning so could be wrong.
 

BobK

Jan 5, 2010
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It is the forward voltage at some current, not the operating current of the LED.

The multimeter will apply a very small current to a diode under test, so the voltage reading will be way lower than the forward voltage at say 20mA, or at 1A or more that might be the operating current of a high power LED.

To measure the forward voltage at the operating current, use a current limited supply with the current turned all the way down, then increase the current until you get to the operating current. Now measure the voltage.

Bob
 

tedstruk

Jan 7, 2012
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Is forward voltage, the voltage after a devices voltage drop?
 

Harald Kapp

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Forward voltage is the voltage drop in the conducting (forward) direction (+ to anode, - to cathode).
Reverse voltage would be the voltage drop in the non-conducting (reverse) direction (+ to cathode, - to anode).
 

altronic

Sep 20, 2017
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The measurement is of forward voltage drop, right? and the setting on the multimeter is the diode setting, it doesn't read the diode backwards right? OK so when you test an LED and it says 1730 and you are supposed to be figuring out the proper resistor to use so you don't burn out the expensive LED array... what reading is the meter giving me? is this 1730 reading in millivolts?
1.7 sounds about right to me
 

BobK

Jan 5, 2010
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But the forward voltage of a diode or LED depends non-linearly on the current through it.

As I stated before, your multimeter is measuring the forward voltage of your LED at some unspecified current. It is not measuring the forward voltage at the rated operating current, which is the number you need to utilize the LED.

Bob
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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The "diode" setting on your multimeter is really an "ohms" or resistance function except, instead of resistance, it displays the voltage across the two probes while providing a very small constant test-current from a low-compliance (typically less than 9 V, often less than 1 V) current source within the multimeter. For example, a typical ohms function test current might be one milliampere, causing a 1000 ohm resistor to drop one volt, which the meter would then interpret and display as 1000 ohms. Because the constant-current compliance voltage is so small, higher resistances must be measured at smaller constant currents. Thus a ten megohm resistor might be measured with a constant current of 0.1 microamperes, resulting in a drop of one volt across a 10 megohm resistor.

Now, switch to the diode function and what happens? First, the meter no longer displays resistance; it displays the voltage between the two probes as a voltage. Second, the current supplied can be larger than the current used for resistance measurements, but is still limited by the internal battery to something on the order of one milliampere. It is definitely less than the typical operating current of an LED. What is your take away from all this? The diode function on a multimeter is good for several things, but testing the operating forward voltage drop of an LED is NOT one of them. Use the diode test function as a Go/No-Go test to find shorted and open diodes and as a way to distinguish germanium diodes from silicon diodes. If you know the polarity of the test probes, you can also determine which terminals of the diode are cathode and anode.

The forward operating voltage of an LED depends on two things: how it is constructed and the forward operating current through it. You can possibly use the diode function to determine how the LED is constructed. Red LEDs will have a lower forward voltage (at any forward current) than blue, green, yellow, or so-called "white" LEDs. You can use that information in conjunction with a datasheet to "ballpark" a safe current-limiting resistor and power supply voltage.

No datasheet? Sneak up on the maximum LED operating current by using a variable constant-current power supply, as @BobK suggested, or start with a high-valued current-limiting resistor and a variable voltage power supply. Work toward lower resistance values. Stop when you reach the brightness you need, or the LED burns out, whichever comes first. Measure the current through, and the voltage drop across, the LED where satisfactory operation occurs. Make a note of it for future reference because there are an infinite number of voltage supply and current-limiting resistor combinations that will work for that particular LED. You can use the information you just determined to calculate them.

It baffles me why so many people on this forum think powering up an LED is akin to rocket science. It's really quite simple, and we have a nice resource here that describes how to do it. Strongly suggest you swing on over there and read it.
 

Harald Kapp

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It baffles me why so many people on this forum think powering up an LED is akin to rocket science.
... because the concept of a "lamp" not being controlled by a voltage (as in an incandescent lamp) but by a current is way off the everyday experience?
 

Audioguru

Sep 24, 2016
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Many new people do not read the datasheet of an LED (maybe because cheeep LEDs on ebay do not have datasheets) so they see the forward voltage is 2.8V then calculate the resistor for a 2.8V LED. But the minimum voltage for some of those LEDs is probably 2.4V so the current will be too high or the maximum voltage for some of those LEDs might be 3.5V then maybe there is not enough supply voltage for it to light.

The entire range of forward voltages should be used to calculate the supply voltage and the current-limiting resistor so that the LED does not burn out and is not too dim.
 

hevans1944

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Jun 21, 2012
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Well said hevans1944 .......

Greg
Thanks, Greg. And welcome to Electronics Point.

It was really astute of you to invest in a new 1973 Harley-Davidson motorcycle and then sell it for a milling machine and a lathe two years ago. I, OTOH, "invested" in a new Honda Shadow ACE with 1100cc v-twin engine, rode it for almost ten years with round trips from Dayton, OH to Atlanta GA and Memphis TN with wife on the back, and then had to retire from riding (because of health) at wife's insistence. She says if I convert it to a trike I can ride again here in Florida. That's not likely to happen because of the cost to do the trike conversion.

Yesterday, while waiting for the traffic light to change in front of the Publix supermarket on Tamiami Trail in Venice, FL I was privileged to see a nicely restored, chopped, Ford deuce coupe drive by. It looked very similar to the photo below:

upload_2017-9-21_13-40-24.jpeg

For me, this car was immortalized by the Beach Boys song Little Deuce Coupe while I was serving in the Air Force from May 1963 through May 1967. I wanted one, but on my enlisted man's pay could only afford a used POS Buick two-door sedan with a cracked engine block. After my Air Force enlistment was finished, plenty of Stop Leak got me and the car, towing a small U-Haul trailer with all my meager possessions, home from Kincheloe AFB, MI to Dayton, OH, whereupon the car promptly died in front of my parent's rented house. I sold it to a neighborhood kid for fifty bux, but he failed to register the transfer of title and the car was later found abandoned on the side of the road. So I wound up paying again to have it towed to a junk yard.
 

pepi

Sep 20, 2017
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Hearing the Beach Boys with that shot also. I grew up in Miami, riding a bike there was great late night, early morning rides, in T shirts. Very relaxing great memories, quick shot of the car and truck, not wanting to turn the forum into car stuff.

His;

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Hers;

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Cheers,
Greg
 

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