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Here are the specs:

  1. Emitted Color: 850nm
  2. Forward Voltage (VF): 1.5-1.7V
  3. DC Forward Current (IF): 500-700mA
  4. Maximum Pulse Voltage: 2.2V
  5. List item

But after hooking to power supply (which I first set at 100mA just to be safe and 1.5V) it would only use 18mA. I increased to 1.7V at 300mA and got about 24mA. Also with with my test light I decided to increase it to 2.2V at 500mA set on power supply, and only got about 80mA.

Why can I not get even close to the 500-700mA as advertised?

Thanks.

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    \$\begingroup\$ Where is the datasheet? \$\endgroup\$
    – PlasmaHH
    Nov 5, 2018 at 21:19
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    \$\begingroup\$ Did you check the voltage with a meter, preferably at the LED terminals? Meters on power supplies, or worse, dial settings on power supplies, just aren't reliable. \$\endgroup\$
    – TimWescott
    Nov 5, 2018 at 21:35
  • \$\begingroup\$ Can you provide a schematic of your circuit? There may be an extra/missing component somewhere. \$\endgroup\$ Nov 5, 2018 at 21:47
  • 4
    \$\begingroup\$ Item from ebay or aliexpress? \$\endgroup\$ Nov 5, 2018 at 21:48
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    \$\begingroup\$ Does your voltage measurement include the ammeter drop or not? Even in seemingly trivial circuits such as this one, a schematic can help. \$\endgroup\$ Nov 5, 2018 at 22:09

1 Answer 1

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Along the lines of @TimWescott s comment, I presume you have a circuit which looks like

schematic

simulate this circuit – Schematic created using CircuitLab

where V1 is a current-limited variable power supply.

Rather than depend on the supply voltage meter, try the following. With the voltage set to five volts or more and the current limit set to zero, connect a DMM set to measure current as the power supply load. Increase the current limit setting until you have something like 200 mA being provided. Short circuit the output and disconnect the meter. Now connect the LED and release the short circuit. Use the DMM to measure the LED forward voltage. Good power supplies have fairly accurate output meters, but there is no guarantee that you have a good one.

Alternatively, if you don't want to depend on your current limit function, get a 100 ohm, 5 watt resistor. Spend a little extra and get a 1% unit. Set the current limit to something like 1 amp. Connect the LED like

schematic

simulate this circuit

Now set the supply voltage to something like 12 volts. You should be reading a load current of about 100 mA. Now measure the LED forward voltage.

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  • \$\begingroup\$ Actually what you described above "connect a DMM set to measure current as the power supply load" is something I did. It verified what the Power supply was reading. Also I did keep the constant current then kept increasing the voltage up to about 3v (which I did not dare go above). At that point it was drawing close to 100ma or more, plus it showed very bright in the camera. But It was my understanding this is supposed to operate at 1.5V at 100mA. \$\endgroup\$ Nov 5, 2018 at 23:03
  • \$\begingroup\$ Yes but did you actually measure the voltage? \$\endgroup\$
    – TimWescott
    Nov 5, 2018 at 23:39
  • \$\begingroup\$ @ShermStebbins - Look - do you trust your power supply to put out a constant current at limit or not? If you do, then you can set the voltage at anything you want without worrying about the voltage at the load. So, "which I did not dare go above" suggests that you don't know how your supply works. So follow the procedure I set out. And actually measure the voltage. Also, get a better meter. If your meter is, as you say 10k input impedance it cannot be a digital meter, but has to be an old (very old) analog unit. Get rid of it and get a real DMM. \$\endgroup\$ Nov 6, 2018 at 6:03
  • \$\begingroup\$ Yes, I've even put the DMM (which I have many and was using my most trusted on)., but I did play with both constant current and constant voltage. I would try one, then try the other.. Unless I cranked the voltage Higher then spec, I could not get much visibility(using a camera to see) and not much higher then 120mA.. Again also double checking with DMM. I will try your advise I do have several spares so I can push one all the way till it dies.. It will be interesting to see what voltage that is at.. \$\endgroup\$ Nov 6, 2018 at 15:30
  • \$\begingroup\$ @ShermStebbins - Ack! From a comment, "The light is lighting up, but I have to use my cell phone to see it." and from another, "Unless I cranked the voltage Higher then spec, I could not get much visibility" Well, yes. You do realize, I hope, that 850 nm is not visible to the naked eye? And cell phones are limited to approximately visible response? The fact that you can see the LED at all on your cell phone is just an indication that it has some response in near IR. Do not pay any attention AT ALL to the apparent brightness of the LED. \$\endgroup\$ Nov 6, 2018 at 17:55

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