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I have just started experimenting with electronics and am concerned that I may have destroyed my LM339 comparator circuit. As a basic test, I had a 3.0 volt power supply hooked up to the Vcc pin and one of the + input pins. Nothing was hooked up to the corresponding - input pin. The long leg of the LED was attached to the corresponding output pin with a 1kohm resistor in between. The short leg of the LED was attached to ground but the comparator GND pin was not. I expected the LED to light up when I flipped the power switch.

When I flipped the power switch, The LED flashed briefly and then went off. After this incident, I verified that the LED was still working. Now I am trying to figure out if the comparator is broken. How can I do this using my multimeter? What are some basic tests I can do to figure out if the comparator is working properly?

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    \$\begingroup\$ It's a 50-cent part. If you think you broke it, throw it away and put a new one in the circuit. If the circuit still behaves the same, the problem was not in the LM339. \$\endgroup\$ – The Photon Dec 16 '13 at 3:39
  • \$\begingroup\$ Before declaring the LM339 broken, connect it properly - both inputs connected to suitable voltages, and the power and ground pins correctly connected - only then will you know if it works or not. (of course, your partial connections may have damaged it, so perhaps it would be best to discard this part, and start over with a new one...) \$\endgroup\$ – Peter Bennett Dec 16 '13 at 3:43
  • \$\begingroup\$ @PeterBennett Could you explain how the partial connections could damage it? \$\endgroup\$ – mushroom Dec 16 '13 at 3:45
  • \$\begingroup\$ @mushroom: with the ground pin not connected, and the LED from output to ground, you will be causing voltages and currents within the chip that were not anticipated by the designer. I don't know whether this would damage the part, but using and IC far outside its intended environment is definitely Not Good. \$\endgroup\$ – Peter Bennett Dec 16 '13 at 3:50
  • \$\begingroup\$ @ThePhoton I unfortunately paid 5 times that much because I don't know what I am doing. Now I see that I can get 10 of them for less than $5. I would still prefer to figure out what happened and how to diagnose problems like this, rather than developing circuits completely through trial and error. \$\endgroup\$ – mushroom Dec 16 '13 at 3:54
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The long leg of the LED was attached to the corresponding output pin with a 1kohm resistor in between. The short leg of the LED was attached to ground but the comparator GND pin was not. I expected the LED to light up when I flipped the power switch.

Seems to me that you have connected the Anode to the output of the comparator and the cathode to the GND, that can not work for an open collector/ open drain output , the way this works is that you have to connect the positive supply to the load (LED and resistor) and then the output of the comparator will provide the ground (sink current) to the load (LED Cathode)

LM339 drive LED

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  • \$\begingroup\$ So, when the +input is higher than the -input the current will flow through the LED and the LM339 to ground. And when the +input is not higher than the -input, no current will flow through the LED? When I set it up this way, the LED lights up. However, now I have been unable to find an arrangement of the inputs that causes the LED to turn off. How can I make the LED turn off? Also, what did you use to create that diagram? \$\endgroup\$ – mushroom Dec 18 '13 at 1:48
  • \$\begingroup\$ @mushroom I didn't create the schematic, I just searched using google to find an image that included what I wanted to show and cropped it. You say that the led is always on? Have you connected both inputs to a known state or you leave one of them floating and apply the voltage to the other? Take a look at ermicro.com/blog/?p=1578 I think it will help \$\endgroup\$ – alexan_e Dec 18 '13 at 8:11

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