I have wired up the LM311 circuit shown in the figure below. It should work as follows (at least to my understanding), when V+ (pin 2) > V- (pin 3) the output (pin 7) should rise to 12V and when V+ < V- the output should fall to Gnd. I have applied a 1kHz square with an amplitude of 1V swinging around Gnd to pin 2 (V+), see scope image on the second picture. I expect a square wave from 0V to 12V on the output but instead it stays at approximately -12V (CH2 on the scope image), so i.e. at Vee. Even when I apply 12V on pin 2, the output stays at -12V.

My question: Why is this happening? Shouldn't it also produce a square wave on the output? Is there an error in my circuit?enter image description here LM311 circuit as wired up on breadboard

  • \$\begingroup\$ The circuit looks OK to me. \$\endgroup\$
    – Andy aka
    Apr 27 '20 at 14:23
  • \$\begingroup\$ You have fitted the pullup resistor R? ? \$\endgroup\$ Apr 27 '20 at 14:23
  • \$\begingroup\$ Are you sure that you did not swap the pin 2 with pin 3? \$\endgroup\$
    – G36
    Apr 27 '20 at 14:29
  • 1
    \$\begingroup\$ Vout cannot be equal -11V if pin 1 is connected to GND. \$\endgroup\$
    – G36
    Apr 27 '20 at 14:36
  • \$\begingroup\$ Yeah I double checked everything, I think the IC is broken \$\endgroup\$ Apr 27 '20 at 15:50

Go over your connections on the hardware and make sure they are connected to the right pins/ wires/ components/etc.

I’ve connected everything together with a circuit I worked on and thought everything was fine, then the circuit is misbehaving. Two hours later I find out a resistor is connected to the wrong pin, changed it and everything was fixed!

  • \$\begingroup\$ Yeah I reconnected the circuit but it still outputs -12V. I bought this comparator on eBay from China, so I suspect it is just a broken IC. \$\endgroup\$ Apr 27 '20 at 15:02
  • \$\begingroup\$ Perhaps the IC received electrostatic damage or was a malfunctioned part. Try another LM311 if you have spare. \$\endgroup\$
    – Leoman12
    Apr 27 '20 at 15:54
  • \$\begingroup\$ Yeah I also thought about that, I tested the whole batch I ordered. None of them seem to work.. \$\endgroup\$ Apr 27 '20 at 16:05
  • \$\begingroup\$ This may do nothing but try using 10V positive peak input and -10V negative peak input. Keep supplies at 12 and -12V. See if this does help. Perhaps you are too close to rail voltages. \$\endgroup\$
    – Leoman12
    Apr 27 '20 at 16:57
  • \$\begingroup\$ @Leoman12 Please take care to really answer the question. If you have only suggestions or want to try the OP something, use the comments. \$\endgroup\$
    – Ariser
    Apr 27 '20 at 17:30

Pins 5 and 6 are floating. Probably not a good idea.

  • 2
    \$\begingroup\$ They should be kept floating according to the datsheet (if unused) \$\endgroup\$ Apr 27 '20 at 14:14

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