LM311P Problem

Good day, all
For this little project, I’m using a LM311P. It is has a collector/emitter output. Because my circuit didn’t function the way it should, I started debugging.
With the collector unconnected the emitter (pin 1) still sinks (11 mA) and sources (20 mA) current, when applying a square wave input. I measured mean dc amperage to either rail, so the max is higher. I cannot explain this behavior.
Doing the current measurement on the collector (pin 7) with an unconnected emitter I get 14 mA when sinking, and none when sourcing. But not following the input signal.

Anyone has suggestions what could cause this? Many thanks!

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Hi Dan
I don’t quite get what you are doing here. The LM311P is described as a voltage comparator (as any OP Amp can be) but with a few refinements. It has an uncommitted transistor output. By uncommitted I mean both the collector and emitter are available. This means it can operate at say 5V and switch or do something else with say 50V without interfering with the basic operation. Similar in some respects to another comparator the 393 (dual) or 339(quad) but these are what is known as open collector where only the collector is available. This can be very useful as 2 or more can be arranged in “a wired or” configuration simply by connecting the collectors together.

What pin are you applying the input signal to. Bear in mind this device will switch, not amplify as such. It will go nearly rail to rail with no in between.

The transistor at the output is probably behaving like any other transistor. If tested with a multimeter it will behave like 2 diodes back to back. NPN (this case) positive to base will conduct to both emitter and collector with the negative lead on each of these. PNP transistors (not this case) will be the opposite. This is only an example as you have no access to the base.

I think to make further comment one would have to know more about your test set up and how you made these measurements

Keep in mind like any OP Amp one input inverts the other is non inverting.
Cheers Bob


Thank you for all replies. :slightly_smiling_face:
Sometimes online stuff makes me laugh. I one saw a toy figure of a Gorilla, it had quite a long tail. It was a good resemblance of the real thing. Except gorilla’s don’t have tails. So some guy in China made a pretty nice Gorilla from probably a photo which “hid” the non existing tail. But how did he determine the tail size, I wonder?
And what kind of creature is my LM311 in reality, I really wonder. I’m now certain I got something else. (An op amp or push pull comparator) Because my circuit (no rocket science) works with a LM211 just fine.
However even in my batch of LM211 I got a lot of fake ones! (They look the most pretty though)
I think I learned my lesson.

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Hi Dan

Some posts make one laugh too. Or at least wonder what the hell is going on.

Where do you get your chips from. Obviously from some backyard outfit in the boondocks of China it seems like. Suggest you change supplier. Or maybe just study up on OP Amps and comparators.
Cheers Bob

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