Lm393N bad how do we know?

[Savetheday]

Hi everyone
I would like to know a bad LM393N How can I do that?There is 70 Ω between GND (4) and VCC (
And If I supply a 4.5V power and check the legs (4-8) 3.8V, (1-4) 1.8V, (2-4) 2.0V and (3-4)1.7V.
Thanks.

[james_s]

Is that in or out of circuit? If you see 70 ohms between Vcc and GND out of circuit then that's a pretty good clue that it's bad.

[Savetheday]

Is that in or out of circuit? If you see 70 ohms between Vcc and GND out of circuit then that's a pretty good clue that it's bad.

Yes out of circuit.
How do can we test it if it's general IC bad?

[Ian.M]

Is that in or out of circuit? If you see 70 ohms between Vcc and GND out of circuit then that's a pretty good clue that it's bad.

The qualifier to that is "... between Vcc and GND, with Vcc positive, out of circuit ...".  Many ICs have structures that will look like one or more diode junctions if supply polarity is reversed.

[james_s]

Testing an IC involves having a bit of knowledge about how the IC is supposed to work. I would plug it into a breadboard and wire it up to a current limited bench supply and slowly turn up the voltage while watching the current draw. Then hook up a very simple circuit, many datasheets include a test circuit.

[perieanuo]

hi,
usually it doesn't work this way
ic's like lm393n can be tested on output side with diode tester, they usually have shorted output transistor, and in his datasheet in the simplified schematic you see some npn or pnp transistor as output transistor. for the input side, it's not the same, as the inputs are more 'complicated' schematics.
so what we can do? for AC, use the scope and see if ac waveform is amplified as it should be, for DC we can verify if he 'triggers' in the right direction. that means we understand very well how opamp works.
that 'shorted output' diagnose works from my experience in maybe less than 50% situations, usually when the designer left the possibility of opamp output to be overloaded on output side (like not protecting him from oscillations, short circuits - i found opamps with short-circuit protection included to be shorted!, bad layout on output side etcaetera). inputs can suffer the same and you got the input protection diodes or input transistors shorted or smoked, in that case you won't see the short with your diode tester.
finally, as @james_s pointed, the test schematic from datasheet is finally the 100% reliable test, but anyway often enough we find that one implemented with +- modifications on our defective board

[james_s]

Another option is to simply try replacing it with a known good new part and see if that behaves differently.