Re: Opamp problem

lerameur wrote:
John Popelish a écrit :


lerameur wrote:

I took a snap shot of my circuit and also posted on
http://www3.sympatico.ca/lerameur/
I used the LM324 becasue I had it, and using the LM393 or LM339 did not
work using simple testing . From what I read these chips should be
doing the same work. Any chips you recommend ??

The main differences between the LM324 and 339 are that one is an
opamp (bidirectional output stage and slow response) while the other
is a comparator (fast response, pull down, only, output).

If you want to alter the circuit to take advantage of the speed of the
comparator, you have to add some pull up mechanism to each of the
outputs. I cannot recommend something specific, without understanding
better what the circuit is supposed to do.


I just want to understand how they work for now. My knowledge was to
put energize the chip (V++, V--) then by putting a voltage at the non
inverting input, gounding the inverting input, I should be getting V++
at the output right ? with any chip above, but i am not getting this
except with the LM 324.

The operating concept for both opamps and comparators is that the output goes low whenever the + input is more negative than the - input. However, this effect takes place, only when both inputs are within the input common mode voltage range (in some cases, only one has to be inside this range).

So, with the inverting (- input grounded, the output should pull low whenever the + input is more negative than ground. But the input common mode limit for these parts is 0.3 volts below the negative supply rail, and the inputs are not allowed to go lower than this, without damaging the chip.

In order to have a positive output voltage when the comparator is not pulling down, you must provide some external pull up mechanism to the output (a 10k resistor to the positive supply rail, for instance).

The follwing step (or problem) involves the circuit explained
originally (logic probe), if I try to use two OP amp (in one chip ) the
output go to high when theoretically they should be low.. just like I
show in my schematic.

I am still skeptical that you have the opamps connected properly. Can you add a photograph of your assembly to the web page?

ken


.

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