Re: Opamp problem


ehsjr a écrit :

lerameur wrote:
John Popelish a écrit :


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?


I added a simple circuit on the bottom of the page, should be easy to
see
http://www3.sympatico.ca/lerameur/

ken


On te LM324 circuit photo - check that the resistor
from row 30 to 35 is 330 ohms. I can't tell the value
from the photo. The green wire in row 38 (pin 11 of
the 324) is supposed to go to ground.
You have it wired to +.


The bottom photo is of a different chip - an LM393 -
in a different circuit. You state in the text below the
photo that you should get about 6 volts on the orange
wire. That is not correct - the circuit in the photo
will never give you +6 on the orange wire.

The LM393 will NOT work in the swansontec circuit. The
393 requires pullup resistors. That's why the orange
wire won't give you +.

thank you
but how do i know if they require pull up resistors ?
What do I have to look at the specs to know that ?

ken

.

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