Maker Pro
Maker Pro

Opamp problem

L

lerameur

Jan 1, 1970
0
hello,

I made a logic probe circuit using Pspice. The theory works, but i
implemented it, the circuit gives me weird values. I am not familiar
ith working with these chips.
I have put a schematic of the circuit here:
http://www3.sympatico.ca/lerameur/
and an explanation below showing where the bug is.

thanks
ken
 
E

Eeyore

Jan 1, 1970
0
lerameur said:
hello,

I made a logic probe circuit using Pspice. The theory works, but i
implemented it, the circuit gives me weird values. I am not familiar
ith working with these chips.
I have put a schematic of the circuit here:
http://www3.sympatico.ca/lerameur/
and an explanation below showing where the bug is.

That's a very high source resistance to be biasing an LM324 from !

Why are you even using such an ancient and rather poor op-amp even ?

I also can't fathom what the circuit's meant to do. There's no feedback round
the op-amps. Do you actually want comparators ?

Graham
 
J

John Popelish

Jan 1, 1970
0
lerameur said:
hello,

I made a logic probe circuit using Pspice. The theory works, but i
implemented it, the circuit gives me weird values. I am not familiar
ith working with these chips.
I have put a schematic of the circuit here:
http://www3.sympatico.ca/lerameur/
and an explanation below showing where the bug is.

The opamp adds a small bias current to whatever its inputs are
connected to. (see the data sheet for the typical value of this current).

http://www.onsemi.com/pub/Collateral/LM324-D.PDF#search="lm324"

It looks like it is around 100 nA. This current should pull the
divider voltage up only about 50 millivolts. So it does not explain
anything close to what you are seeing. I suspect you have got
something wired up wrong. The pin out shown on your schematic is
right for only one of the 4 amplifiers in the pack. What pinout did
you use for the second amp?
 
L

lerameur

Jan 1, 1970
0
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 ??

ken


Eeyore a écrit :
 
J

John Popelish

Jan 1, 1970
0
lerameur said:
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.
 
L

lerameur

Jan 1, 1970
0
John Popelish a écrit :
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 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.

ken
 
J

John Popelish

Jan 1, 1970
0
lerameur said:
John Popelish a écrit :




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?
 
L

lerameur

Jan 1, 1970
0
John Popelish a écrit :
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).


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
 
E

ehsjr

Jan 1, 1970
0
lerameur said:
John Popelish a écrit :




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 +.

Ed
 
D

Don Bowey

Jan 1, 1970
0
John Popelish a écrit :


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 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.

ken

You need to put a couple capacitors between V+ and V-. Try 47 mFd and 0.1
mFd.

Don
 
E

Eeyore

Jan 1, 1970
0
John said:
You have both power pins connected to the positive rail. Pin 11 needs
to be changed to the negative rail.

Nice one.

Graham
 
L

lerameur

Jan 1, 1970
0
ehsjr a écrit :
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
 
J

John Popelish

Jan 1, 1970
0
lerameur said:
ehsjr a écrit : (snip)



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 ?

On the data sheet:
http://www.fairchildsemi.com/ds/LM/LM393.pdf#search="lm393"
look at the schematic on page 2. The output connects to only a single
transistor collector, Q8. There is no pull up part of the output,
like there is in every opamp.

On page 3 there is a spec for output sink current, but no spec for
output source current.

Such outputs are very common in comparators, but some do have normal
logic output stages, especially those made with a CMOS process.

To see some of the variations, go to Digikey:
http://www.digikey.com/digihome.html
and search for [comparator].
Then select comparators under the category of Integrated Circuits.
The second selection column is Output Type. Unfortunately, you will
not find the LM393 under [OC open collector], where it should be. For
some reason, it is listed under [TTL,DTL,ECL,MOS,CMOS], all the logic
families someone thinks you can drive with this open collector output.
A very unhelpful and undescriptive category.
 
D

Don Bowey

Jan 1, 1970
0


Between the two of them, they filter signals having a wide range of
frequencies. If you do not filter the power, your opamp can be affected by
external signals and lack of filtering can permit your circuit to oscillate.

Many circuits that don't work right may be found to be oscillating.

Don
 
P

Peter Wing Larsen

Jan 1, 1970
0
Don Bowey said:
Between the two of them, they filter signals having a wide range of
frequencies. If you do not filter the power, your opamp can be affected
by
external signals and lack of filtering can permit your circuit to
oscillate.

Many circuits that don't work right may be found to be oscillating.

Is this always the case in opamp circuits? In other words whenever I'm using
an opamp, do I have to filter the power?
 
D

Don Bowey

Jan 1, 1970
0
Is this always the case in opamp circuits? In other words whenever I'm using
an opamp, do I have to filter the power?

It's true for all circuits with active electronics, even tube circuits. If
you have an oscilloscope, look at the DC voltage with and without the
capacitors and you may see the difference.

The will need larger capacitors if your circuits use high current.
 
Top