Opamp problem

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

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).

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?

I took a snap shot of my circuit and also posted on

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 =E9crit :

ound

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.

John Popelish a =E9crit :

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

t

The operating concept for both opamps and comparators is that the=20 output goes low whenever the + input is more negative than the -=20 input. However, this effect takes place, only when both inputs are=20 within the input common mode voltage range (in some cases, only one=20 has to be inside this range).

So, with the inverting (- input grounded, the output should pull low=20 whenever the + input is more negative than ground. But the input=20 common mode limit for these parts is 0.3 volts below the negative=20 supply rail, and the inputs are not allowed to go lower than this,=20 without damaging the chip.

In order to have a positive output voltage when the comparator is not=20 pulling down, you must provide some external pull up mechanism to the=20 output (a 10k resistor to the positive supply rail, for instance).

I am still skeptical that you have the opamps connected properly. Can=20 you add a photograph of your assembly to the web page?

John Popelish a =E9crit :

I added a simple circuit on the bottom of the page, should be easy to see

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

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

Don

You have both power pins connected to the positive rail. Pin 11 needs to be changed to the negative rail.

Nice one.

Graham

ehsjr a =E9crit :

not

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

(snip)

On the data sheet:

look at the schematic on page 2. The output connects to only a single=20 transistor collector, Q8. There is no pull up part of the output,=20 like there is in every opamp.

On page 3 there is a spec for output sink current, but no spec for=20 output source current.

Such outputs are very common in comparators, but some do have normal=20 logic output stages, especially those made with a CMOS process.

To see some of the variations, go to Digikey:

and search for [comparator]. Then select comparators under the category of Integrated Circuits. The second selection column is Output Type. Unfortunately, you will=20 not find the LM393 under [OC open collector], where it should be. For=20 some reason, it is listed under [TTL,DTL,ECL,MOS,CMOS], all the logic=20 families someone thinks you can drive with this open collector output.=20 A very unhelpful and undescriptive category.

Why?

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

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.