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Ridiculously easy opamp circuit not working

Ridiculously easy opamp circuit not working

Ridiculously easy opamp circuit not working

(OP)
At my wits end here. I must be missing something incredibly basic. Thanks in advance.

I have wired up a simple non-inverting opamp circuit to a LM324A. I have a 10k potentiometer connected across the inverting input and output of the opamp, and a 2k resistor between the inverting input and ground.  

Next I have powered it with +9v and ground.

With only the two resistors and power connected, the output is pegged high (7.8V) no matter what I change the potentiometer value to (verified with a multimeter). What could be causing this?

When I feed it an input signal, nothing happens because it is pegged.

spec sheet:
http://www.fairchildsemi.com/ds/LM/LM324A.pdf

RE: Ridiculously easy opamp circuit not working

While I believe there are voids in the universe that pass over my workbench (and others)on occasion that cause the rules of physics to violate themselves, this state doesn't last very long.   The first thing to rule out is the belief that "I've done nothing wrong."  Maybe a bad chip, wires are on the wrong pins, started with pin 1 as the wrong pin.  First thing to try is to set one input at mid voltage with a simple voltage divider (power to ground).  Then use the pot as a second voltage divider on the other input.  About the middle of the pots rotatation you should see the output flip as the op amp acts as a simple comparator.  This transition occurs when the difference is less than a milivolt.  Something to remember!  In most circuits, seeing a measurable difference between the two inputs indicates something is wrong.

RE: Ridiculously easy opamp circuit not working

You say nothing about your non-inverting input. Where is it connected before you apply the input signal? Try connecting it to V+/2 if you haven't already. There was a problem with latch up in the original (NS) LM324 if you left the inputs floating - may still be there, the schematics look very much the same now as back in the seventies.

Another thing that you should check is power. Remember that Gnd goes to the "top" pin and Vcc to the "bottom" pin. That is opposite to intuition and often connected wrong.

Gunnar Englund
www.gke.org

RE: Ridiculously easy opamp circuit not working

Good idea opera.

My thoughts:

If the pot is really a 100K pot then the gain would be so high that any signal will peg the output.

The pot may be open/faulty.  This makes your amp a comparator with the reference voltage set at zero.  Hence any signal will trip the comparator pegging the output.

RE: Ridiculously easy opamp circuit not working

The inverting input should be connected to a regulated +4.5V virtual ground, not ground, or the (-) supply should be connected to a -9V supply.



Mike Halloran
Pembroke Pines, FL, USA

RE: Ridiculously easy opamp circuit not working

Yes, all of the above.

If you are using a single 9v supply referenced to ground, then both the input signal and the output signal plus feedback should all be referenced to +4.5v somehow.

Another way to do it is to leave the circuit as it is, and supply the +ve pin from one battery (-ve connected to ground), and the -ve supply pin from a second battery (+ve connected to ground).

The LM324 then operates from +ve and -ve supplies and both the input signal, and the output signal will be with respect to ground.

RE: Ridiculously easy opamp circuit not working

GUYS, Guys, guys, you need the +V/2 ONLY IF you want to amplifier an AC signal it is most definitely NOT needed for a DC non-Inverting amplifier application.

Thomas Frederiksen would be shocked to find out you needed a split rail..  (LM324 designer)

RE: Ridiculously easy opamp circuit not working

Smoked, smoked, smoked...

It is a 10 k pot. The gain will not be more than 1 + 10/2 = 6 times if the pot is at 100 %. (And no more than 51 times if it were a 100 k pot).

Connecting V+/2 is a good test. You can forget about (possible) latch-up and also offset voltage playing tricks. The latch up thing is very real - the amplification changed sig if you went above a certain common mode voltage. Haven't used the 324 for decades so I do not remember the details. They are hopefully better nowadays.

Still think that power is connected wrong way. Most guys that aren't used to this particular pin out do it wrong first time.

Gunnar Englund
www.gke.org

RE: Ridiculously easy opamp circuit not working

Yeah skogs I've reversed those on occasion.  Its right up there with putting in EPROMs backwards and the pretty light that comes out of the window signaling total annihilation.

And I agree AS A TEST V+/2 might be entertaining...3eyes  

RE: Ridiculously easy opamp circuit not working

(OP)
Thanks for all the help. I've done some more testing using your suggestions and am still not able to get the thing to work. I've tried setting it up as a simple voltage follower, with +/- supplies, and with V/2 into the non-inverting input. Still pegged.

I tested the same circuit on a breadboard with another opamp and it works fine, so I think I may be having a problem with the circuit layout on the pcb or with the way I am powering it.

RE: Ridiculously easy opamp circuit not working

Well heck look at the circuit board and check it.  Tiz not hard...

But truly the most likely problem then is the part is blown! That "old" part blows very easily!!  Look at the data sheet, there are warnings all over it..... This means while messing with them the likelihood of killing them is pretty high.. A slipped scope probe, etc., is all it takes.  Replace the IC and proceed.

Remember cut all the leads off the part then remove each pin out of the board independently.

RE: Ridiculously easy opamp circuit not working

(OP)
Well I resoldered a chip on there and used a wrist strap this time. It's working fine now. Argh.

Thanks for the help!

RE: Ridiculously easy opamp circuit not working

heheheh BOO-YAH!

I have a product with 9 of those suckas on it.  Calibration and set up require I insert and remove several of them with the power on.  I have a problem with one about every 200 I go through.

Carry on.infinity

RE: Ridiculously easy opamp circuit not working

Hint for the day:

For a first attempt at any new design on a printed circuit board, always fit the IC's into sockets.  It can save a LOT of frustration.

RE: Ridiculously easy opamp circuit not working

Warpspeed has an invaluable point about using sockets on prototypes. I never go without one. An 8-pin device is desolderable but try doing desoldering something with more pins - no fun if not impossible for >- 32 pins.  

RE: Ridiculously easy opamp circuit not working

Ah yes the good old dips... I don't think I've been able to use DIPs in the last 3 years.  :(

RE: Ridiculously easy opamp circuit not working

See?  Surface mount devices have their advantages :)  If I need to remove one, I slip a thin piece of magnet wire under all of the pins, heat them all at once with a bar, then gently tug on the wire... instant separation.

Dan
Owner
http://www.Hi-TecDesigns.com

RE: Ridiculously easy opamp circuit not working

An interesting point.  How on earth do you prototype something that uses surface mount components located on both sides, and a 99 layer circuit board inbetween ?

It is one of many reasons why I am now very glad to be retired.

RE: Ridiculously easy opamp circuit not working

Nice tip mac... I will give that a try!

Warp... Often it ain't pretty..  Trace cutting is a standard.

Actually you try extra hard to not botch things up in the original design...  None-the-less I just left a bar off over an enable for a 4051..cry  I jumpered the enable pin to VCC with a trace completely under the part.crycry

In this case I just had to unsolder the pin, bend it up then bridge it to the pin next to it to tie it to ground.  Bridging is simple with smt parts.  Just solder the pin without being extra careful.. It'll be bridged.  lol

RE: Ridiculously easy opamp circuit not working

You proto-type using a PCB, the somewhat finished design. To prove a hardened design requires the actual PCB be used because PCB layout is very critical to proving noise immunity and such. When I proto a SMT device, we always use the actual layout and populate using the SMT equipment. Of course, for a real rough proto, you can always breadboard with through hole parts just to prove the design works without the concerns for environment and equipment near by (immunity).
I have never done a board with more than three layers so not sure if there is something special for the guys who build multilayer boards. I am betting its done with the SMT equipment as well, the only real way to do it.

RE: Ridiculously easy opamp circuit not working

I used to prototype most of my circuits using through hole components simply because they are much more convenient to work with, and then use surface mount where appropriate for the production item.  I suppose it really depends on the nature of the circuit.

It is just too much work for the production guys setting up and programming their robots to make just one prototype board for the loony engineer upstairs to play with. They may have to scrap ten boards before they can get their system sorted. It also disrupts whatever production run they may be doing at the time. A very quick way to lose friends down on the production floor.

I was mostly into analog, instrumentation, and switching power supplies. Hard core digital and RF is a whole different ballgame.  EMC compliance is another aspect that adds to the fun too.

RE: Ridiculously easy opamp circuit not working

I hand build the first two SMT protos myself as there is always some little detail to find anyway.  Like messed up foot  prints or poor pad layouts or through-hole diameters/pads not quite right.  If someone one else does these you may never find out about it or worse on the first rushed batch of 250 pieces someone comes up and taps you on the shoulder.. banghead

The other thing is there is always some mechanical issue like connector placement or board holes that you would like to improve and if I use a thru-hole proto you miss this opportunity up front.

Lastly half the parts I need don't come in thru-hole these days.

RE: Ridiculously easy opamp circuit not working

itsmoked,

I agree with you!  I too always build the first prototypes by hand.  You are right about there always being something you want to change, even if it is component designator in the silkscreen that is ambiguous.  Depending on the complexity of the board it only takes about a day or two to completely populate it.  I do stay away from BGA packages, though, since I don't have the equipment to do those properly.  I guess I just consider myself lucky to have talked the boss into my Pace MBT250 and a Meiji stereo microscope smile though there are times I wish I had a hot air tool that would insert and remove those types of parts sadeyes.

RE: Ridiculously easy opamp circuit not working

Hi Noway2;

I designed a full featured PID controller that had to be two boards to keep it the size of a large postage stamp.  I would blow the layout up to 4X to examine it and that was even small!  Then I had to hand build 30 of them, sticking 88 parts on each.  It has four, 20 pin connectors with 12mil leads and 8mil spacing... bomb  They are true misery to solder, taking 20 minutes a connector!  I finally had to get a stereo microscope too.  Boy is it a joy to use!

I have a hot air pen.. They aren't all that great.  I used it about 10 times but was never happy with the results.

RE: Ridiculously easy opamp circuit not working

I know people who swear by the hot air tools, but I personally prefer the conductive (iron) methods.  I might could mention that the person I know who swears by it the most also uses break cleaner to remove the flux banghead but thats another story.  

I find that with an iron I can go a lot quicker and don't have nearly as much problem getting the parts to align or bridging issues.  Part of my problem is that the air tool tends to blow the small parts off the board or out of alignment if I don't hold it just right, though this means that I could be using too much air volume.

With the iron, I have soldered as small as an 0201, thankfully though I never had to use one in a design,  I find that for myself, the "dry tack" method works best.  I have the most trouble with SOT-23 transistors where one of the leads connects to a ground plane.  A nice IR pre heater would solve that problem, but that is more $$.

RE: Ridiculously easy opamp circuit not working

0201 Ugh! I've never been victimized by that..  lol

I use a small Weller Pen set to 730F with the longest skinniest tip with the last 50mil bent 30degrees. This bend happened slowly by itself and I started finding that it was a great advantage!!  So I have always kept it.  I have no problem soldering (an smt pin) to a ground plane with it.  On the other hand I generally design with planes and so always run a short trace to a via which makes the plane connection which thermally disconnects the pin from the plane.

My problem is always blowing the 3-pin SOT-23 footprints.. The board I just laid out I got both wrong. cry  I had to pull them both and rotate them 120 degrees (got lucky?).

RE: Ridiculously easy opamp circuit not working

You reminded me of my "winner" with an SOT-23 package.  I was designing a board that used a FET and I remembered from school that we were taught that the Drain and Source are interchangeable, in comparison to the collector - emitter.  Well thats all good and well, until the you realize that you turn the device on by bring Vgs above the threshold and you are attempting to control Vgd and the Source pin is floating curse.

RE: Ridiculously easy opamp circuit not working

The actual source and drain are interchangeable.  But, what's usually done is that the substrate is tied electrically to the source, which makes the transistor asymmetrical.

TTFN



RE: Ridiculously easy opamp circuit not working

Some power MOSFETS are not symmetrical from the get-go, since drain is physically designed for a large BVdss.  

Int. Rect.'s V-groove devices were that way.

TTFN



RE: Ridiculously easy opamp circuit not working

Yet another hand raised for the group who builds the first proototypes by hand... and one final proto-production unit (using the exact layout/mask that production will use).  I've found a few last minutes pad changes that nearly put me in the SOL camp, such as a 2mm connector that (for whatever reason) had holes half as big as they should have been).

I've never had to hand solder any individual component smaller than 0603, so I suppose I'm lucky there.  BGAs have been out of my domain due to lack of equipment, too, but I've been sorely tempted to make one of those do-it-yourself hotplates for prototyping.  Can't count the number of times an SOT-23 pad placement has gone awry... don't forget you can also "deadbug" the suckers if rotating 120 doesn't give you the desired pinout.

Dan
Owner
http://www.Hi-TecDesigns.com

RE: Ridiculously easy opamp circuit not working

Dead bugging... Hadn't thought of that.. Wish I had, once.
With rotation and dead-bugging I think you could correct any screw up.  Nice.

I have sort of standardized on 0603 for everything.  Seems kind of diminishing return on the parts smaller than that.  Not to mention, power dissipation, (grain-of-sand = nada).

I've used reversed BJTs many times never done a FET that way though.

RE: Ridiculously easy opamp circuit not working

Source and Drain are generally interchangable for run of the mill JFETs.

Some JFETs designed for HF use may not be totally symetrical so as to reduce Cgd.

On some of the JFETs I was familiar with, the substrate formed the gate contact.

Power Mosfets have a built in reverse biased diode from drain to source, so if you reverse the fet the diode turns on...

RE: Ridiculously easy opamp circuit not working

On the dead bugging front, some sot23 devices are available in "R" versions, with the two connections interchanged :O)

My favourite cockup was an SSOP device.

I gave the layout guy what I thought was the correct SSOP dimensions, 25 thou pitch.

Unfortunately the device had 40 thou pitch.

100 boards. Ooops.

Gave the layout guy the correct dimensions for the next 100 boards. He uses 50 thou pitch.

Ooooops. But 40 thou will spread to 50 thou with a little "encouragement".

3rd try was correct. 40thou pitch (actually 1mm).

Then the chip was made obsolete. :O)

RE: Ridiculously easy opamp circuit not working

zeitghost laughtears

Gawd that must've been hard to live down once the bean counters started rattling the cage.

That's why I never use FETs backwards!!  I forgot about that internal diode in this discussion...  I almost always use power mosfets so I never come to that junction in the road..

RE: Ridiculously easy opamp circuit not working

It would have helped if the owner had actually ordered prototypes: he only ever ordered 100 of anything.

Another fine mess was when he created a 63 pin SM package... didn't believe in using footprints, just created them on the fly using pads. Oooops. Wasn't too bad since it wasn't populated in the main product, but a real pain for the prototype it went in, green wire time.

On the same 4 layer board he managed to short the power planes together with a mounting hole... that was cured with a drill.

All of which was due to producing a prototype for an exhibition in 5 weeks.

I was down the road about a month later clown

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