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
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
RE: Ridiculously easy opamp circuit not working
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
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
Mike Halloran
Pembroke Pines, FL, USA
RE: Ridiculously easy opamp circuit not working
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
Thomas Frederiksen would be shocked to find out you needed a split rail.. (LM324 designer)
RE: Ridiculously easy opamp circuit not working
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
And I agree AS A TEST V+/2 might be entertaining...
RE: Ridiculously easy opamp circuit not working
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
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
Thanks for the help!
RE: Ridiculously easy opamp circuit not working
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.
RE: Ridiculously easy opamp circuit not working
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
RE: Ridiculously easy opamp circuit not working
RE: Ridiculously easy opamp circuit not working
Dan
Owner
http://www.Hi-TecDesigns.com
RE: Ridiculously easy opamp circuit not working
It is one of many reasons why I am now very glad to be retired.
RE: Ridiculously easy opamp circuit not working
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..
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.
RE: Ridiculously easy opamp circuit not working
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
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
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
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
RE: Ridiculously easy opamp circuit not working
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...
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 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
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.
RE: Ridiculously easy opamp circuit not working
RE: Ridiculously easy opamp circuit not working
TTFN
RE: Ridiculously easy opamp circuit not working
Int. Rect.'s V-groove devices were that way.
TTFN
RE: Ridiculously easy opamp circuit not working
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
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
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
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
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
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