Ridiculously easy opamp circuit not working 1

[monkeysolder]

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

 

[Warpspeed]

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.

 

[itsmoked]

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..

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.

 

[Noway2]

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 though there are times I wish I had a hot air tool that would insert and remove those types of parts .

 

[itsmoked]

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... 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.

 

[Noway2]

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 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 $$.

 

[itsmoked]

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

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. I had to pull them both and rotate them 120 degrees (got lucky?).

 

[Noway2]

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 .

 

[IRstuff]

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

 

[IRstuff]

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

 

[MacGyverS2000]

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

 

[itsmoked]

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.

 

[zeitghost]

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...

 

[zeitghost]

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)

 

[itsmoked]

zeitghost

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..

 

[zeitghost]

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 )