555 monostable pulls up pin 5 a bit!

[Circlotron]

I make this particular circuit board at home (I have made about 3000 of these things over the last 15 years) and one small part of it has a 555 monostable with 10k and 100nF that gives about 1.1mS output with a 15us trigger pulse. In recent times I have had two boards that gave an output of about 3mS instead of 1.1mS and it turns out that the pin 5 voltage which normally sits at 2/3 of the 5v supply rail (and is bypassed with a 100nF cap) gets pulled up by about 1 volt and so the pin 6+7 RC voltage needs to go a lot higher to make the mono time out. If I disconnect everything from pin 5 and even cut the pin from the board you can clearly see it going high. It is being pulled up from within the chip. Put in a brand new device and it does the same thing. My only "solution" has been to scrap the boards. Anybody got any suggestions?

[Enzo]

See if this happens only with certain brands or variants of the 555.

Different factories make them, and they come in CMOS and other varieties. Look at an old working board and see if the parts installed on the new board are different.

[Circlotron]

The original 555 was dead, which may be significant. I put in a replacement and it behaved as described in the first post. Then another and it did exactly the same thing. Then a third, all from the same tube, but this time put the third one in a known good board and it worked just fine. All three replacement 555s were TI brand from the same batch.

I would begin to think that maybe there was some kind of partial short circuit inside the board laminate, but the fact that when you totally =isolate= pin 5, i.e nothing connected to it at all because it is pointing in mid air and it still goes from 2/3 to nearly full rail in time with the output pulse makes me think that it must be something inside the chip, but what???

[jan.didden]

Since pin 5 is on a resistive divider from the supply, could it be a supply spike in that particular board? Defective regulator?
Or maybe even a flaky ground that has high impedance?

jan didden

[Circlotron]

There is an earthed ground plane on both sides - really went nuts with earthing because it operates in an automotive environment.

It does have a 100nF SMD bypass right at the chip but I'll have a look at the rails anyway. Good point.

[benb]

Originally Posted by Circlotron

The original 555 was dead, which may be significant.

Could some of the other components connected to it have changed value? Can you measure them? I can't see how this would affect the behavior of pin 5, but it may be good for a sanity check.

I put in a replacement and it behaved as described in the first post. Then another and it did exactly the same thing. Then a third, all from the same tube, but this time put the third one in a known good board and it worked just fine. All three replacement 555s were TI brand from the same batch.

I would begin to think that maybe there was some kind of partial short circuit inside the board laminate, but the fact that when you totally =isolate= pin 5, i.e nothing connected to it at all because it is pointing in mid air and it still goes from 2/3 to nearly full rail in time with the output pulse makes me think that it must be something inside the chip, but what???

What does pin 5 go to? Is it a .01 cap to ground, as shown in the data sheet?

And speaking of which, I suppose it's this one (apparently the ORIGINAL TI 555):
http://www.ti.com/lit/ds/symlink/ne555.pdf
instead of this one (the former National Semiconductor 555):
http://www.ti.com/lit/ds/symlink/lm555.pdf

Do you have an original 555 you can pull from an old board and put into this? If it does what the other new boards do, that should be convincing that it's the board and not the 555 (which like others is what I was first suspecting - a new batch operating slightly differently).

Sync a scope on the output, and with the other channel look at each pin on the new and old board, and see what the difference is. Is it more than just pin 5? Can you measure the current into and out of each pin? Perhaps something connected to the output has partially shorted (to the VCC, maybe?), causing an extra load on the output, reflecting through the internal VCC or ground to affect pin 5.

[Elvee]

With the 555, I have already noticed that the substrate isolation "tubs" behave funnily as soon as a pin receives a voltage above or below the supply rails; especially below.

You would say: normal. Yes, except there is no need to pass current in the isolation junctions for funny things to begin to happen: as soon as something is drawn slightly negative, currents begin to flow at unexpected places of the circuit, a sort of ghost field effect transistor.
I would check every pin, including the outputs for such a condition.

[Circlotron]

Originally Posted by Elvee

as soon as something is drawn slightly negative, currents begin to flow at unexpected places of the circuit, a sort of ghost field effect transistor.

Yes, I know for example that HC series CMOS many years ago (maybe not now) had a parasitic SCR as part of the output mosfet pair. If you raised the output pin above the supply rail this SCR would turn on and short circuit the supply rails. Not good.

[Circlotron]

Originally Posted by benb

What does pin 5 go to? Is it a .01 cap to ground, as shown in the data sheet?

100nF.

Originally Posted by benb

Do you have an original 555 you can pull from an old board and put into this? If it does what the other new boards do, that should be convincing that it's the board and not the 555

Yep. That might be the next thing to try.

[Circlotron]

Originally Posted by benb

Perhaps something connected to the output has partially shorted (to the VCC, maybe?), causing an extra load on the output, reflecting through the internal VCC or ground to affect pin 5.

Righteo then. Compared a good and bad board and the 555 has a 5 volt supply and seeing it is a bipolar version the output swings to about +3.8 volts. It splits two directions and drives a 10k to a transistor base, and directly to the gate of a 10 amp 60 volt mosfet. Now the bad board had the 555 output swing all the way to 5 volts just like a CMOS version of the chip would, but it should not. The mosfet was still operating okay but I cut the gate lead and it all suddenly came good.

So it seems there was a bit of leakage from drain to gate in the mosfet and this was pulling up on the output pin 3. Some stray currents must have been finding their way through a now forward biased pn junction somewhere inside the 555 and pulling up on pin 5, making the threshold of pin 6 go from 2/3 rail up to nearly full rail.

Hooray.