Easy latchup demonstration using Numitrons!

[Moses]

Newcomers to the electronics hobby may not know of latchup. It happens to ICs for various reasons and can result in a short usually between the power supply rails! There is plenty of material on the internet about it for reading.

I did just find an easy method of demonstrating it! Take a half dozen numitrons, hook them up to '595 shift registers and let the inputs float, omit smoothing or decoupling caps. Because of the floating inputs the display will usually shift around in a beautiful dazzle! But eventually will latch up and cook your ICs!

Even if you have a microcontroller driving the inputs.. it might not come up to output state fast enough to prevent a latchup, and the driver ICs will just sit there and cook themselves. An easy trap to get into for a newcomer not familiar.

The spikes in the power rail caused by the rapid on/off sequence causes this. A smoothing capacitor pretty much fixes it and a couple of 100nf decoupling capacitors will get rid of it entirely. Use decoupling caps on each IC and you should never see this.

An excuse to buy a thermal camera and make a video? hmm..

Be well!

Regards,

-Moses

[Mark Moulding]

Interesting!  I guess I've been lucky for the inputs to not have been floating, or perhaps the decoupling caps I've usd have been enough, because I've never seen that particular problem.  (Now that I think about it though, I wonder if that's what's going on in another circuit I'm building?   Lots of hash on the power supply rails until the power-up reset completes.  Hmmm...)

Harbor Freight introduced a thermal camera a few months ago: https://www.harborfreight.com/thermal-camerainfrared-digital-imaging-thermometer-58111.html  At $220 it's not a Fluke, but it successfully found the loose breaker wire that was causing my AV projector to flake out.

~~

Mark

[Moses]

I had space for decoupling caps on the PCB for each IC, but omitted them on this particular prototype. I had space on the PCB for 7 decoupling caps, the six 595 shift registers and one 74125 logic shift IC. I don't remember the last time I had seen latch up like this so I was curious how many decoupling caps or smoothing caps it would take to get rid of the issue. One or two decoupling caps and it was difficult to get latchup, but with all 7 decoupling caps installed (and no smoothing cap) I left the inputs floating for days and it did not latch up. The smoothing capacitor will likely make it even more resilient.

Some more info if you are curious. I had prototypes with both TPIC6C596N and TPIC6C596N (high power versions of the '595) and both would latch up. It varied how many of the ICs would actually latch up, anywhere from 1 to all 6. Each of of them would use up 360ma at 5v, or about 1.8w. I verified latchup was between VCC and GND. I left one latched for a few minutes and surprisingly it was not destroyed, I'm sure it's not good for it though! It needed a complete power down to reset.

Yes, keep an eye on your power supply noise. I know my lab power supply was not happy with the disturbances as it would blink the current overload (CC) icon, even though the slowly updating display showed it no where near the set current limit.

Indeed I was looking at what a nice thermal camera you can get now for a few hundred. I remember not too long ago we were amazed at the "inexpensive" handheld flirs that were coming out for *only* a few thousand dollars!

Regards,

-Moses

[Mac Doktor]

Concerning latch-up, isn't the 741 immune? At least that's what I heard over the years.

On Dec 30, 2022, at 12:23 AM, Moses <mo...@neonixie.com> wrote:

Indeed I was looking at what a nice thermal camera you can get now for a few hundred. I remember not too long ago we were amazed at the "inexpensive" handheld flirs that were coming out for *only* a few thousand dollars!

I'm not sure if it's as good but Seek Thermal makes a lo-res IR camera for smartphones. You can take both visible and IR photos at the same time. Around $250 retail for the entry-level model, on sale periodically at Amazon. I got mine for $195 seven years ago. There's probably more competition now.

Eats battery.

Terry Bowman, KA4HJH
"The Mac Doctor"

https://www.astarcloseup.com

"If only you could see what I've seen with your eyes."—Roy Batty, Blade Runner

[Moses]

Only the '595 and 596' would latch up. And a correction to my previous post.. I am using the TPIC6C596N and TPIC6C595N. I never saw the 74125 latch up.

Lots of thermal camera choices now for sure. Will have to wait for a bit, I'm a bit over budget on equipment :/

Regards,

-Moses