7805 failure mode

Just wondering if all 7805's are the same circuit?

martin

Dumb question, but after observing this, did you try the same experiment with a new 7805, but wired correctly before applying power?

Thanks, Rich

What are you measuring with (O'scope?, voltmeter?) What is the duration of the spike?

Either way, this sounds like a busted 7805. Or, you have something else hung on the output, intentional or otherwise.....

-mpm

Yes, we can wonder. This was a Fairchild part, and Paul has given me the rest of his stock, two pieces.

I have not, but I have the two remaining parts from the manufacturer. I also have the bad part.

It certainly appears busted, all right.

The probable failure mechanism was...

Turn on without a ground. While it had no reference, it should have current-limited without damage.

But the failure occurred when it was unpowered and the output capacitor dumped thru the device to the input.

...Jim Thompson

Some clarification needed: by 'the ground pin' you are referring to the

When the pin was properly grounded, did the 7805 resume normal operation? If so, I wouldn't call what you experienced a 'failure'. It would be 'unspecified behaviour'.

AFAIK they have no reverse discharge diode protection. Might have been killed on the first turn-off. Anyhow, I have pretty much completely adopted the LM317 for almost everything positive. It can take at least modest spikes in reverse.

It's the things that don't show on the schematic... isolation junctions.

...Jim Thompson

That's my thought (the 11V or so on the output would be enough to break down E-B junctions and maybe there would be enough current to cause damage if the input had a heavy load or short to ground), although I don't see from the schematic what could make it act as described.

Best regards, Spehro Pefhany

We had a funny failure with 7908s a few (15?) years ago. One batch of ST- made devices gave an output of -10 volts, within a few mV, absolutely consistent. We thought we had been given mismarked units until we found that ST didn't make 10V versions, so we tested further, and found that the exact -10V was pure coincidence, and that above a certain forward voltage, the output tracked exactly the input, just a few volts down. As the PSU was a kludge (a 12V supply made to give 8V by heating up the world), the input was over this threshold. Problem solved by using someone else's devices, and we never bothered investigating further.

Paul Burke

Or a too small area for the reverse jolt from the output cap upon turn-off. IIRC that's next to nothing for these regulators.

Yes.

It operates normally, except for the short turn-on voltage spike every time the power is applied.

Yes, it appears the current-limit finction should work properly with the common floating, but the substrate would be pulled well above the output-pin's voltage by the startup and over-temp shutdown circuit.

I'll check with Paul, but I don't believe such a shutdown-current path exists in this case. Also, doesn't that kind of mistreatment destroy the part? Paul says this one works normally once it's running. But it makes a voltage spike each time it starts up.

Some data sheets show a diode from the output, back to the input to prevent damage.

Win, Have you examined Paul's circuit to make sure it doesn't have some design defect... perhaps loaded under the minimum?

...Jim Thompson

I wouldn't call that normal if the device specifications state that it should maintain voltage within a certain envelope following input supply transients. Unless the device spec actually addresses operation with a floating ground pin (doubtful).

I also wouldn't call it a failure if, once the pin is hooked up properly, the device commences operation within spec. On the other hand, it would be a device failure if, once the pin is properly connected, this anomalous behavior continues.

Sorry if I'm a bit of a terminology Nazi here. But I've done quite a few FMEAs for avionics and other stuff. The FAA (and others) really pick nits about which is a failure and what its effects (subsequent failures vs operation out if spec) are.

There's no minimum load for the 7805 (unlike the 317)... there's an internal 5K or so to ground (the feedback divider chain) so it should even be able to sink a bit of current, at least at room temperature.

Of course if an input filter e-cap was connected from input to output rather than from input to ground you'd get the spike on turn-on but it would spike negative (or not at all) on turn off.

It really seems a bit odd that the output should spike right to 12V (the input voltage) on turn on from some internal fault... usually those things have a drop of a volt and change even at zero output current, but perhaps Paul was speaking loosely.

Best regards, Spehro Pefhany