7805 failure mode

My co-author Paul described an interesting failure mode for a 7805 regulator IC, which was in a circuit with a modest load current, and with about 1uF of load capacitance.

The failure symptom is that when 12V power is first applied, the output voltage first goes to 12V, before dropping back to 5V and operating properly. When the power is turned off, the output voltage again spikes high.

One possible clue: when the 7805 was first used in this circuit, a hand-wired prototype, the ground pin was left floating. But when the failure mode was observed this pin was properly grounded.

Reply to
Winfield Hill
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Just wondering if all 7805's are the same circuit?

martin

Reply to
martin griffith

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

Thanks, Rich

Reply to
Rich Grise

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

Reply to
mpm

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

Reply to
Winfield

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

Reply to
Winfield

It certainly appears busted, all right.

Reply to
Winfield

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

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

[snip]

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

7805 common pin, correct?

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'.

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Paul Hovnanian     mailto:Paul@Hovnanian.com
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Reply to
Paul Hovnanian P.E.

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.

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Regards, Joerg

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Reply to
Joerg

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

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
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.

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Best regards, Spehro Pefhany

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Reply to
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

Reply to
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.

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Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

Yes.

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

Reply to
Winfield

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.

Reply to
Winfield

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

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Reply to
Michael A. Terrell

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

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
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.

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Paul Hovnanian	paul@hovnanian.com
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Reply to
Paul Hovnanian P.E.

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

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Reply to
Spehro Pefhany

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