Saw something wacky today

Speakerguy · Dec 31, 2007

I was making a simple bipolar supply +/-15V out of stuff from the scrap box. I was using a uA7915 from TI on the negative rail. Its data sheet claims 'no external components necessary". With no output cap or just a 0.1uF ceramic cap, I got a 1Vpk-pk sine wave @ 1.8MHz on top of the 15V regulated output. Sticking a 100uF electrolytic on the output fixed it. I've never seen a standard/non-LDO have stability problems without output capacitors, nor have I seen an old school regulator need them for compensation. Has anyone seen this behavior before? With the 100uF it was a nice steady regulated DC voltage.

Nigel Goodwin · Dec 31, 2007

As far as I'm aware ALL 78/79 series regulators specify they need capacitors on the input and output, as close to the pins as possible. If you don't fit them you run the risk of oscillation as you've found.

100uF would be considered a bit on the large size, I would normally use between 0.1uF to 10uF.

Speakerguy · Dec 31, 2007

100uF was what I had laying around. Actually I think the positive rail got 220u and the negative rail 100u, or vice versa. The data sheet for it (uA7915 from TI) explicitly said no extra components required (though I admit I almost always use them anyway). It's an obselete part, but I thought it odd that the entire tube of them I had showed the same thing contrary to the data sheet.

ETA: I found the application note that made me think that they weren't necessary on standard (NPN darlington pass transistor) regulators, see pg19:

**broken link removed**

It uses the word 'usually'. Guess I found the non-usual case

ericgibbs · Jan 1, 2008

speakerguy79 said:
100uF was what I had laying around. Actually I think the positive rail got 220u and the negative rail 100u, or vice versa. The data sheet for it (uA7915 from TI) explicitly said no extra components required (though I admit I almost always use them anyway). It's an obselete part, but I thought it odd that the entire tube of them I had showed the same thing contrary to the data sheet.

ETA: I found the application note that made me think that they weren't necessary on standard (NPN darlington pass transistor) regulators, see pg19:

**broken link removed**

It uses the word 'usually'. Guess I found the non-usual case

hi,
I also have seen application data stating that caps are not required on the 7805/9 because of the internal transistor configuration.
In practice I have found it necessary to fit at least a 0.1uF, close to the pins, to ensure its dosnt oscillate.

Hero999 · Jan 1, 2008

There's a big difference between a 100nF capacitor and a 100:mu:F capacitor though. I woud have thought it would be alright with 100nF, perhaps you have a dud?

I wonder if it's still oscillating even with the 100:mu:F capacitor and you just ca\n't see it beacause all the AC current is being shunted to 0V.

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