Saw something wacky today - Electronic Circuits Projects Diagrams Free

speakerguy79 · 31st December 2007, 07:47 PM

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** · 31st December 2007, 08:03 PM

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.

speakerguy79 · 31st December 2007, 09:15 PM

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:

http://www.national.com/appinfo/power/files/f4.pdf

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

ericgibbs · 1st January 2008, 11:40 AM

Quote:

Originally Posted by speakerguy79

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:

http://www.national.com/appinfo/power/files/f4.pdf

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 · 1st January 2008, 11:54 AM

There's a big difference between a 100nF capacitor and a 100µF 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 100µF capacitor and you just ca\n't see it beacause all the AC current is being shunted to 0V.

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31st December 2007, 07:47 PM	(permalink)
speakerguy79	Saw something wacky today 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.

31st December 2007, 08:03 PM	(permalink)
Nigel Goodwin	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. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

31st December 2007, 09:15 PM	(permalink)
speakerguy79	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: http://www.national.com/appinfo/power/files/f4.pdf It uses the word 'usually'. Guess I found the non-usual case Last edited by speakerguy79; 31st December 2007 at 09:22 PM.

1st January 2008, 11:54 AM	(permalink)
Hero999	There's a big difference between a 100nF capacitor and a 100µF 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 100µF capacitor and you just ca\n't see it beacause all the AC current is being shunted to 0V. __________________ I also post at the following sites: http://www.stop-microsoft.org http://www.heated-debates.com Screen name: Aloone_Jonez