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gerry
Regular Contributor
Username: gerry
Post Number: 29
Registered: 01-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, 11 August, 2007 - 05:50 pm: | 
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I've been following the recent Circuit Surgery articles. Although I understood them, I was wondering what are the symptoms of an 'unstable' regulator? I appreciate that it'll oscillate, but if it's at such a high frequency surely this wouldn't show up on a multi-meter? Or do regulators simply not work when they oscillate??
I've been using regulators for years and, to my knowledge, have never had this problem, but these articles have me curious now!
(Message edited by Gerry on 11 August, 2007) |
gordon
Frequent Contributor
Username: gordon
Post Number: 286
Registered: 05-2005
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, 12 August, 2007 - 07:18 pm: |
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I have also used several types of voltage regulator, and normally if the data sheet is followed on the external capacitors needed, they perform very well.
I guess if the regulator is oscillating at a high frequency, then the output voltage may be out of spec. or the voltage regulation will be poor under load.
I agree the EPE article was very interesting, and quite technical in explaining how voltage regulators could be unstable. |
epithumia
Frequent Contributor
Username: epithumia
Post Number: 466
Registered: 06-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, 12 August, 2007 - 10:24 pm: |
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I just tried it out. 9V regulated source feeding a uA79M05 via two 60cm leads. A further 60cm (single) flying lead to an analogue DVM. No capacitors. Oscilloscope on the regulator output (doesn't matter which end of the 60cm lead).
(Note this is a NEGATIVE regulator - it was what I had to hand.)
Here's the result:
2V/division, 0.5us per division.
-5 volt level added in red.
The DVM reads 6V.
When I add a capacitor to the regulator output the DVM reads 5V and the oscillation stops.
Obviously the output is not symmetrical around 5V. Possibly this is because the regulator can pull the voltage higher (in this more negative) quickly, but the voltage can only drop more slowly as the circuit stray capacitance discharges through the DVM.
Epi
If you need me, Neil and me will be hanging out with the Dream King. - Tori Amos
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gerry
Regular Contributor
Username: gerry
Post Number: 30
Registered: 01-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, 12 August, 2007 - 10:35 pm: |
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Interesting. Thanks for that Epi! |
john_becker
:::: Super User ::::
Username: john_becker
Post Number: 1526
Registered: 05-2005
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, 13 August, 2007 - 01:24 am: |
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Ian Bell comes in to look at this site from time to time. I hope he will follow up on this. Ian ??
J |
zeitghost
Frequent Contributor
Username: zeitghost
Post Number: 839
Registered: 01-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, 13 August, 2007 - 08:50 am: |
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I had a 78l05 regulator singing away quite happily on a breadboard at 45MHz not so long ago...
The output voltage was a lot of things, none of them being 5V... |
zeitghost
Frequent Contributor
Username: zeitghost
Post Number: 840
Registered: 01-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, 13 August, 2007 - 08:57 am: |
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I also remember a regulator based on the 7663 that oscillated gently at about 20kHz.
This didn't matter until the power control for a UHF transmitter changed from being ground referenced to being +5V referenced...
It went totally unnoticed until one day I increased the span on a spectrum analyzer & found two sidebands at +/- 20kHz re the carrier...
Ooooooooooooooooooooops. |
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