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Old 18th July 2009, 09:01 PM   #1
rman is offline rman  Canada
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Default Broskie regulator oscillates

Hi.

I built this regulator from the gassware webite.Broskie reg

It oscillates at about 3MHz and 0.3v pp. I was just wondering what causes this. I know that an oscillator needs a resonant circuit and positive feedback to work. I just don't quite see how it happens here.

Here are some observations I made about it on another thread.
Quote:
The load for this test is a big wire wound pot set to 12.5k (20ma. @250v). It was intended for a phonostage, the regulator that is. Interestingly, when the load is adjusted to about 12.1k, the oscillation stops. The Mosfet is an IRF BC40 (found on shop floor at work). swapped in an IRF 540 and the amplitude of the oscillation rises. I really don't' know enough to account for all this. I can only conclude that layout is critical, and that mine is not great.
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Old 18th July 2009, 09:06 PM   #2
Colt45 is offline Colt45  Serbia
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Throw a cap on the output...?
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Old 18th July 2009, 09:16 PM   #3
rman is offline rman  Canada
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I forgot to mention that I tried that as well. Some small valued caps. the amplitude of the oscillation actually went up! I don't really need this unit right now and am not just looking for a quick fix, just trying to understand how it happens in the first place
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Old 18th July 2009, 10:40 PM   #4
athos56 is offline athos56  United States
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Did you try stopper resistors? Everything I've made with mosfets needed to be encased in stopper resistors.
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Old 18th July 2009, 10:51 PM   #5
rman is offline rman  Canada
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Hi Athos.

The original design show a 200r gate stopper. When I saw the oscillation on my scope, I upped it to 1k and moved it as close to the mosfet's gate as I could get it. The oscillation waveform only became distorted.

When you say encased in stoppers, do you mean something more Than just a resistor on the gate?
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Old 18th July 2009, 10:56 PM   #6
Gordy is offline Gordy  United Kingdom
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I think this guy did the same design, and has documented his experiences:
http://gboers.xs4all.nl/daisy/home/g...oltagepsu.html
http://gboers.xs4all.nl/daisy/home/g...wersupply.html
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Old 18th July 2009, 11:04 PM   #7
iko is offline iko  Canada
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Try a generic 470uF capacitor on the output. Something not with a low ESR.

Could you please give a few more details? What opamp are you using? You might want to try a few different opamps. When I played with various opamp based regulators I found that some did not oscillate so readily. Another thing to try is a 200pF to 10nF capacitor on the G-S pins of the mosfet. I would not use that in the long term, just temporary to troubleshoot the circuit.
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Old 18th July 2009, 11:54 PM   #8
pointy is offline pointy  United Kingdom
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as I am very rusty and maybe a bit mad........................OK ?

I think the feedback is the diode loop just to the left of the marking +295 in red.

the resistance is ESR of the wire.

and if I remember right you can move the oscillation by putting in a resistor or something.

PS..... I have been awake for more than 24 hours or so
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Old 19th July 2009, 03:20 AM   #9
rman is offline rman  Canada
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Hi guys.

Thanks for all your suggestions. I hope pointy is getting some sleep. After messing around some more I still see a signal at around 1MHz. Then I notice it is still there only bigger with the power turned off! also it is pulsating. I must be looking at an AM radio station.
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Old 19th July 2009, 10:12 AM   #10
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This is an old problem. Most constructors layouts are designed without any RF consideration. The mosfet is oscillating parasitically at will without one knowing about it and so high that most scopes won't show it. The decoy is the scope probe often stops it.
Fit small ferrite beads in both gate and source leads. I fit in both, some may find the gate is enough to dampen. Make sure they aren't touching anything else and that should keep the device quiet.
Try it !

richy
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