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Old 12th September 2009, 08:05 PM   #1
bigwill is offline bigwill  United Kingdom
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Unhappy LM317 oscillating in cathode circuit

I have a simple LM317 CCS circuit in the cathodes of my output tubes - it seems to work well, apart from a millivolt level oscillation problem I discovered on my scope. Attached is the circuit. It seems to be oscillating into the Mhz region, I can't remember which scale I had the scope on, but it was near the limits of my 20Mhz bandwidth. The amplitude of the oscillation is about 20mV.

Each LM317 is programmed for 12.5mA, which may actually be too low. My decoupling capacitor is a 1uF electrolytic (not 10uF like in the schematic, sorry about that), which may be the problem too.

Perhaps the problem could be solved with a smaller ceramic type decoupling capacitor?

I have attached the circuit
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File Type: gif problem.GIF (5.9 KB, 383 views)
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Old 12th September 2009, 08:06 PM   #2
bigwill is offline bigwill  United Kingdom
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I forgot to mention, the oscillation was seen as probed from the input on top of the 220R resistor, and below it. The waveform was slightly different either side.
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Old 12th September 2009, 08:13 PM   #3
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Hi,

That's because you can't really use 317's like that. They need voltage across them to work properly.

I suggest a "real" CCS, unless you want to toss in a -V supply to make the 317's work right.

Cheers!

**afterthought**

Try losing the LED's. Maybe an extra 1.7V will "give them a kick"

Last edited by Geek; 12th September 2009 at 08:17 PM.
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Old 12th September 2009, 08:22 PM   #4
bigwill is offline bigwill  United Kingdom
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Quote:
Originally Posted by Geek View Post
Hi,

That's because you can't really use 317's like that. They need voltage across them to work properly.

I suggest a "real" CCS, unless you want to toss in a -V supply to make the 317's work right.

Cheers!

**afterthought**

Try losing the LED's. Maybe an extra 1.7V will "give them a kick"
Oh well they do have about 7V across them - the 220R resistor is just there to drop some of the voltage. Surely an LM317 is OK with drawing 12.5mA over 7V?
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Old 12th September 2009, 08:46 PM   #5
bigwill is offline bigwill  United Kingdom
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What I did notice when poking around - if I touched the cases of SOME of the ICs - but not all - the amplitude of the oscillation increased. The cases is Vout on these regulators. Maybe the output 100R resistor needs bypassing?
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Old 12th September 2009, 08:57 PM   #6
bigwill is offline bigwill  United Kingdom
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Okay, probing the outputs of the regulators that change when you touch them reveal themselves to be the ones that are oscillating!
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Old 12th September 2009, 09:59 PM   #7
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why not just look to SY's "Red Light" amp -- and by the way, an LM317 has impedance and phase.
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Old 12th September 2009, 10:17 PM   #8
bigwill is offline bigwill  United Kingdom
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Originally Posted by jackinnj View Post
why not just look to SY's "Red Light" amp -- and by the way, an LM317 has impedance and phase.
Most likely, but it's only having to sink DC - it is bypassed by a 470uF capacitor. It is your standard cathode bias circuit, but with a CCS
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Old 13th September 2009, 06:20 AM   #9
Svein_B is offline Svein_B  Norway
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You have to expect some noise, both from LEDs, zeners, and the LM317.

I observed a little noise from my LED referenced CCS. Zooming in on the scope I got this image. Seems similart to what is described here.
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File Type: gif LED-noise.gif (61.7 KB, 298 views)
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Old 13th September 2009, 08:29 AM   #10
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Sounds as if there might be a bit of radio pickup since you are an excellent radio antenna and you increase the oscillation. Try replacing the LED's with 10R resistors right on the pins.

Despite what people will say they work brilliantly with almost no sonic signiture.

Shoog
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