Help with HV adjustable power supplies for radiation detectors.

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Cai Arcos
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Re: Help with HV adjustable power supplies for radiation detectors.

Post by Cai Arcos »

This linear regulator are toys, with terrible regulation under load and tremendously easy to pop. I have finished the supply now, using a classic series regulator with a HV transistor from a CRT TV.
If anyone is interested in pursuing low noise power supplies, GET A DIFFERENTIAL PROBE! I can not tell you how many times I've being fooled by common mode noise and how many hours wasted tracking inexistent noise supplies. If you don't have it, you are handicapped (like me). Short ground clips (use the tiny ground springs) and always triggering the scope on the input waveform are a must. If the "noise" also does not trigger, or/and changes amplitude changing the lenght or shape of the ground clip, then you should most likely ignore it.
And do some serious indepth reading about noise before going down the rabbit hole.
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Richard Hull
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Re: Help with HV adjustable power supplies for radiation detectors.

Post by Richard Hull »

Sounds like you have a good grip on the effort related to noise. I bought a Tektronix differential probe system at a hamfest at a "silent key" sale of $10.00 (the sellers did not know what it was. It was listed at over a thousand dollars by Tek!!). It is a high voltage differential probe and can handle 1kv. Sweet! I have only needed it a couple of times since I bought it. You are right about triggering and grounding issues went poking about for noise. Did I mention, I hate noise!! I hate chasing it down.... Fortunately, noise is not an issue in most of my current work.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Joe Gayo
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Re: Help with HV adjustable power supplies for radiation detectors.

Post by Joe Gayo »

Why not filter with a 2-stage RC filter, like what every preamp uses? Since the bias current is low the resistor values can be MOhms.

http://sim.okawa-denshi.jp/en/CRCRtool.php
Cai Arcos
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Re: Help with HV adjustable power supplies for radiation detectors.

Post by Cai Arcos »

Joe:

If you use a fluorescent lamp inverter and this filter (just like Jim Williams used) the noise will be indistinguishable from background. However I wanted to add feedback as to make the supply regulated, and it always seemed to introduce noise. As of right now, and trying to learn as much as possible about feedback compensation and such, and are experimenting with a simple circuit that seems to work, more or less.
One of my main learning points have been that you can not say simply "add feedback to make it regulated" to a noiseless supply and expect it to stay that way without additional care.
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Joe Gayo
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Re: Help with HV adjustable power supplies for radiation detectors.

Post by Joe Gayo »

Feedback will cause instability if the gain loop crosses 0dB at more than -20db/decade
Cai Arcos
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Re: Help with HV adjustable power supplies for radiation detectors.

Post by Cai Arcos »

The problem is that I dont know the characteristic function of the CCFL-Inverter and bridge-filter themselves, so planning for compensation is, well, impossible.
The best I'm doing is inserting various capacitance values between the feedback and the output pin of the TL072 until the noise (which very clearly is caused by oscillations since by killing the bandwidht by putting 0.1uF at the output of the opamp eliminates it) goes down to acceptable levels and no oscillation is present.
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Joe Gayo
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Re: Help with HV adjustable power supplies for radiation detectors.

Post by Joe Gayo »

Not impossible. It can be measured.
Cai Arcos
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Re: Help with HV adjustable power supplies for radiation detectors.

Post by Cai Arcos »

How can this be done?
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Joe Gayo
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Re: Help with HV adjustable power supplies for radiation detectors.

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Cai Arcos
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Re: Help with HV adjustable power supplies for radiation detectors.

Post by Cai Arcos »

If I understood correctly, my main objective using this technique is to observe the phase shift at 0db and make sure it is high enough (between 45 or 50 degrees).
If I make my function generator float, then the transformer is unnecessary, right?
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Joe Gayo
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Re: Help with HV adjustable power supplies for radiation detectors.

Post by Joe Gayo »

Basically, yes.

I would use a cheap 120V to 12V 60Hz transformer.
Cai Arcos
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Re: Help with HV adjustable power supplies for radiation detectors.

Post by Cai Arcos »

Thank you very much!
Cai Arcos
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Re: Help with HV adjustable power supplies for radiation detectors.

Post by Cai Arcos »

Well, no mater what I do, I am not able to observe a sinewave on the scope. All I see is 50hz hum and the noise of the supply. I will have to try something else
EDIT: I also appear to have destroyed my last TL072, so until Monday, I'm not going to be able to do anything else
EDIT EDIT: Searching in my drawer, I found another TL072! Tomorrow will do more tests
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Richard Hull
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Re: Help with HV adjustable power supplies for radiation detectors.

Post by Richard Hull »

Those FET front-end op amps....Great in working circuitry.....Need a pile to really experiment with. Thank goodness they are cheaper by the dozen or thousands. I just bought a pack of 100, 2N7000's....Great little TO-92, low end sub-microamp input mosfet for turning on stuff in the .1 amp range using the joule energy from a mouse fart. When the turn-on energy just isn't there, the 2N7000 is.

I have used the OPA128 op amp to make single IC electrometers. It takes only 75 fempto amps to tickle the input. On a scope trace, on a winter's night, in DC follower mode with a 4-inch diameter aluminum ball on the input, my cat padding across the rug 5 feet away drove the trace off screen. I had to stay real still to not move the horizontal trace more than .5 volts one way or the other. Even manual shorting out was tough as moving away after grounding sent the trace off screen. I see why my vibrating reed Kiethley gold electrometer head has a small needle grounding solenoid to facilitate a remote grounding arrangement from the instrument box and a 6 foot cable going to the remote head.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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