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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 18th Jun 2020, 2:24 pm   #21
GMB
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Default Re: High voltage regulator circuit.

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Originally Posted by dave cox View Post
AFAIK, the above circuit can be turned into a 'foldback' limiter with just 1 more resistor!
That has to be the best solution - it looks to me like it would work very well indeed.

The reason I go for a doubler is that it limits the current even if the MOSFET melts down to a short. But I now see that this circuit is not being run off the mains - so not relevant for that situation.
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Old 19th Jun 2020, 5:44 am   #22
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Default Re: High voltage regulator circuit.

Thanks all. Posted replies but the post seems to have disappeared, that or I didn't click submit. Anyhoo, I'm plugging away at this, have several versions including the Sussex PSU jotted down and will post again when I've had a play.

Andy.
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Old 7th Jul 2020, 7:14 am   #23
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Default Re: High voltage regulator circuit.

Right, I've done some more work on this and tried a few circuits. First off I built the anode/screen supply out of the Sussex. Essentially its the same as the one I built but it has a CCS supplying a zener string, this gives us a hopefully stable reference voltage, this goes into a voltage divider, which sets gate/OP voltage.

It worked pretty much first time, but like mine isn't stable, OP V drifts a little as the circuit warms and V out drops with increased current demand.

Next I tried a few floating regulator types, a few based on the LM317 application notes on HV voltage regulator's, sometimes called a Maida regulator on DIY audio. I also re-visited a similar circuit out of The Art of Electronics that used a LDO LT3080. All use a similar approach, a mosfet to drop excess V followed by a floating regulator.

I built three circuits, The AOE one, the 21st Maida and the LM317 HV reg. I'd built the AOE one previously and found it unstable, this was my fault, I left off a 2.2u cap off the OP. This time I found a 10u 450v film cap in my stash, this improved things, but the circuit was still unstable, everytime I tried to adjust it, the OP oscillated. All of these suffered from the same fault too, adjustment resulted in the OP jumping to the lowest V setting, then no I was able to make no more adjustment. All of these circuits were a pain to get working.

So, I write these floating types off, due to their instability and the need for massive brick size expensive low ESR. HV film caps.

Conclusions, for a simple HV reg the 1st circuit and the Sussex version works ok, but they're not 100%. To make a proper HV reg I think a more complex approach is needed, IE opamp foldback which means a separate PSU. For what I need at present this is a bit OTT and I haven't the room. In future I'd definately go down the valve route; valves laugh at HV & HV V reference valves give stable results, a string of zeners drift too much with temp.

Just my inexpert view, Andy.
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Old 7th Jul 2020, 2:01 pm   #24
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Default Re: High voltage regulator circuit.

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All of these suffered from the same fault too, adjustment resulted in the OP jumping to the lowest V setting, then no I was able to make no more adjustment. All of these circuits were a pain to get working.
The Maida does work but it is easy to damage it. Both of the circuits you included are NOT short circuit proof. The slightest slip with your crocodule clips and the regulator dies. The 317 version can even fail if there is no load!
IME it is better to build the Maida reg with no capacitors on the regulator IC 'in' or 'adj' pins. Those capacitors just create problems.
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Old 7th Jul 2020, 2:47 pm   #25
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Default Re: High voltage regulator circuit.

Actual ZENER diodes are only up to a couple of volts. Above that you get avalanche diodes although everyone still calls them Zeners. What goes on inside the diode may be different, but how they look to the world is very similar.

Avalanche diodes have significant temperature coefficients that depend on their voltage. What's interesting is that it crosses over from a positive to a negative signed coefficient a little above 5v.

So 'Zeners' around 5v ara much more stable, as a percentage, over temperature

This is why you see designs all over the place using 5.1v zeners as references theat get multiplied up by resistor ratos (Nice stable resistors, Mmmmm )

Motorola Made a family of 'Voltage Reference Diodes' with very low tempco at several voltages. Inside each was the avalanche diode and a straightforward silicon diode in series. The avalanche part was engineered to have a controlled tempco that matched and cancelled that of the plain diode. The resulting thing was like a 'zener' but with a most unusually low tempco.

1N823A is the 6.2v part, probably the best one of the series. Like any other zener, you have to pay attention to specified current for accuracy and tempco best behaviour.

They were the best reference there was until the three terminal IC references using bandgap references came along. IC fabricators also sometimes use 'buried zeners' as references.

David
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Old 7th Jul 2020, 3:12 pm   #26
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Default Re: High voltage regulator circuit.

Thanks both. Re zeners, or avalance type diodes, I wonder if a PTC thermistor or maybe an LED and LDR would serve to adjust for temp? I know you can get better voltage references but there are non I'm aware of that'll do a few hundred volts. Having said all that it's probably best to use gas valve VR in combo with SS regulation. Probably talking out of my hat, just pondering.

I'll keep dipping into this subject and do some more testing etc when the chance occurs., Andy.
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Old 7th Jul 2020, 3:45 pm   #27
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Default Re: High voltage regulator circuit.

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Thanks both. Re zeners, or avalance type diodes, I wonder if a PTC thermistor or maybe an LED and LDR would serve to adjust for temp?
How accurate do you need it to be? The tempco of Zener diodes is normally more than good enough for valve circuits. My guess is you are seeing the effects of the current-limiting resistor, not Zener variation.
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Old 7th Jul 2020, 4:46 pm   #28
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Default Re: High voltage regulator circuit.

Yea, the original design (and I agree, pretty much the same as the one I saw from the Sussex) will drift thermally and with load current. My mod for 'fold-back' would make that much worse since it increases the output resistance (in order to get enough voltage drop to reduce the current into a short circuit).

To get better performance you will need feedback but that doesn't have to be an op-amp

dc
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Old 8th Jul 2020, 5:36 am   #29
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Default Re: High voltage regulator circuit.

+/- 5% David according to yon datasheet for the 807. However this is for AB2 conditions, so I don't need that level of accuracy right now but while I'm at it.... "My guess is you are seeing the effects of the current-limiting resistor, not Zener variation" could be.

"To get better performance you will need feedback but that doesn't have to be an op-amp" I'm all ears Dave 2.

Andy.
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Old 8th Jul 2020, 8:15 am   #30
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Default Re: High voltage regulator circuit.

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Originally Posted by dave cox View Post
but that doesn't have to be an op-amp
There were valve opamps. Operational amplifier really describes an application for an amplifier.

Look up GAP - George A Philbrick. Rob Pease cut his teeth working there.

Then came discrete transistor designs, then monolithic ones.

David
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Old 8th Jul 2020, 11:16 am   #31
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Default Re: High voltage regulator circuit.

Andy, I knocked this up quickly last night, just an idea to see what it would involve without using too many parts. Looking at it now I could probably use 1 less transistor.

There are multiple problems as it is.
it will only work down to ~35V and the regulation is not brilliant (~ a volt at 350V).
Having 350V across a pot anyone ?
I didn't have a MOSFET model so used a bipolar for the series element.
The top right current source will be a pain, the others can be replaced with resistors (one sinking several watts).

There must be a better way ...

dc

PS I tried to attach the model but I couldn't upload it
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Old 8th Jul 2020, 11:35 am   #32
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Default Re: High voltage regulator circuit.

The fog is clearing ...

LTSpice model attached.
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Old 8th Jul 2020, 12:26 pm   #33
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Default Re: High voltage regulator circuit.

A few more issues ...

Should be a 6V2 zener not 6V8 for best thermal drift, but in any case there is not enough current through it to work properly. Ditto the 30V zener. More current is a problem as its being pulled rather wastefully from 400V. 1V2 shunt regulators are good and cheap (band-gap) BUT that wouldn't leave any headroom for the long tailed pair. Is there a cheap / available ~ 10V shunt regulator ? Or even a 30V ?

Maybe Q2 would get killed by a shorted load

dc
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Old 8th Jul 2020, 1:45 pm   #34
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Default Re: High voltage regulator circuit.

Could the reference diode be replaced by a TL431?

Take care if you build a circuit with high voltage on a variable resistor and make sure it can handle the power dissipation in the resistive element.

Reverse biased power diodes across the output terminals and the pass transistor are usually recommended.
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Old 8th Jul 2020, 2:09 pm   #35
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Default Re: High voltage regulator circuit.

TAA550 is a varicap tuning supply shunt regulator IC, temperature compensated 33V. A bit old hat but still around. High voltage "Zeners" have rotten temperature stability, you need to try them in a test rig to appreciate just how bad they are even well within their dissipation rating.

With PSUs, it's wise to imagine what happens if a variable resistor wiper goes open circuit (or maximum resistance if rheostat-connected to one end of the track)- generally, output going low is preferable to output going high!
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Old 8th Jul 2020, 2:24 pm   #36
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Default Re: High voltage regulator circuit.

Yea,
TL431 could replace the 6V2 zener (6V8 on the schematic)
Should work at 2mA.
Drop the ref to 2.5V so no additional resistors needed.
Let R3=2K5.

The pot with the voltage is NOT nice

dc
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Old 8th Jul 2020, 3:46 pm   #37
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Default Re: High voltage regulator circuit.

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+/- 5% David according to yon datasheet for the 807.
Sussex can do that easily. You seem to be trying to run before you can walk. If you can't make the Sussex work properly, you definitely won't be able to make that SPICE circuit work.
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Old 9th Jul 2020, 6:25 am   #38
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Default Re: High voltage regulator circuit.

Thanks Dave. Is Q3/4 a current mirror? i'll try and find time to do some work on this but have a tfmr to rewind by hand today. I've noted all the IP, have a TL341 etc. Re pots across HV, the schematic I posted back in post #1 (re- posted here)uses a pot across 350v, a 470/500k. I've used two of these PSU's in a homebrew tester with no issues. i put two 10M from wiper to each end if I remember right.

Got you David, the Sussex PSU is essentially the same as the attached schematic, but uses a CCS, zener (avalanche) string to get a HV V ref then uses a potential divider instead of the pot. I only briefly tried it, saw how the zeners drifted, moved on. David said of the circuit " IT isn't actually a regulator. It's a pot with a current booster and a current limiter. Any variation of the input voltage comes through proportionately onto the output." I thought it possible to do a bit better by adding foldback. i'll admit to not being 100% on this stuff but have some idea, thanks for your concern though.

Elektor published such a circuit that has a opamp and Farnell used a similar approach in their E350 ( though that use valves as the series pass element) and uses discreet FB circuit. If I can get Dave's schematic working it would be nice.

Andy.
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Last edited by Diabolical Artificer; 9th Jul 2020 at 6:44 am.
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Old 9th Jul 2020, 6:48 am   #39
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Default Re: High voltage regulator circuit.

Quote:
Originally Posted by Diabolical Artificer View Post
Thanks Dave. Is Q3/4 a current mirror? i'll try and find time to do some work on this but have a tfmr to rewind by hand today. I've noted all the IP, have a TL341 etc. Re pots across HV, the schematic I posted back in post #1 (re- posted here)uses a pot across 350v, a 470/500k. I've used two of these PSU's in a homebrew tester with no issues. i put two 10M from wiper to each end if I remember right.

Got you David, the Sussex PSU is essentially the same as the attached schematic, but uses a CCS, zener (avalanche) string to get a HV V ref then uses a potential divider instead of the pot. I only briefly tried it, saw how the zeners drifted, moved on.

Andy.
Its only the early Sussex that used a chain of zeners.
"Version Nine, a daughter board (LR8 board) is mounted on main PCB which removes the series Zener Diodes, ie ZD1...ZD7 arrangement and uses a high-voltage, low-output current adjustable linear regulator (U1). to control two Power MOSFETs (TR2 & TR4) for the Anode and Screen grid voltages."
The Schematic is in this post
https://www.vintage-radio.net/forum/...postcount=1477

Peter
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Old 9th Jul 2020, 9:44 am   #40
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Default Re: High voltage regulator circuit.

Thanks Peter for the 'heads up' on the amendment to the Sussex,
I think both Andy and I had only seen the original.

dc
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