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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 29th Sep 2019, 11:55 am   #21
turretslug
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Default Re: HV regulation.

A small output decoupler ( a few uF) to eliminate tendency to HF "springiness" on the output of the Darlington?
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Old 29th Sep 2019, 5:03 pm   #22
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Default Re: HV regulation.

Thanks both, I'll try that and report back.

Andy.
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Old 29th Sep 2019, 9:17 pm   #23
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Default Re: HV regulation.

An idea I've only seen applied to KV regulation ( as used on submarine cable PSU) is to use saturation of the core via a second winding to keep the transformer output volts stabilised. The two systems I have in mind fed circa 500mA at a few KV regulated on cables carrying approx five 12 channel groups at 3khz spacing with other circuits. Th idea is that as output volts from the transformer rise, the saturation current is increased to make the transformer less efficient and thus reduce the output volts.
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Old 3rd Oct 2019, 1:43 pm   #24
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Default Re: HV regulation.

I think that the capacitive multiplier circuit shown in post #19 has the potential to become a pulse generator under certain conditions.

If you look at the circuits on Rod Elliott's website you will see that he has placed a resistor between the base of the driver transistor and ground.

This creates a voltage divider which gives more control over the differential voltage between input and output.

His circuits seem to be designed for much lower voltages so you will have to modify the design to suit your application.
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Old 4th Oct 2019, 7:02 am   #25
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Default Re: HV regulation.

Thanks. This is on the back burner for the moment, but I'll give it a try when I drop back on it.

Andy.
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Old 4th Oct 2019, 7:12 am   #26
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Default Re: HV regulation.

Quote:
Originally Posted by Oldcodger View Post
An idea I've only seen applied to KV regulation ( as used on submarine cable PSU) is to use saturation of the core via a second winding to keep the transformer output volts stabilised. The two systems I have in mind fed circa 500mA at a few KV regulated on cables carrying approx five 12 channel groups at 3khz spacing with other circuits. Th idea is that as output volts from the transformer rise, the saturation current is increased to make the transformer less efficient and thus reduce the output volts.
It's called a magnetic amplifier.

Careful design of the core and where windings are located is used to inimise coupling of the AC power into the DC control winding.

Saturable cores are also used to make variable high power inductors. Resonated ith a capacitor it can be tuned onto or away from resonance at the operating frequency to regulate an output.

These can be used to individually regulate each output of a multi-output switch-mode supply. The variable inductors go under the name 'transductors'

David
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Old 20th Oct 2019, 1:46 pm   #27
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Default Re: HV regulation.

Finally got round round to building a HV PSU using valves this weekend, see att for schematic. It worked straight off without any mucking about which surprised me. I used what valves I had to hand. The 12E1 is overkill for a 300v 40mA ish supply but I wanted a beefy octal valve, anyroad it works.

Performance isn't too bad, 20mA ripple, which could be further filtered prior to any high gain stage as is normal. Another positive is that it's slow powering up, negatives that you can't bung loads of capacitance on it to surpress ripple.

Some questions, re the two pentodes/b tets G2 max, 100v for the 6BR7, 300v for the 12E1. I presume this is 100v with regard to cathode? That's be 60v ish for the both at present. Are there any things I've overlooked or any mistakes?

Andy.
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Old 24th Oct 2019, 8:12 am   #28
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Default Re: HV regulation.

To wrap this up after building various circuits and mucking about I found the best solution was an CLC filter, EG 100u after the diodes, L then 820u = ripple of less than 1mV P-P. No idea what that would mean in terms of PSU Z.

Out of the various circuits I tried the cap multiplier was disappointing and produced some odd results especially the mosfet version which produced massive ripple with 2nd H.

So though not regulated I think it's the best fit for the job in hand, space etc. Still, it's been an interesting exercise.

Thanks to Molly foor typing this out, Andy.
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Old 24th Oct 2019, 8:56 am   #29
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Default Re: HV regulation.

There is something to be said for the CFE* approach - capacitors are quite cheap these days, it's very simple so there's little to go wrong and you don't need all sorts of exotica like multiple floating heater supplies for different valves etc. One thing to bear in mind though is that 820uF @ 300V stores 37 joules which will give you a pretty nasty belt if you should touch it. It may also take a long time to bleed down after you've switched off.

With a capacitor the psu impedance depends on frequency. If the capacitor doesn't have a large ESR then it will be very low, except at deep bass frequencies, and even down there it can be pretty low. You can estimate it from the formula for the reactance of the last capacitor i.e. 1/(2 x pi x freq x C). So at 20Hz your 820uF would have a reactance of just under 10 ohms (or at least the dimension of ohms - if you draw current at 20Hz you will get a ripple voltage of 10 times the current but, unlike a resistive source, with a capacitative one the voltage and current will be 90 degrees out of phase).

Cheers,

GJ

*Informal military term for Capacitor, Flippin' Enormous (I paraphrase slightly ).
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Old 25th Oct 2019, 8:21 am   #30
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Default Re: HV regulation.

"There is something to be said for the CFE* approach" Indeed, I like the simplicity. I always put a bleeder resistor across big caps, it produces a nice blue arc when discharging with my cap discharger.

Thanks for explaining the Z, I thought it was more complicated than that, having to take into account the resistance of the tfmr, L etc.

I've put two VR150/30's as a simple shunt reg to clamp the HT whilst the heaters warm up. Not getting 300v, more like 330v but have to play with the limiting resistor - 10k at present. I'd forgotten how nice these look lighted up, a nice blue ish glow, not orange like the smaller 85A2's.

Andy.
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Old 27th Oct 2019, 9:23 am   #31
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Default Re: HV regulation.

Having issues with the shunt regulator, it's not working as it should. I'm using two VR150/30's in series, so should get about 300v +/- 10v right? I'm getting 340v only 20v ish difference from HT loaded - 356v.

I'll own to some confusion about how to work out the dropping resistor value, I thought it was load current + a bit for the reg valves, which is 5mA - 40mA each. So by that reckoning 300/0.030 = 10k, that's 5mA each valve 20mA load I. Tried this, VR valves struck, glowed, voltage out 340v ish. So tried 5k, same, at one point tried 33k and also tried two R's, one to the bottom VR, one on top, same result.

If in doubt read the manual. I have an STC application report which covers the subject so had a read. It says your basic formula to determine Rs = Vi-Vs/Is+IL ... Vi is Vin, Vs is stabilised voltage, Is is VR current and IL is load current. Did the maths, got 1k6 as the result, used two 3K3's, same %*&!? result - 340v.

Another issue I had was the VR's valves flickering after the thing had been powered for about 10mins but I think this was down to dodgy resistors failing in my circuit.

I remember when I first got these valves, I tried powering them and they worked. I also tried two other VR150's, same result.

Andy.
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Old 29th Oct 2019, 9:35 am   #32
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Default Re: HV regulation.

Time to put this one to bed... After more experimentation I've found getting the dropper resistor that limits VR150 current right takes some doing, but have started to get the hang of it. There's a sweet spot to ensure proper regulation and supplying enough current to the circuit powered or load. Another aspect is supplying enough current to the VR valves so that they glow a nice deep blue, but this obviously effects the final regulated voltage, raising it.

In the end I'm not too bothered about the final regulated voltage in this application ( 10-20v either way makes no real difference to the final gain of the preamp) , the VR valves are really only there to clamp the HT whilst the heaters warm, so that Vmax of the smoothing caps isn't exceeded, and the blue glow is a bonus.

Andy.
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