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 Post subject: Voltage regulator
PostPosted: May Mon 28, 2018 10:00 pm 
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I am needing a voltage regulator that can handle an input of around -2KV and regulate it to -1.6KV.

B- current is 1mA.

This is to be used with an oscilloscope clock I put together as a change in B- voltage affects the image size which is undesirable.

I thought of a string of zener diodes, but that would require several diodes and I don't know how much current they will draw nor do I know what current the HV winding is rated at so I'm reluctant to go that route unless it is the only way to do it.

Fixed voltage is fine, but variable would work as well or a way to make the fixed one regulate at a different fixed voltage with a component change.

I can get this zener, but would require 10 of them and they have a 5% tolerance which means the voltage can vary anywhere between -1.520KV through -1.680KV. That was figured on each zener having a +/- voltage tolerance up to 8 volts 8 volts and 10 in series would get a +/- voltage tolerance up to 80 volts.

https://www.mouser.com/ProductDetail/ON ... ntETaf8%3d


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 Post subject: Re: Voltage regulator
PostPosted: May Mon 28, 2018 10:42 pm 
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Tube-type color TVs used a 6BK4 as HV regulator. I don't have any circuit ideas.

Rich


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 Post subject: Re: Voltage regulator
PostPosted: May Mon 28, 2018 11:33 pm 
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You can't beat the zener diodes for cost!

The ones you picked have their ratings specified at 8 mA. They get considerably worse at lower current. Perhaps you should pick a lower power device so that it is operating at a current closer to where it performs best.

This assumes that the 1 mA you referred to is mostly going through the voltage divider network and only a small part through the CRT. If so, you can eliminate most of the divider and just take off the voltages you need directly from the zener string. That should considerably reduce the overall current draw.

The tolerances you mentioned don't involve voltage changes as a result of line voltage or load current changes. They are the variations you get from one device to another a fixed current of 8 mA. That is probably irrelevant to your application. What you need to be concerned about is the zener impedance rating which tells you how much the voltage can change as the current changes.

Don't forget to heed the heat sinking specifications.

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 Post subject: Re: Voltage regulator
PostPosted: May Tue 29, 2018 12:38 am 
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Echoing Jim - I think in this case voltage accuracy is less important than stability. Does it really matter if the B- is 1520V or 1680V, as long as it's stable?

I'd disagree a bit with Jim that "They get considerably worse at lower current" - from the datasheet, the I/V curve for the 1N5384 is pretty damn flat & appears to be within +- a volt or two from 0.1mA to 10mA. Better than most zeners, in fact. Wouldn't imagine that's too much of a problem.

But if you really want accuracy, the usual way @ high voltages is to design the supply with proper feedback regulation - either a feedback winding on the step-up transformer, or a resistive divider on the output, feeding back to control the V or duty cycle of the low-voltage side.

If EHT is from a straight transformer or multiplier, I might consider something like a Maida/Natsemi type 'floating' regulator (as described here; a somewhat updated version can be found in various places e.g. here)). It'd have to be adapted to suit, though - I'd suggest a 600-800V MOSFET for the pre-regulator, and a high-voltage regulator (e.g. TL783) in place of the LM317H. And depending on how/if the EHT B- is related to the rest of the supply, you might have to convert to the negative versions of each.


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 Post subject: Re: Voltage regulator
PostPosted: May Tue 29, 2018 12:50 am 
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Here's the power supply for the CRT.

Attachment:
power supply.png
power supply.png [ 3.83 KiB | Viewed 674 times ]


Just realized that measuring the voltage across the 470K resistor to derive the current might not give a proper reading given the filter cap after the 470K resistor.

I know that about zeners, but was just stating that I could have a final B- voltage at anywhere between those two voltages based on device tolerances which is why I would prefer not to use the zeners plus I doubt the HV winding or voltage doubler could handle the necessary minimum zener current.

Perhaps I could do some sort of oscillator such as what is done in most nixie tube clocks to generate the HV.

I did use four 20 volt zeners across the intensity pot for an 80 volt drop and they seem to work ok with the voltage across the diodes being 79.4Vdc so maybe I could do the 10 160 volt zeners. Without the zeners the voltage across the 100K pot is -91.9Vdc. So that is a B- current of 922uA.


Last edited by Tube Radio on May Tue 29, 2018 1:13 am, edited 1 time in total.

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 Post subject: Re: Voltage regulator
PostPosted: May Tue 29, 2018 1:11 am 
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So, what feeds that? If it can be completely isolated from the rest of the supply (e.g. separate winding) until after a regulator section it's a lot easier.

What are the actual focus & intensity voltages needed? It's unclear if you want to feed that doubler with 1.6kV, or expect to get 1.6kV out to feed the focus/intensity supplies.

Going by the datasheet, you really only need 0.1-0.2mA zener current to regulate to spec. If you can't spare that, you might need to completely rethink things.

It'll be even easier to make suggestions if you show us the whole thing, from power in to CRT...


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 Post subject: Re: Voltage regulator
PostPosted: May Tue 29, 2018 1:20 am 
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I can spare the 100-200uA of current I'm sure.

The output of the doubler is 2KV.

Attachment:
CRT.png
CRT.png [ 12.95 KiB | Viewed 669 times ]


Looks like it is going to be as simple as using 10 160 volt zeners in series and reducing the value of the 470K resistor.


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 Post subject: Re: Voltage regulator
PostPosted: May Tue 29, 2018 1:42 am 
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There are high voltage gas regulator tubes. Believe numbers in 6xxx.

Here is a Russian tube:

https://www.ebay.com/itm/SG302S-302-vol ... 4b228cb9f5

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 Post subject: Re: Voltage regulator
PostPosted: May Tue 29, 2018 1:47 am 
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Location: Australia
O...K...

I'll put it this way: I have a number of practical and safety issues with the circuit as drawn and, considering the voltages & possible currents involved, I wouldn't feel happy going any further into those issues without face-to-face discussion. So I'll bow out and leave others to assist you further.

Quote:
Looks like it is going to be as simple as using 10 160 volt zeners in series and reducing the value of the 470K resistor.

I will however say that this is a fair start.


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 Post subject: Re: Voltage regulator
PostPosted: May Tue 29, 2018 2:00 am 
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Agreed the currents and voltages involved are quite dangerous and the circuit should only be built by those experienced with HV.

If you don't mind me asking what are the practical and safety issues?

Rod, thanks for what help you provided.

Norm, I like the gas regulator tube.

EDIT:

I'll try the 10 160 volt zeners.

I've got another 470K resistor I can parallel with the current one and see if 235K is low enough to keep the zeners operating over a B- voltage variation with varying the voltage +/- 5 volts from 117Vac using a variac.

If it is too low then I'll use a higher value resistor.

I also have two other CRTs this circuit is good for each with a little higher B- voltages so I might order a couple extra 160 volt zeners so I can see what works best for those CRTs then update the relevant schematics for those CRTs.

This is part of a project I'm doing to help the designer of the circuit get it working good for 5" CRTs while improving the design where I see deficiencies. As I get the opportunity I'm testing with different 5" CRTs and modding the circuit to work best with the particular CRT used. So far it's been a 5DEP1 (the one used in the scope clock I have), 5ADP1 and 5BP4.


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 Post subject: Re: Voltage regulator
PostPosted: May Tue 29, 2018 5:39 pm 
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What's the purpose of the NE-2 bulb in the negative supply? it maintains a more or less constant voltage across itself so any change in supply voltage gets passed straight through to the CRT divider. If tit were replaced by a resistor the change would be reduced by the voltage divider ratio.

I assume the other NE-2 is a pilot light. Otherwise I see no purpose for it.

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 Post subject: Re: Voltage regulator
PostPosted: May Tue 29, 2018 7:41 pm 
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The NE-2 in the - supply serves three purposes.

1. Pilot light.
2. Indicates the presence of B-
3. Puts the focus control more at the center of its range for proper focus.

The other NE-2 serves to indicate that B+ is present.

They both are of course optional.

The other NE-2 originally wasn't there in an earlier revision, but the B+ voltage took a good bit of time to decrease to a lower voltage after removing power that I figured an indicator would be best for if I was doing something with the circuit I'd know if B+ was still high enough to shock me. a later revision has B+ dropping a bit quicker so the light is technically not needed, but given it is already mounted to the front panel I just left it there.


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 Post subject: Re: Voltage regulator
PostPosted: Jun Sat 02, 2018 7:59 am 
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The ten 160 volt zeners did the trick.

If my HV probe is right I'm at 1.552KV and it will hold to that voltage within a volt or two from 110-120Vac as shown on the variac dial. Also the zeners don't get the least bit warm. Thought I'd have to reduce the value of the 470K resistor, but it is ok as is.

Now here's where I should have ordered an extra 160 volt zener or two so I could test the regulator with the other two CRTs.

To see how close the probe is I'll measure the voltage across the intensity control, focus control and neon with its 1 meg series resistor using the regular test leads then add them up.

B- is 1.561KV which is well within tolerance. I measured across each zener and added the voltages together as the meter was possibly loading down the circuit measuring across the other stuff.

I may lower the 470K resistor to 235K so there's more voltage to work with so the regulation is good.


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