### Author Topic: 0-350V 0.2A Bench Power Supply  (Read 17753 times)

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#### james_s

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##### Re: 0-350V 0.2A Bench Power Supply
« Reply #250 on: December 05, 2017, 10:49:32 am »
If you're worried about the pass device failing, install a meter on the panel connected across the output so you can monitor it. Any HV power supply should be considered live and dangerous regardless what the output is set at. It's like a firearm, always treat any gun as if it is loaded even if you know that it's not. There are other ways for the output to go to maximum uncommanded besides fail shorted of the pass element. A bad pot to set the voltage, a resistor going open, bad connection, someone bumping the knob, foreign object causing a short, this stuff happens.

#### Hero999

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##### Re: 0-350V 0.2A Bench Power Supply
« Reply #251 on: December 05, 2017, 07:55:35 pm »
Quote
The circuit I posted is no more dangerous, than the one you mentioned. In both cases, the pass device(s) can fail short circuit, causing the output to sit at the full voltage.
Not at all, it's a problem of probability: the probability of a tube to fail in short circuit is practically nil.
A tiny ingress of air could cause the valve to arc over internally and the probability of it happening is double because there are two in parallel.

Actually, the first thing I'd do, is use a centre tapped transformer, to give 200V and 400V supplies use another TL431 as a comparator to switch between them, depending on the output voltage  Here's a quick sketch. I haven't done an in depth simulation, hence why I haven't attached the .asc file. It probably needs some modifications to be a practical solution.

Pretty reasonable.

Tweaks like threshold current (TL431 is only accurate above 1mA) come down to merely pushing around resistors.

Or use TLV431, since it's cascoded in both instances and the lower voltage limit is not a hindrance.  Saving a couple mA helps a lot at this voltage.

Tim
I agree, the TLV431 is much more suitable!

If you're worried about the pass device failing, install a meter on the panel connected across the output so you can monitor it. Any HV power supply should be considered live and dangerous regardless what the output is set at. It's like a firearm, always treat any gun as if it is loaded even if you know that it's not. There are other ways for the output to go to maximum uncommanded besides fail shorted of the pass element. A bad pot to set the voltage, a resistor going open, bad connection, someone bumping the knob, foreign object causing a short, this stuff happens.
Exactly.

#### oldway

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##### Re: 0-350V 0.2A Bench Power Supply
« Reply #252 on: December 05, 2017, 08:17:52 pm »
Quote
A tiny ingress of air could cause the valve to arc over internally and the probability of it happening is double because there are two in parallel.
Another wrong answer....!!!! You said you don't know nothing about vacuum tubes and you proove it... Tubes have a getter to eliminate tiny ingress of air....Breakdown voltage in air is 3KV/mm.....distances between anode and cathode (and their connections) is far more than a mm !

Tubes like P(E)L500, PL504, PL509, PL519 works normally with 6KV peak voltage without any damage....
807 tubes can withstand several KV's.....
EL34 is used in public address audio amplifiers with 700V anode voltage in push pull schematic, that means it withstand an anode / cathode voltage of at least 1400V.....No match at all with semi-conductors !

About safety, primary protection is to prevent dangerous events, not only signalizing it.

Signalizing a danger is only a secundary safety, not the principal one.

Further more, for the same output current and same max output voltage, you must use more Mosfets in parallel than tubes, the risks are also greater for this reason....
« Last Edit: December 05, 2017, 08:36:36 pm by oldway »

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#### T3sl4co1l

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##### Re: 0-350V 0.2A Bench Power Supply
« Reply #253 on: December 05, 2017, 08:36:31 pm »
It used to be that vacuum tubes such as the humble EL509 held one final advantage over semiconductors: performance at high voltages.

Parts like this approach it:
https://media.digikey.com/pdf/Data%20Sheets/IXYS%20PDFs/IXTx02N250(S).pdf
2500V 200mA, 450 ohms.  An EL509 has closer to 50 ohms on-resistance, and around 7kV peak plate voltage handling.

But since then, parts like this have been introduced:
http://ixapps.ixys.com/DataSheet/DS100458B(IXTL2N450).pdf
at a price competitive with a lone EL509 even, let alone including heater and screen supply.

For still-more-specialty applications, there are SiC MOSFETs up to 10kV (though not generally available anywhere, AFAIK), which are significantly better than hard modulator tubes.

Hmm, they're probably not generally available because pulse generators are "munitions".  I suppose because of radar and detonator applications?  Who knows.

Tim
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#### oldway

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##### Re: 0-350V 0.2A Bench Power Supply
« Reply #254 on: December 05, 2017, 08:43:32 pm »
Using a pentode with (relatively !) low voltage G2 is Intrinsically safe because it limits the short circuit current without need of additional components.

If you want to use semiconductors, you must go to the SMPS technology, not the linear technology with pass Mosfet .....
« Last Edit: December 05, 2017, 09:01:58 pm by oldway »

#### james_s

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##### Re: 0-350V 0.2A Bench Power Supply
« Reply #255 on: December 06, 2017, 07:27:32 am »
The getter will absorb tiny quantities of residual gas but it will quickly be overwhelmed by any sort of leak. I've certainly seen gassy tubes that arced internally, happened to one of the output tubes in my friend's guitar amp several years ago. I've also seen it in a CRT once, a microscopic leak somewhere allowed enough air in that the neck glowed bright purple when it was energized.

I would agree that a tube is less likely to fail shorted than a semiconductor but IMHO this is a moot point.

#### oldway

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##### Re: 0-350V 0.2A Bench Power Supply
« Reply #256 on: December 06, 2017, 07:57:56 am »
It is extremely rare.....

it's a problem of probability: the probability of a tube to fail in short circuit or arcing is practically nil.

Guitars amplifiers are often abused, transported in trucks without any care, sometimes they even drops them, so it may happen that the glue of the octal socket comes off and there are breaks in the glass at the socket connections which can cause an air inlet.

This can not happen with a bench power supply that is not subject to this kind of mechanical stress.
« Last Edit: December 06, 2017, 08:02:00 am by oldway »

#### T3sl4co1l

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##### Re: 0-350V 0.2A Bench Power Supply
« Reply #257 on: December 06, 2017, 10:53:15 am »
it's a problem of probability: <snip>

Guitars amplifiers are often ... <snip>

"It's not a problem. Except when it is."

Nice conditional probability there.  I give it Bayes out of 10.

Tim
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#### Ian.M

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##### Re: 0-350V 0.2A Bench Power Supply
« Reply #258 on: December 06, 2017, 01:59:39 pm »
Anyone who thinks the probability of failure of two tubes from the same batch, run in parallel, in adjacent sockets isn't strongly correlated is a fool.

Gassy CRTs are *RARE*.   It requires enough of a leak to let sufficient air in for the tube to support ionised conduction, without there being enough oxygen present for the filament to fail.  In a lifetime in the service trade you might see a few cases, but mostly a leaky tube will be very obvious - flat milky white getter and a blown filament.

If you want to 'gold plate' what was originally intended to be a cheap but usable HV bench supply,  use a dual gang pot and add a crowbar circuit for output overvoltage >10%, and if it uses tubes, loss of negative grid bias rail, or excessive G2 current.

If multiple tubes are used, to prevent overloading, their filaments should be in series so they all cut out if one fails.

#### floobydust

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##### Re: 0-350V 0.2A Bench Power Supply
« Reply #259 on: December 06, 2017, 04:41:33 pm »
Tube vs Solid-state reliability, each has their own aging and failure mechanisms.
Should we go biplane or helicopter?

If I needed something built in an hour, this is a junkbox tube design.
If I'm designing this professionally, it's SS with much greater effort, cost and risk.

There is no guidance on how much to derate low-cost switching MOSFET's SOA.
Using a bunch in linear mode, subject to thermal instability due to hot spots, with no DC SOA spec. is like walking into the casino.
Getting around this SS issue is expensive - or use tubes.

We need a "burning HV power supply" contest, with a scope giveaway and DMM's for the runners up.
If community members could place wagers on which design will win, my kind of casino

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#### oldway

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##### Re: 0-350V 0.2A Bench Power Supply
« Reply #260 on: December 06, 2017, 10:37:09 pm »
I found a bunch of Ls50 tubes. Looks good, isn't it?
How to add transistor driver to it?

I heared what some kepco in 70' use some sort of transistor driver and current limiter with two stage pass tubes (one is for preregulator and the second is for stabiliser)
Nothing against the LS50 tube, it is a 40W transmitting pentode, it can be used as pass device without any problem. Heating voltage is 12.6V.
But the socket seems more expensive than the tube itself.

No way to find schematics or service manual of Kepco products  as they are copyrighted.
##### Re: 0-350V 0.2A Bench Power Supply
« Reply #268 on: December 12, 2017, 12:14:46 pm »

How to transfer years of experience and knowledge about tubes to beginners in the field, who, at most, have made a single project with tubes in their life?

Here's a good start:
http://www.john-a-harper.com/tubes201/
« Last Edit: December 12, 2017, 12:25:04 pm by Alex Eisenhut »

#### floobydust

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##### Re: 0-350V 0.2A Bench Power Supply
« Reply #269 on: December 12, 2017, 12:39:00 pm »
You would put the pro's and con's of the part/architecture, weighing things like cost, size, reliability, complexity etc. into  a 'decision matrix' to choose.
The forum (format) can't accomplish this- the requirements are poorly defined and the endless debate continues...

We're up against the stigma of an old technology where people don't know why it's "old"- it is to be scorned. Although it's solid and proven to work.
We're up against the marketing hype of new technology where people don't know the datasheets are full of deceptive bullshit. It's guess work as far as reliability. SPICE SOA models are unfortunately not there yet.

#### oldway

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##### Re: 0-350V 0.2A Bench Power Supply
« Reply #270 on: December 13, 2017, 10:05:22 pm »
I continue to look for high voltage power supplies and technical solutions adopted on commercial gears.

I found Kikusui HV power supplies 350V 3.5A and 600V 2A.
http://www.kikusui.co.jp/en/product/detail.php?IdFamily=0003

How do they work?
https://www.kikusui.co.jp/common/product/pdf/pan-a.pdf
http://www.kikusui.co.jp/kiku_manuals/P/PAN_A_VE3_E6.pdf

First, they are solid state linear power supplies that use a phase-controlled pre-regulator, which reduces the risks since in normal operation, the voltage difference on the pass transistors is low.
In case of short circuit of the pass elements, overvoltage will be reduced to only a tenth of volts over the adjusted output voltage.
The phase control is made by MOSFET's, not by SCR's.

On the other hand, Kikusui does not use Mosfets as pass devices, but bipolar transistors.

« Last Edit: December 13, 2017, 10:07:20 pm by oldway »

#### floobydust

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##### Re: 0-350V 0.2A Bench Power Supply
« Reply #271 on: December 14, 2017, 05:43:12 am »
The Kikusui HV power supplies look interesting, 50usec response time.
I did chuckle at their weight though: 350V 3.5A  (2,100VA in) 79lbs/36kg. I'd need a forklift
That, and using a universal manual for the 16V-600V products... "Connect an electrolytic capacitor (C) with a capacity of a few thousands of uF to a few tens of thousands of uF across the load terminals."
Any idea on cost?

I' ve used Xantrex HV power supplies, now under the Ametek/Sorensen brand. Inside they have SMPS and use ZVS, modern designs.

The XG 850W 600V 1.4A is USD $1,600, about$2/watt.

#### oldway

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##### Re: 0-350V 0.2A Bench Power Supply
« Reply #272 on: December 14, 2017, 07:38:28 am »
My interest in talking about kikusui power supplies was to see which technology was used in a linear solid state commercial HV power supply.

This does not change my point of view that, for a hobby application and home-made power supply, the vacuum tube solution is the simplest and safest.

However, I note that those who advocated to use a solid state solution neglected 2 important points: the use of a pre-regulator to avoid semiconductors to operate  under a high voltage difference, and secondly, the choice of bipolar transistors instead of Mosfets.

As for the need for additional protections, like the adjustable crowbar, it was actually mentioned.

#### eliocor

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##### Re: 0-350V 0.2A Bench Power Supply
« Reply #273 on: December 14, 2017, 09:13:23 am »
I'm asking for some schematics suggestions.
My minimum requirement are the following:
- 0-350 Vdc (higher can be better)
- 0-30 mA (my MAXIMUM needs)
- regulation: CV/CC
- low ripple
- good stability

At this date for my experiments I'm using a Tennelec TC-952 power supply (0-3000Vdc, 10mA, 2mV ripple typ.) but it has no current regulation.
I also own a Fluke 415B (0-3000Vdc, 30mA, even more precise and with lower ripple) but it also has no current regulation; BTW this one uses a valve/tube, just to satisfy people who love them!

What I'm asking is for some good suggestions on a schematics which can satisfy my requirements: output voltage can even be higher than 350V but current limits will never be higher than 30mA.
Otherwise any hint on a not too expensive instrument (used or new) is welcome.

P.S.: as you can see, from the output voltage of my power supplies, I already have some experience working with high voltage...

#### Hero999

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##### Re: 0-350V 0.2A Bench Power Supply
« Reply #274 on: December 14, 2017, 08:08:42 pm »
My interest in talking about kikusui power supplies was to see which technology was used in a linear solid state commercial HV power supply.

This does not change my point of view that, for a hobby application and home-made power supply, the vacuum tube solution is the simplest and safest.
I'd choose a switched mode power supply myself. It's not that difficult nowadays, even for hobbyists, with the vast range of controllers and high performance MOSFETs available.

Quote
However, I note that those who advocated to use a solid state solution neglected 2 important points: the use of a pre-regulator to avoid semiconductors to operate  under a high voltage difference, and secondly, the choice of bipolar transistors instead of Mosfets.
I posted a design, using a tap changer to halve the voltage across the pass device. It might not be as good as a proper pre-regulator, but it's a lot simpler.
http://www.eevblog.com/forum/projects/looking-for-yours-opinions!-hv-stabilized-power-supply/msg1365710/#msg1365710
And regarding BJTs, rather than MOSFETs: I think BJTs were ruled out awhile ago, because at high collector-emitter voltages, the safe operating area, limits the maximum current to too lower level. Plenty of MOSFETs are available, which can dissipate >400V, at >200mA.
« Last Edit: December 14, 2017, 10:35:11 pm by Hero999 »

Smf