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Old 7th February 2012, 04:01 PM   #1
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Default looking for a lineair high voltage regulator .

im looking for a lineair regulator that can supply 100 milliamp at around 350-400 volts . down to 0 volts .

the only requirement is that it has good load regulation . and solid state is a huge pro as tubes take up to much space for my application . and need negative bias to operate .

it seems you can make a good regulator whit a HV mosfet and a feetback loop through an opamp . but i dont have the slightest idea how .

thanks .
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Old 17th April 2012, 02:27 AM   #2
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Quote:
Originally Posted by v4lve lover View Post
im looking for a lineair regulator that can supply 100 milliamp at around 350-400 volts . down to 0 volts .

the only requirement is that it has good load regulation . and solid state is a huge pro as tubes take up to much space for my application . and need negative bias to operate .

it seems you can make a good regulator whit a HV mosfet and a feetback loop through an opamp . but i dont have the slightest idea how .

thanks .
Did you get anywhere with this project?

Being able to turn the voltage all the way down to 0 volts seems extreme. Perhaps it could be done with a rotary switch and some resistor networks for rough ranges.

You don't need a negative supply voltage for a tube-based regulator. You should be able to simply and easily set up a tube with self-bias, especially in a power-supply scenario and not worry about the diff between self- and fixed, since the power supply is all about regulation and stability, not passing audio signals.

I can't remember seeing any extra -ve voltage supplies to run pass-tubes. Voltage is relative.

"100 milliamp at around 350-400 volts " is about what you'd need to run a small 10 watt power amp. But on my test bench, I've rarely needed any 'continuous' variation supplies. Just a few that I can ballpark voltage ranges with via extra loads and shunts.

Once you commit to using a small HV transformer anyway,
you aren't going to save much space skipping a regulator tube. ...
If you want space savings, maybe a switching supply is the way to go. But I always ended up just using a big clunky transformer for the voltage, and bridge and caps for a raw supply, with some shunts to drop a hundred volts or so, into a heatsinked array of high wattage resistors.
Yep. You lose some bench real-estate space if you want a HV supply.
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Old 17th April 2012, 07:47 AM   #3
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ive breadboarded a regulator wich uses a potentiometer wich is protected against supply variations by means of a LT783 current source . EG im using a 100 kilo ohm's 10 turn pot set at 3.5 milliampere it will give a constant voltage of 350 volts accross it. independant from load variations . allthrough if the voltage falls below the minimum 350 volts the regulator won't regulate current very good.

the pot wiper is then connected to a mosfet gate folower wich has a transistor over it as a current limiter . sensing the differential voltage. if it rises over 0,6 volts the gate is cut off . the maximum output current can be set whit different value resistors.


ive breadboarded it works a treat .

edit i have set my unit at about 140 milliampere cut off . to ease dissipation im going to force air cool it whit a fan .
worst case dissipation is estimated at 20-30 watts for a few minutes. ive also put a 100 ohms resistor in series whit the power supply limiting the maximum surge current . i think im going to be all right .



v4lve.

Last edited by v4lve lover; 17th April 2012 at 07:59 AM.
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Old 17th April 2012, 08:58 AM   #4
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This one does what you want: a very good regulator and adjustable with a single resistor value between 0 and 300 or 400V.
I'm revising the PCB to use cheap, regular MOSFETs like the IRF740.

http://www.linearaudio.nl/t-reg-1.htm

jan didden
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Old 18th April 2012, 12:33 AM   #5
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can you use our rectangular cutting cores?
i think the good can solve your problem

regards
Engineer Jay
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Old 18th April 2012, 11:47 AM   #6
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this is what i came up whit sofar .

Click the image to open in full size.

circuit description .

C1,C2 can be omitted provided that you already have a well filtered supply .

the constant current source TL783 sets the pot at a fixed voltage . R1 sets the current source
Eg. when i have a 100 kilohm pot and i set the current 2.5 at milliampere the current source will bias the pot at 250 volt (provided there is a little over 250 volts available to account for dropout voltage , And that the pot is exactly 100K

beware not to bias the pot outside of the regulator's in-output differential Eg 125 volts in-out for the TL783

the mosfet acts as a folower folowing the voltage across the wiper .
R4 sets the maximum current before the NPN transistor starts conducting normally .6V for a silicon unit the transistor then essentially grounds the wiper voltage across the load . limiting current

on a short circuit across the output the current drops to zero.


any thoughts, ideas , suggestions , please don't hesitate to post . I'd like some feetback

edit D1 and D2 are there to protect the transistor from over voltage . you could also add a zener over the transistors emitter but overcurrent protection wouldnt work . i myself use 300 volt mspa 42 transistors . for my 0-300 unit




v4lve.

Last edited by v4lve lover; 18th April 2012 at 12:13 PM.
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Old 18th April 2012, 01:18 PM   #7
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Nice. Q: what's the voltage drop minimum across the TL783 you need for the current source?
That current limiter will work but when it gets active, you can have the full DC input voltage across the TL783...

jan
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Old 18th April 2012, 02:11 PM   #8
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the mimimum voltage drop required is around 5 volts "headroom"

indeed when it shorts the gate to ground on a dead short .it means there is another 100k resistor paralel across the 100k pot to ground effectively becoming a 50k resistive load . wich whit the current source set to 3 milliampere means 150vdc on the pot wiper . from a 350vdc filtered supply that would mean 200 volts in>out to much for the 783

bypassing the regulator whit a 100 volt zener (3w) fixes this problem. under overload the zener turns on. zener current will be 350/50k 0,007A
zener dissipation will be 100*0,007 =,7W

100k potentiometer dissipation will be 250*0.0035 = .875w nothing special for the 2w pots im using .

the reworked version Click the image to open in full size.



tell me what you think.

v4lve.
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Old 20th April 2012, 06:34 AM   #9
nazaroo is offline nazaroo  Canada
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I was wondering what values/changes one would have to make to use such a circuit for say 450, 700 or even 1,200 volts?

say examples of 160-200 mA, and/or 320 mA?
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Old 20th April 2012, 10:01 AM   #10
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maybye it could be done .

for anything higher than 600 volts i suggest using a multi tap transformer and stacking power supplies as im not sure mosfets can stand up to the constant high voltage potential


for a 600volts version you would need a high value potentiometer . something in the range of 500k-1mohm you would then need to work out the current at witch it will bias correctly

i suggest increasing R3 to account for the higher value pot . i would let R3 equal the pot value . as this means when R3 is grounded on a dead short the parallel resistance to ground . halves the total resistance witch will halve the voltage output the current source will try to pushout . now the zener activates dropping 100 volts bypassing the current source .

the gate of the mosfet now sees a very low voltage the 100k resistor the trasistor diode junction and the 100 ohms resistor essentially form a voltage divider the gate is now at the lower end of the divider giving a very low voltage . the mosfet however will try to soak up the rest of the voltage essentially giving a dissipation of Vin *Iset


D1 D2 are high voltage zener diodes. if you use high voltage NPN transistors like Mpsa 42 they protect from transistor destroying overvoltage



lets say your filtered supply is at 620VDC load (maximum 725 unloaded or you will blow the 783 you could use a zener -resistor string to protect the IC from overvoltage(as the maximum and minimum current is known) and the diode protects from undervoltage

you set your 500k pot at 1.2 milliampere giving 600 volts accross it .

your goal is 300 millamperes that would give a worst case dissipation of 250 watts more than one mosfet could possibly handle . and the voltage would shift while they heat up.

for that you probably could best use a ARRAY of 10 mosfets in paralel mounted on a forced air cooled heatsink . the mosfets should be equipted whit current sharing resistors .

you could probably make the current limiter variable . by using a low value high power pot . lets say a 100R 5 watt type . on the 100R setting they would limit the current to about 6 milliampere on the 10 ohm ohms setting the maximum current is 60 milliamperes , you would need to put a resistor of around 2 ohms in series whit it as 2 ohms limits the current to 300 milliamp ( putting the pot on 0 ohms removes the current limit )


V4lve .
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