24vdc to 300vdc adjustable regulated converter

Thread Starter

tubeguy

Joined Nov 3, 2012
1,157
Hello...

I'd like to build a regulated boost convertor for use in vacuum tube projects.
Yes... I said vacuum tubes..:)

Specs:
Input = 24VDC
Output = adjustable 50-300VDC @50ma, regulated within 5% or so

If possible I would like to stick with common components, not special IC's or transformers.

Attached is a circuit idea with calculations.
This circuit appears to be workable, but I need help with regulating it.
Is it possible to use a 555 or op amps for PWM along with some sort of feedback??
 

Attachments

Papabravo

Joined Feb 24, 2006
21,159
It is typically done from an AC line transformer with multiple secondaries. One secondary supplied 6.3 or 12.6 VAC directly to the filaments. Another winding for the plate, aka B+. In some cases there were high voltage batteries for this purpose. The relatively low current requirements mean that smaller diameter wire can be used for those windings. Typically a half wave rectifier and a big filter cap are all that is required. Since there is not much difference between no load and full load further regulation may not be required.
 

crutschow

Joined Mar 14, 2008
34,285
You can use a 555 to generate a PWM signal and regulate the voltage with feedback, but the feedback circuit needs compensation to prevent oscillations of the feedback loop. Much easier to buy a boost switching regulator IC which includes the compensation network design.
 

Thread Starter

tubeguy

Joined Nov 3, 2012
1,157
Hello,

I want the capability of regulated and adjustable voltage from 50 to 300 vdc so I can both test and match audiio tubes at various operating voltages in real world circuits.

Standard 60hz transformers and 100uf @450v capacitors are becoming costly. With those as a starting point an adjustable high voltage regulator circuit would still be required.

I already have some high voltage mosfets and shottky diodes, so this circuit is intriguing because it requires only a small inductor, and high voltage caps of less than 0.1uf
 

Thread Starter

tubeguy

Joined Nov 3, 2012
1,157
I hadn't considered oscillations in the feedback loop.
I did research some boost regulator IC's but they all seemed to require a special HF transformer, not just an inductor. Any suggestions on a particular and easily available IC would be greatly appreciated.

Thanks.
 

Papabravo

Joined Feb 24, 2006
21,159
Designing you own inductors for a DC-DC converter will be more challenging than buying an HV (HF(?)) transformer to work at 60 Hz.. You can stack capacitors in series to up the capacitance and the voltage rating of the string. Think 10, 10 μF @ 50VDC in series to get 100 μF at 500 V.
 

t06afre

Joined May 11, 2009
5,934
If you somehow can handle the power dissipation. You could make a linear power supply using LM723. Since you live in USA this will require a 1:2 transformer from the mains. Anyway using the mains directly is a no-no
 

Papabravo

Joined Feb 24, 2006
21,159
Any tube project requires both filament and plate supplies. Solving one without the other won't get you very far and as has already been pointed out, choosing a set of tubes whose filaments add up to 120VAC for direct connection in series to the mains is a Bozo no-no.

Remember the 50C5?
 

Papabravo

Joined Feb 24, 2006
21,159
Hmmm. I was thinking of capacitors in series. The capacitance does not add, but I think the working voltage does, since each capacitor in a series circuit "sees" half the applied voltage.

Capacitors in parallel add their capacitance and have the same working voltage as the lowest working voltage of any unit in the parallel combination.
 

Thread Starter

tubeguy

Joined Nov 3, 2012
1,157
Hello,

Interesting discussion... Thanks for the input.

I want the capability of regulated and adjustable voltage from 50 to 300 vdc so I can both test and match audiio tubes at various operating voltages in real world circuits.

Standard 60hz transformers and 100uf @450v capacitors are becoming costly. With those as a starting point an adjustable high voltage regulator circuit would still be required.

I already have some high voltage mosfets and shottky diodes, so this circuit is intriguing because it requires only a small inductor, and high voltage caps of less than 0.1uf
I'm looking outside the box here. making an adjustable regulated hi-frequency PWM supply using simple components I already have.

I am well aware of normal linear power supply design, filament supplies etc.
I'm looking for small, and efficient. Just need help with regulation. Need the regulation for close matching of power tubes.

The circuit attached above page 2 is running at 50khz and needs only very small value cap and inductor.

Any help with regulation ideas would be greatly appreciated. Thanks in advance
 

Thread Starter

tubeguy

Joined Nov 3, 2012
1,157
Hmmm. I was thinking of capacitors in series. The capacitance does not add, but I think the working voltage does, since each capacitor in a series circuit "sees" half the applied voltage.
Yes, the capacitor voltage does add in a series string so 10 50-volt caps would equal a 500 volt cap. Its also a good idea to include a string of balancing resistors across the capacitors.
If each capacitor in a series string is equal value then the total capacitance is equal to CapValue/CapQty.
 

Papabravo

Joined Feb 24, 2006
21,159
The OP did ask for a DC-DC converter with 24 VDC input and 50-300V output. I'm not sure if a design meeting that criteria will be forthcoming, but it is hard to argue with the simplicity of the the linear design. Actual performance will require testing of course.
 

Thread Starter

tubeguy

Joined Nov 3, 2012
1,157
I agree, it is hard to argue with the simplicity of linear supplies.
I do need the adjustability. Scott, the circuit you provided is perfect for that with the added benefit of current limiting, and so simple!

I probably don't need the regulation, because a meter will handle the voltage setting and even when matching tubes, they just need to be stabilized at a particular current setting, so no real need for extra regulation.

I'll have to break down and buy a transformer and some caps :):).

First forum I've participated in - lots of fun !!!

(I started in electronics before the Internet, the computer of choice was a
Commodore 64.)

Thanks for the help !
 

Papabravo

Joined Feb 24, 2006
21,159
Glad you enjoyed it.

Mine was Big Iron. An IBM 7090 with Model 729 II 9-Track Magnetic Tape Drives and a 1410 with punch card input.
 

bountyhunter

Joined Sep 7, 2009
2,512
Hello,

I want the capability of regulated and adjustable voltage from 50 to 300 vdc so I can both test and match audiio tubes at various operating voltages in real world circuits.
You could try the LM2577 simple switcher as a boost and see if the free software will kick you out a design. Check TI's website.
 
Last edited:

bountyhunter

Joined Sep 7, 2009
2,512
Designing you own inductors for a DC-DC converter will be more challenging than buying an HV (HF(?)) transformer to work at 60 Hz.. You can stack capacitors in series to up the capacitance and the voltage rating of the string. Think 10, 10 μF @ 50VDC in series to get 100 μF at 500 V.
Nope, that's wrong. Caps stacked in seriews REDUCES the total capacitance, parallel increases. Two 10uF caps in series give you 5 uF.
 

SgtWookie

Joined Jul 17, 2007
22,230
Designing you own inductors for a DC-DC converter will be more challenging than buying an HV (HF(?)) transformer to work at 60 Hz.
For such a low current level, it's not going to be so bad. Our OP could make either a boost or a flyback supply without working too hard.
You can stack capacitors in series to up the capacitance and the voltage rating of the string. Think 10, 10 μF @ 50VDC in series to get 100 μF at 500 V.
Ahh, I'm afraid you got that backwards. Capacitors in series are calculated like resistance or inductance in parallel; 10 10uF, 50v caps would get you 1uF @ 500v, and you'd need to have high-value resistors across them to balance the network.

Tubeguy,
Have a look at Ronald Dekker's great "Flyback Converters for Dummies" page:
http://www.dos4ever.com/flyback/flyback.html
There are both a boost converter and a flyback converter presented. One thing you'd have to do for operation on 24v is to add a regulated 10v to 16v circuit to power the 555 timer, as most of them max out at 16v. The R4/R5/R6 network provide feedback to control the output voltage. You'd need to decrease R4/increase R5 to allow a lower output range. Make sure to increase R6 correspondingly.
 
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