Need input on high voltage regulator design

[Scott Miller]

I got bored yesterday and started working again on a Geiger counter design I
came up with a couple of years ago. The high voltage power supply was
mostly cribbed from a couple of other designs I found, since I'm a digital
guy and high voltage isn't my thing. As it stands now, the microcontroller
puts out a square wave that drives the primary of a transformer via a
MOSFET. A voltage doubler on the secondary gets the voltage into the
required range (500+ volts in this case), and a couple of varistors in
series form a shunt regulator.

It's the shunt regulator I'm looking to replace right now. Since the input
to the inverter is provided by the MCU's timer channel, it can be easily
pulse-width modulated. I just need to get some feedback from the HV side so
the MCU knows how to adjust the duty cycle. My first thought was to use a
pair of resistors to form a voltage divider and feed maybe 1/120 of the
output voltage into the MCU's A/D converter. But with any reasonable
resistor values, there's too much load on the supply to maintain the needed
voltage. Higher values might work, but the thick film resistors are
expensive and I'm not sure how much current the ADC needs to function
correctly.

Any suggestions? I'm thinking an op amp might help, with its high input
impedance. That still means using expensive thick film resistors, though.
And again, I'm a digital guy and I'm a bit out of my element here.

I'm really trying to keep the cost and component count down to make it an
easy hobby project, but I'd also like it to not suck a battery dry in 2
hours. If anyone's got any suggestions on how to implement a better
regulator, or maybe a different supply design entirely, let me know.

Current schematic is at: http://n1vg.net/geiger/images/gc1-9v.png. The HV
supply is on the right-hand side - everything from MCU pin 13 to the
Geiger-Mueller tube. The pin header at left is for an LCD display.

Thanks,

Scott

 

[Active8]

On Sun, 28 Nov 2004 12:33:30 -0800, Scott Miller wrote:

I don't know how tight your voltage spec is, but would a separate
sense winding do it?

 

[Joerg]

Hi Scott,

This is frequently done with a Texas TLV431 or similar from other
sources such as National. Cheap, small, available at almost every street
corner but best of all, well under a uA on the ref pin. You wouldn't
really need the ADC here.

As for resistors, it is customary to connect some in series in HV
applications. It would be hard to obtain anything below 5% when you are
above the 10M region.

Regards, Joerg

 

[Pooh Bear]

I got bored yesterday and started working again on a Geiger counter design I
came up with a couple of years ago. The high voltage power supply was
mostly cribbed from a couple of other designs I found, since I'm a digital
guy and high voltage isn't my thing. As it stands now, the microcontroller
puts out a square wave that drives the primary of a transformer via a
MOSFET. A voltage doubler on the secondary gets the voltage into the
required range (500+ volts in this case), and a couple of varistors in
series form a shunt regulator.

It's the shunt regulator I'm looking to replace right now. Since the input
to the inverter is provided by the MCU's timer channel, it can be easily
pulse-width modulated. I just need to get some feedback from the HV side so
the MCU knows how to adjust the duty cycle. My first thought was to use a
pair of resistors to form a voltage divider and feed maybe 1/120 of the
output voltage into the MCU's A/D converter. But with any reasonable
resistor values, there's too much load on the supply to maintain the needed
voltage. Higher values might work, but the thick film resistors are
expensive and I'm not sure how much current the ADC needs to function
correctly.

Any suggestions? I'm thinking an op amp might help, with its high input
impedance. That still means using expensive thick film resistors, though.
And again, I'm a digital guy and I'm a bit out of my element here.

I'm really trying to keep the cost and component count down to make it an
easy hobby project, but I'd also like it to not suck a battery dry in 2
hours. If anyone's got any suggestions on how to implement a better
regulator, or maybe a different supply design entirely, let me know.

Current schematic is at: http://n1vg.net/geiger/images/gc1-9v.png. The HV
supply is on the right-hand side - everything from MCU pin 13 to the
Geiger-Mueller tube. The pin header at left is for an LCD display.

Any PSU using a *shunt* regulator is hopelessly inefficent.

What regulated voltage do you need ?

Using 'spare cycles' on an MCU is a truly lousy way to do it btw. Far better a
dedicated controller chip.


Graham

 

[Joerg]

Hi Graham,

Any PSU using a *shunt* regulator is hopelessly inefficent.

Yes. Should be a nice PWM control instead. Of course, when it is cold a
shunt regulator does have its merits ;-)

Using 'spare cycles' on an MCU is a truly lousy way to do it btw. Far better a
dedicated controller chip.

Why is that lousy? If the MCU has spare horse power why spend the money
on a PWM chip? Of course, to be safe this would have to run reliably.
Within its own interrupt routine or something like that.

Regards, Joerg

 

[Jonathan Kirwan]

I got bored yesterday and started working again on a Geiger counter design I
came up with a couple of years ago. The high voltage power supply was
mostly cribbed from a couple of other designs I found, since I'm a digital
guy and high voltage isn't my thing. As it stands now, the microcontroller
puts out a square wave that drives the primary of a transformer via a
MOSFET. A voltage doubler on the secondary gets the voltage into the
required range (500+ volts in this case), and a couple of varistors in
series form a shunt regulator.

On a Geiger counter I built many years ago from scratch, I used the recommended
idea of a string of series NE-2s. Worked great. I still have that Geiger
counter and, aside from replacing the tube once, it's still working just fine!

Jon

 

[Fred Bloggs]

I got bored yesterday and started working again on a Geiger counter design I
came up with a couple of years ago. The high voltage power supply was
mostly cribbed from a couple of other designs I found, since I'm a digital
guy and high voltage isn't my thing. As it stands now, the microcontroller
puts out a square wave that drives the primary of a transformer via a
MOSFET. A voltage doubler on the secondary gets the voltage into the
required range (500+ volts in this case), and a couple of varistors in
series form a shunt regulator.

It's the shunt regulator I'm looking to replace right now. Since the input
to the inverter is provided by the MCU's timer channel, it can be easily
pulse-width modulated. I just need to get some feedback from the HV side so
the MCU knows how to adjust the duty cycle. My first thought was to use a
pair of resistors to form a voltage divider and feed maybe 1/120 of the
output voltage into the MCU's A/D converter. But with any reasonable
resistor values, there's too much load on the supply to maintain the needed
voltage. Higher values might work, but the thick film resistors are
expensive and I'm not sure how much current the ADC needs to function
correctly.

Any suggestions? I'm thinking an op amp might help, with its high input
impedance. That still means using expensive thick film resistors, though.
And again, I'm a digital guy and I'm a bit out of my element here.

I'm really trying to keep the cost and component count down to make it an
easy hobby project, but I'd also like it to not suck a battery dry in 2
hours. If anyone's got any suggestions on how to implement a better
regulator, or maybe a different supply design entirely, let me know.

Current schematic is at: http://n1vg.net/geiger/images/gc1-9v.png. The HV
supply is on the right-hand side - everything from MCU pin 13 to the
Geiger-Mueller tube. The pin header at left is for an LCD display.

Thanks,

Scott

You could do something like shown below with a JFET input OA. This
circuit uses a resistor string to convert the HV into a current, 10uA at
HV=500V, and then an operational amplifier current-to-voltage converter
to produce the A/D sample. The output impedance of the circuit is 10K
which most embedded A/D can handle. If you need less impedance then go
to a TL082 and use the other OA as a buffer. The output clamps at 5V
until the input exceeds approximately HV=400V, and for voltages greater
than this the output subtracts ~25mV from 5V per volt of HV input in
excess of 400V- landing you somewhere around Vout=2.5V for HV=500V.


View in a fixed-width font such as Courier.

5V
|
+------+-------------+
| | 9V |
/ / | /
100K 100K | 10K
/ / | /
\ \ | \
| |TL081| |
10M 10M 10M 10M 10M | | |\| |
HV IN >-/\/\-/\/\-/\/\-/\/\-/\/\-+----------|-\ |
| | | >-|<|--+--> V
5x 1/4 Watt | +---|+/ | out
| |/| |
| 1.2M | |
+----/\/\----|-------+
| |
| 620K |
+----/\/\----+
|
|
---

V
out
|
V = 5V for HV < 400V 5V+---------
out | \
| \
(HV-400) 2.5| - - - - - -\
Vout= 5V- -------- x 1.2Meg | |\
50Meg | \
+--------+---+-----------
500V
400V

HV-->

 

[Scott Miller]

I'm really not sure. There's still a doubler after the secondary winding
that's going to introduce some loss. The bigger problem is finding the
right transformer. Winding a custom one isn't an option. I'm currently
using a $3 CCFL driver transformer - it's compact and works pretty well.

Scott

 

[Scott Miller]

How would you set one of these up in a high voltage circuit? And wouldn't
any shunt regulator still be pretty inefficient?

Scott

 

[Ken Smith]

I got bored yesterday and started working again on a Geiger counter design I
came up with a couple of years ago. The high voltage power supply was
mostly cribbed from a couple of other designs I found, since I'm a digital
guy and high voltage isn't my thing. As it stands now, the microcontroller
puts out a square wave that drives the primary of a transformer via a
MOSFET. A voltage doubler on the secondary gets the voltage into the
required range (500+ volts in this case), and a couple of varistors in
series form a shunt regulator.

What does the ADC use as a reference?


How good does the regulator have to be?

Two ideas:


You could sample the primary windings voltage some time after the MOSFET
is switched off. The waveform will look like this:

A B C D E
! ! ! ! !
V V V V V
....*.......................
....**......................
....***.....................
....************............
....***........*............
....**.........*............
....*..........**...........
....*..........**...........
....*..........*****........
....*..........**..*........
....*..........*...*........
***...............*********


(A) the MOSFET turns off and the voltage goes rocketing up, overshoots and
rings a bit.


(B) the voltage flattens out at some highish level and droops slightly as
we go towards (C)

(C) the energy is all out of the inductance and the voltage drops
suddenly, undershoots and rings.

(D) the voltage settles down to Vcc (or maybe it doesn't get time to.

(E) the MOSFET turns on again.


The voltage you want for this sort of feedback is the one at (C). The one
at (B) is almost as good. From (B) to (C) is almost a straight line.

You should be able to get fairly good regulation. The tricky bit is that
the micro needs to fiddle with the ADC timing to get the right point for
the feedback. The nice thing is that the load doesn't change much so the
software can take a bit of time.




2nd Idea:

R1 R2 R3
----/\/\/----/\/\/----/\/\/\----+-------
! !
! !
/ --!+\
\R5 ! >----+-- To ADC
/ --!-/ !
! ! !
GND ! !
+--/\/\----
! R4
\
/ R6
\
!
Vref


The ADC will give a zero output up to some voltage and then go up towards
full scale as the volatge increases above that point. This allows the
ADCs bits to be put to better use since you know that the voltage must be
near 500V

R1, R2, R3 are a string of however many resistors you need to make the
impedance high enough.

(1+ R4 / R6) sets the gain for the op-amp

The divider of R5 and (R1+R2+R3..) sets the fullscale point.

Decide on a full scale.

Determine R5 and (R1+R2...) to make R5 have Vref on it at full scale

Decide on a bottom value for the ADC Lets say 450V

Figure out the voltage on R5 at 450V


The op-amp's input see the impedance of R5 in parallel with the (R1+R2)

You want the parallel combination of R4 and R6 to equal that impedance and
you want them to make the op-amp have a gain of (Vref-VR5)/VR5.

The TL071,2,4 type op-amp should work for you if you don't need to
accurate of voltage.

 

[Scott Miller]

Any PSU using a *shunt* regulator is hopelessly inefficent.

Yeah, that's why I'm trying to get rid of it. I know the GM tube doesn't
draw much current - it spends most of its time not conducting at all. And
the polypropylene capacitors seem to hold a charge pretty well - I've been
zapped by one that had been charged and accidentally left lying on my bench
for at least half an hour.

What regulated voltage do you need ?

For this application, 500 volts. Some tubes take 900 volts. I'm sticking
with 500 volts because it requires fewer components, most of the tubes I'm
considering are 500 volts, and Electronic Goldmine's selling tiny 500 volt
tubes (I'm using one in testing right now) for $35, which is a decent price
for single-unit quantities.

Scott

 

[Scott Miller]

Using 'spare cycles' on an MCU is a truly lousy way to do it btw. Far

Why is that lousy? If the MCU has spare horse power why spend the money on
a PWM chip? Of course, to be safe this would have to run reliably. Within
its own interrupt routine or something like that.

I might leave in a shunt regulator to keep it from exceeding 550 or 600
volts in the event of a processor lockup, as long as the leakage below that
threshold isn't much. The idea is to charge the output capacitor to the
tube's operating voltage, then scale the duty cycle WAY back to just make up
for leakage and whatever the tube uses. It can tolerate a fair amount of
ripple, the load is relatively constant, and I think a simple loop running a
few hundred times a second should be adequate to maintain regulation. Not
sure of the exact algorithm I should use, though.

Even without a regulator, I don't think this circuit is capable of
generating more than about 600 volts from a 9 volt input. Not enough to
cause any damage. Of course, I've been wrong before - a previous,
higher-voltage version of this circuit managed to arc through an FR4 circuit
board, and as far as I can tell it shouldn't have been able to do that.

Scott

 

[Scott Miller]

On a Geiger counter I built many years ago from scratch, I used the

recommended
idea of a string of series NE-2s. Worked great. I still have that Geiger
counter and, aside from replacing the tube once, it's still working just
fine!

The neon lamp tubes? I've seen that done. I'm using 250v varistors
instead. Doesn't seem like it would be any more efficient - it's still a
shunt regulator, and will put a constant drain on the PSU even when the GM
tube's not conducting, right?

Scott

 

[Joerg]

Hi Scott,

How would you set one of these up in a high voltage circuit? And wouldn't
any shunt regulator still be pretty inefficient?

Shunts are inefficient because they regulate by 'burning off' the
excess. I'd just use it just as a feedback. How to do that with a TLV431
is explained in a TI app note about isolated flyback converters. I
forgot the number but you should be able to find it.

Basically a divider is hooked up to the high voltage so that the TLV
starts to pull current when Vref is reached. That signals the MCU or
whatever is on the generating side to throttle down. The TLV431 needs a
minimum 'cathode' current of 100uA. Since your application appears not
to be an isolated one this feedback circuitry will become much simpler
than in the TI note.

Regards, Joerg

 

[Pooh Bear]

Yeah, that's why I'm trying to get rid of it. I know the GM tube doesn't
draw much current - it spends most of its time not conducting at all. And
the polypropylene capacitors seem to hold a charge pretty well - I've been
zapped by one that had been charged and accidentally left lying on my bench
for at least half an hour.


For this application, 500 volts. Some tubes take 900 volts. I'm sticking
with 500 volts because it requires fewer components, most of the tubes I'm
considering are 500 volts, and Electronic Goldmine's selling tiny 500 volt
tubes (I'm using one in testing right now) for $35, which is a decent price
for single-unit quantities.

Well.... for next to no current draw you could choose a simple inverter. Simply
choose the turns ratios to suit the supply and take into account Vce sat on the
primary driver. Wind it so the primary takes next to no magnetisation current.

Should do the trick. And you could still tickle it with your MCU !

Graham

 

[Scott Miller]

You could do something like shown below with a JFET input OA. This circuit

uses a resistor string to convert the HV into a current, 10uA at HV=500V,
and then an operational amplifier current-to-voltage converter to produce
the A/D sample. The output impedance of the circuit is 10K which most
embedded A/D can handle. If you need less impedance then go to a TL082 and
use the other OA as a buffer. The output clamps at 5V until the input
exceeds approximately HV=400V, and for voltages greater than this the
output subtracts ~25mV from 5V per volt of HV input in excess of 400V-
landing you somewhere around Vout=2.5V for HV=500V.

Hey, that's pretty slick. I'll have to try that. I know the A/D will work
with 10K impedance. I was also thinking of trying a comparator, but the
buffered A/D scheme only uses the one input pin, and at < 50 cents for the
TL081 that sounds like a cheap, flexible solution. Though it'd be nice to
find some cheap carbon film resistors in > 10 megohms to reduce the parts
count. The high-value thick film resistors go for about $4 at Digi-Key.

I'll do a full writeup on this project when I get a chance. Most of the
hobby kits and published designs I've seen have been pretty simplistic, with
just a speaker and maybe a meter, and the kits sell for about $150 and up.
Mine's got an LCD display and serial interface, and can be scrounged
together for under $50 in its current form.

Thanks for the input...

Scott

 

[Pooh Bear]

Hi Graham,



Yes. Should be a nice PWM control instead. Of course, when it is cold a
shunt regulator does have its merits ;-)

Only if it's a multi kW supply !

;-)

Why is that lousy? If the MCU has spare horse power why spend the money
on a PWM chip? Of course, to be safe this would have to run reliably.
Within its own interrupt routine or something like that.

That's the sort of thing I meant.

Graham

 

[Fred Bloggs]

Hey, that's pretty slick. I'll have to try that. I know the A/D will work
with 10K impedance. I was also thinking of trying a comparator, but the
buffered A/D scheme only uses the one input pin, and at < 50 cents for the
TL081 that sounds like a cheap, flexible solution. Though it'd be nice to
find some cheap carbon film resistors in > 10 megohms to reduce the parts
count. The high-value thick film resistors go for about $4 at Digi-Key.

I'll do a full writeup on this project when I get a chance. Most of the
hobby kits and published designs I've seen have been pretty simplistic, with
just a speaker and maybe a meter, and the kits sell for about $150 and up.
Mine's got an LCD display and serial interface, and can be scrounged
together for under $50 in its current form.

Thanks for the input...

Scott

The TL082 is still available from Radio Shack- as well as 10Meg 1/4W
carbon film 5-packs - so that's a plus, and why I chose that one.

 

[Frank Bemelman]

I got bored yesterday and started working again on a Geiger counter design I
came up with a couple of years ago. The high voltage power supply was
mostly cribbed from a couple of other designs I found, since I'm a digital
guy and high voltage isn't my thing. As it stands now, the microcontroller
puts out a square wave that drives the primary of a transformer via a
MOSFET. A voltage doubler on the secondary gets the voltage into the
required range (500+ volts in this case), and a couple of varistors in
series form a shunt regulator.

It's the shunt regulator I'm looking to replace right now. Since the input
to the inverter is provided by the MCU's timer channel, it can be easily
pulse-width modulated. I just need to get some feedback from the HV side so
the MCU knows how to adjust the duty cycle. My first thought was to use a
pair of resistors to form a voltage divider and feed maybe 1/120 of the
output voltage into the MCU's A/D converter. But with any reasonable
resistor values, there's too much load on the supply to maintain the needed
voltage. Higher values might work, but the thick film resistors are
expensive and I'm not sure how much current the ADC needs to function
correctly.

Any suggestions? I'm thinking an op amp might help, with its high input
impedance. That still means using expensive thick film resistors, though.
And again, I'm a digital guy and I'm a bit out of my element here.

I'm really trying to keep the cost and component count down to make it an
easy hobby project, but I'd also like it to not suck a battery dry in 2
hours. If anyone's got any suggestions on how to implement a better
regulator, or maybe a different supply design entirely, let me know.

Current schematic is at: http://n1vg.net/geiger/images/gc1-9v.png. The HV
supply is on the right-hand side - everything from MCU pin 13 to the
Geiger-Mueller tube. The pin header at left is for an LCD display.

Could you replace the D10 diode with a transistor, emitter to ground?
Then you get a digital signal that tells if the voltage is above
or below 500V, which you can use to increase/decrease the pwm.

I assume you are trying to keep current consumption to a minimum,
running everything from a 9V battery?

 

[Fred Bloggs]

The TL082 is still available from Radio Shack- as well as 10Meg 1/4W
carbon film 5-packs - so that's a plus, and why I chose that one.

If you go with the TL082 and don't need the buffer, then a good use for
it would be as low-pass filter and buffer for the 5V shown in the
schematic...this will eliminate your 5V switching noise from the MCU and
other digital switching you have going on there.