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Variable step up dc-dc converter.

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steve_j83

New Member
How do i go about stepping up from between 9-12v dc to around 300-500v dc?

Its for powering a geiger muller tube. Would i essentially invert the input run it through a transformer then rectify the output?
 
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Grossel

Well-Known Member
First of all:
This is very easy to google and find some hints.

Some components you might need (chematics also possible to google)
* Variable pulse width generator. The most easy way to achieve this is to make a triangle generator and use a comparator.
* Other input to the comparator should come from a potmeter (if you manually adjust it).

One major thing to think about about before further planning is if the load will be constant? If not, you might need to regulate.
 

steve_j83

New Member
Thanks Grossel, i did manage to find something that could be suitable.

9026-hv-supply-joseph-diverdi.png


A couple of questions though, can i remove the rectifier and use a 12v dc input? Not necessary but i have a 12vdc 1200ma supply available.

And also, Can i replace the transformer with something more readily available?

Might be a stupid quiestion but as 240vac-12vac transformers are readily available (and are roughly the same scale) could i use one of these in the circuit instead?
 

BrownOut

Banned
can i remove the rectifier and use a 12v dc input? Not necessary but i have a 12vdc 1200ma supply available.

Sure. You might requre more current than your PS supplies though.

Can i replace the transformer with something more readily available?

I doubt that would be a problem. Keep an eye on inductive spikes coming from the transformer primary.

This circuit appears to use bang-bang regulation, rather than pulse width modulation. It works, but could be better.
 
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steve_j83

New Member
Sure. You might requre more current than your PS supplies though.

Even though my amps are 400ma above the input on the schematic? Edit, is it because i have doubled the transformer, do i need to double the amperage?


I doubt that would be a problem. Keep an eye on inductive spikes coming from the transformer primary.

Is there anyway to level out the spikes? Isnt that what the capacitors after the transformer are doing?

A bit of text that compaines the schematic. Might be of use.

If the specific transformer mentioned in the schematic is not available to you, any transformer with the specifications primary 117VAC, secondary 6.3VAC CT (center-tapped) should work.
In this case you might have to choose a different drive frequency in order to operate at the sweet spot of the transformer: "[...] I found in driving a transformer as is done in this project that the overall performance varied significantly as I varied the drive frequency. In a series of experiments I loaded the transformer directly with a variable resistor (no rectifier used), measured the AC voltage across this resistor, measured the DC current into the transformer, and varied the drive frequency. I looked for a frequency where the output voltage was highest and the input current was lowest. [...] 'ripple' is roughly +/-25V for output voltage >200V. This ripple isn't sinusoidal at all. It is basically a spike then monotonically decays until the next spike."

Im a little unsure of what is ment by varying the drive fequencey?
 
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BrownOut

Banned
I didn't notice the 800mA on the input. With that, you have just under 10 W of power available, before the losses in the circuit. Maybe that's sufficient. It appears you have to do some experimenting to get the frequency right for your transformer. If you don't understand how to do that, you need to look at some material on the timer IC1. Read This: Look at the section on astable multivibrators. The spikes may or may not be a problem. I was thinking your transformer might have more primary inductance than the one specified, so you might need to just keep in eye on that. C6 will suppress some of it, but your circuit has to tolerate a certain amount of inductive kiciback to get the high voltage you want. So, you have to balence the kickback against the output voltage requirement. It'll probably be fine, just something to keep in the back of your head. Many of these circuits published on the web have insufficient spike protection. I learned that the hard way.
 
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Chippie

Member
A TL494 would be a better chip than the 555 arrangement and would give pwm control...
 

steve_j83

New Member
Ok, i will check the data sheet of the TL494, will probably come back with questions as to how to implement it.

Thanks
 

steve_j83

New Member
Ok so looking at implementing the TL494 is a bit over my head,

With the 555 i gather i need to experiment with the values of R1 or R2 (or both) and maybe C1, but i will get back to that, i am starting to realise that i will need to be checking the HV output as i tune the drive frequencey so i am gathering i will be needing a osciliscope capable of inputing high voltages. Is this correct?
 

BrownOut

Banned
That's the best way I know. Be careful to not exceed the maximum voltage for your 'scope. When I tune a switcher, I start with a low duty cycle, and trun it up slowly. Monitor the power rails to ensure no HV spikes can damage your circuit.
 

steve_j83

New Member
Ok, no worries, am not quite there yet, as i will need to get my hands on a scope, my uni department might have one suitable (sound) but am not sure.

Going back to tuning the drive frequencey, is there any specific frequencey range i should be looking into? Or is it just a case of trial and error till i find what works best?

Edit BrownOut you mentioned the scope is the best way you know of, if i cant get my hands on one is there an alternative?
 
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audioguru

Well-Known Member
Most Helpful Member
You found a powerful circuit. Its high power makes it complicated and expensive.

But your geiger tube does not need high power. It needs a simple little (cheap) circuit.
 

Blueteeth

Well-Known Member
You found a powerful circuit. Its high power makes it complicated and expensive.

But your geiger tube does not need high power. It needs a simple little (cheap) circuit.

Audioguru has hit the nail on the head there. Geiger-muller tubes require high voltage, yes, but notmuch current at all. You could easily get away with less than a few mA on the output.

Take a look here:
Geiger Counter Circuits
https://www.galacticelectronics.com/Images/Schematics/GMCounter.PNG

For 500v+ ata low current, I would recommend the 'easiest' route being a simple self resonant converter, followed by a voltage multiplier. A single inductor boost converter 'could' be used, but its efficiency would be woeful, so transformers are always a good option.

As someone else pointed out, a CCFL inverter is a great and convenient HV source (ebay has loads for a fwe bucks), although its output voltage is not fixed, and probably too high for your tube - you could always limit the current of its power supply, which, in your circuit, would determine the output voltage. That is how they 'dim' CCFL lamps.
 

captainhannes

New Member
Blueteeth,

you're right that CCFL modules have no fixed voltage, but the circuit I mentioned has a regulation for a stable DC voltage and also allows HV adjustment.
 

chemelec

Well-Known Member

MrAl

Well-Known Member
Most Helpful Member
This is an OLD Posting on this subject, But this info might help others in the future.
And as Stated by Audioguru, Geiger-muller tubes only require Low current.
Just in the Low Micro-Amps.

https://chemelec.com/Projects/Geiger-Counter/Geiger.htm


Hi there,

I took a quick look at your circuit and i cant find a return path for the dual transistor emitter. Perhaps you can explain this a bit.

The transformer presumably isolates the DC supply (battery) from the power supply circuit for the tube, then the drop across the 100k resistor presumably drives the base of the dual transistor, but then there is no distinct path from the emitter back to the tube power supply other than the small capacitance of the transformer itself, which if this circuit works as is, would be why it is working at all. Perhaps a more distinct current path would be a good idea but i'll wait for your explanation.

It's a little unclear what kind of transformer that is too, is it really isolated?
And why the need to step up the voltage even more, it is because it is using a more common transformer that is easy to get (perfectly understandable).

I believe your circuit is (or will be) better than the one originally shown in this thread.
 
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chemelec

Well-Known Member
Here is the Origional Circuit.

I Assume that the Leakage through the Transformer In Both these circuits is Completing the path.

If I Connect the Common of the Transformer to the Battery Negative it Makes NO Difference in the Operation of Either of these.

YES the Transformer is ISOLATED between Primary and Secondary.
 
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