Options to get 300~400 VDC from 9~12VDC

Nick Waldvogel

I am looking for a circuit to yield 300 to 400 VDC from 9 to 12VDC. I have looked at several transformer style circuits on the web but they just seem to raise more questions. Most of the examples I found don't list the specs of the transformer and the sites that do list the specs are often outdated or not available anymore. Here are a few questions that I have. Feel free to give any other advice you have on the subject.

The circuit is to feed a CDI ignition system of a car. The storage and triggering system is already made. I just need a way to boost the car batteries 12VDC to 300~400VDC.

Here are some of the factors that I am working with:

1) Input voltage from a lead acid car battery
2) Small footprint is helpful
3) Low interference to the surrounding electrical components.
4) Working temperature will be around 110~120F. The less heat generated by the circuit the better.

Here are my questions that I have worked up as I read more and more on the subject.

1) I have seen some articles about using Mosfets to raise voltage. Is this a better option than a transformer?
2) I know my input voltage, but how do I calculate what my output will be with a transformer or mosfets? There must be a formula but I can’t find a source.
3) Some Transformers only have one primary winding and others have two. What are the advantages and again how do you calculate the input/output of a transformer with two primary windings?
4) How do you calculate the amount of DC voltage you will have when you send the AC voltage into a rectifier?
5) I was in Radio Shack and there transformer all say AC on them. Do they say AC on them because that is what comes out on the secondary side or do they not work with DC voltage?

Thanks for your input.

Nick

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TonyWaite

Hi Nick,

There are a few different ways of doing this conversion and plenty of circuit ideas can be found by
googling "dc voltage multiplier circuits".

It would be *really* easy to electrocute yourself while prototyping the design. From your questions I
would guess that you are new to the high-voltage game and would recommend against a DIY
implementation because it is just too dangerous!

Regards,

T o n y

Added PS on edit:

By way of example: I've taken two 250V shocks in my life - blinding blue light and a nasty jolt only; and a
single 415V belt when working on a supposedly-disconnected isolation transformer. This time, the high-voltage
flung me across the room and gave my heart such a fearsome punch that I've never forgotten.

Post Edited (TonyWaite) : 2/11/2010 4:24:25 PM GMT

kwinn

The simplest and most reliable circuit to do what you want is a boost regulator. Basically it uses an inductor, a fet switch, a rectifier, and some control circuitry to produce a higher voltage. There are many circuits to choose from. Look at the TI, Maxim, and National web sites and search for "boost regulators". I have seen several similar projects in car magazines over the years so you may want to check them out as well.

I would stay away from the 60HZ transformers for this project. They are bulky and limit the frequency and efficiency of the system.

Agent420

Nick Waldvogel said...
I was in Radio Shack and there transformer all say AC on them. Do they say AC on them because that is what comes out on the secondary side or do they not work with DC voltage?

(giggles) Transformers are inherently ac only devices

A big part of this equation is how much current you require - that will dictate what type of solution and what parts you will need.

While diy is often quite satisfying, don't discount re-inventing the wheel where you can avoid it (especially for complex or economic reasons)... check out Jaycar, they seem to focus on this area.

http://www.jaycar.com.au/
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Phil Pilgrim (PhiPi)

For a quick solution using all Radio Shack parts, you might consider getting a 12VDC -> 120VAC inverter. To the output of that, you can add a voltage doubler. If the inverter produces a sine-wave output, a doubler should be all you need to get 300V+, since what gets doubled is the peak voltage. However, if it outputs a square wave, the peak and RMS voltages will be equal, and you may get no more than 240V from it and have to go to a voltage tripler. Also, due to the fact that the inverter's output frequency is 60Hz, you will need fairly large caps to keep the voltage up under load, so this will be a bulkier solution than, say, a DC-DC converter that works at much higher frequencies.

IMPORTANT: I can't reemphasize enough the prior warnings about working with high voltage. The voltages you're considering can be LETHAL!

-Phil

Agent420

Half the parts you need can probably be scavanged from·a dead ups...

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Nick Waldvogel

Thanks for the suggested googles. They have helped a lot in understanding what I am looking at and how the example circuits are operating.

Your warnings are not going unheard. I understand that there are dangers involved.

I take risks every day. If we don't take some we will never move forward. It doesn't mean that we shouldn't be cautions in our surroundings....

Nick

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deadeye2021

I like Phil Pilgrim's idea. (plus I work at Radioshack heh heh) but that's far beside the point. Using an inverter would be optimum since it definitely would get you close to the voltage you want to be. The flaw however is your waveforms. Granted you can triple your voltage using such devices as voltage multipliers OR you could add an additional transformer. There's some tricky math involved (email me and I can do the math for you) but you would get your 400 volts. NEXT you have to worry about your 400VAC with 60Hz meaning you have a sine wave still. This is easily 'rectified' using exactly that: a "Full Wave Bridge Rectifier". These devices are quite simple; 4 rectifier diodes arranged in opposing pairs --->|--- ---|<--- and ---|<--- --->|--- respectively. You can wire these yourself or cut it simple and just buy one. Radioshack has them. You'll have to ask for it specifically as most stores don't normally keep them on hand.

A short lesson on diodes: Diodes tend to have a "clipping" effect on AC waveforms in that the manner in which they are implemented into the circuit ie. forward or reverse bias, determines whether you "clip" the + or - half of the cycle. Diodes configured as a Bridge Rectifier will clip the waveform to give you ALL + or ALL - half cycles all in a row. In other words, you'll have a frequency of 120Hz if you have 60Hz on the input. This is no less than what is called Pulse DC. But this is still useless to you. What you need next is a capacitor. Depending on what your current levels are going to be will determine what diodes and capacitor you use. If you're using high current in excess of 20 amps, you need HUGE diodes and a HUGE capacitor on the Farad level. A 2 amp or less can get by with a 47uF 400VDC capacitor and 4 small 3-amp rated rectifier diodes (again, readily available from our friends at Radioshack). This is actually a common configuration that I'm going to adapt for a device I'm building.

Why a capacitor? The way it charges... Those pulses of DC coming in will eventually charge that thing up to full capacity. Due to the nature of capacitors (and resistors involved before and after its implementation in your rectifier circuit) it charges faster than it discharges. In essence, you'll have a sharp up-slope as it charges when the first pulse comes in but it slowly releases it so the down-slope is VERY VERY shallow or virtually non existent IF the capacitor is the right size. 9-volt 2-amp systems use a 400V 47uF cap. And if you fear a surge in pulses before the capacitor is charged, dont worry. The capacitor will only discharge when it reaches its full capacitance in relation to the input. So all them pulses add up to one slow discharge at a very steady level of basically PURE DC. No waveforms. Resistors slow this and help the discharge process move more slowly and smoothly.

But in answer to your question, a transformer would work best to boost your voltage. They are more efficient and less prone to heating (with the right one of course). The BRF arrangement will convert back to DC at 400 volts and the capacitor will smooth out those waveforms.

Hope this helps.

allendlight2003@yahoo.com

RobotWorkshop

One thing I didn't see mentioned is how much current you are going to need at that high a voltage and how closely you need to regulate it.

If you've run out ideas you can always take a step back and look at older technology....

I've seen some old gear that used a DC motor to run another motor setup as a generator. I suppose you could rewind a motor to generate some high voltages.

I know this works since my father and I have an old WWII searchlight with the original servo positioning system intact. The whole thing is run off a large DC generator. One the light they have a small motor running an AC generator on the light and also another pair of motors running separate generators (with controlable windings) that can generate well over 400V at times. That is just part of the control system.

Robert

stamptrol

All the cautions about high voltage and its unforgiveness apply.

You're looking for a step-up converter which is the heart of every capacitive discharge ignition. The usual transformer is a toroidal unit with a 24 volt center-tapped primary and a 350 - 400 volt secondary which is then rectified. A two transistor oscillator switches the primary, aided by "tickler" windings to help with start-up.

You can experiment with an ordinary 24 volt to 240 volt transformer by googling "step up inverter".

Cheers

PJAllen

The word from the OP on this matter was > 1 year ago.

ElectricAye

PJ Allen wrote: »

The word from the OP on this matter was > 1 year ago.

Indeed. And his last post before disappearing from the forum was a few weeks later, on 03-04-2010, in which he asked about a battery charger.

Alas, maybe he wasn't as cautious with high voltage as he should have been.