# OT? How to build a Simple DC capacitor voltage transformer



## rmay635703 (Oct 23, 2008)

Maybe I am missing something so I will post here, a portion of a circuit I want to build uses contactors and capacitors to change the source voltage up to another. In other words a DC current source is latched to a set of 3 of identical capacitors in parallel, unlatches from the source and then latches the three capacitors in series connected to the load. This triples output voltage with no transformer and limits current and duration.

How would I convert this from analog to a solid state circuit? Mosfets I don't believe can work as a true switch, not sure how to make this simple circuit. I'm not dealing with AC here and the purpose is totally different.

The circuit need only operate at a fixed low frequency (dozens to hundreds of hertz) and not be variable.

Sounds simple enough but gets complicated moving from physical switches to an automated digital circuit.

Any ideas if this is possible?


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## Coulomb (Apr 22, 2009)

Are you talking tripling 5 V to ~~ 12 V (with losses) at a few mA, or hundreds of volts? Significant current?

MOSFETs are actually pretty close to ideal switches, but you may need another DC-DC converter for the gates!

When you say it's "not AC", do you mean you are only interested in square waves, not PWM'd sine waves?

Depending on the details, it certainly sounds doable.


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## TigerNut (Dec 18, 2009)

rmay635703 said:


> Maybe I am missing something so I will post here, a portion of a circuit I want to build uses contactors and capacitors to change the source voltage up to another. In other words a DC current source is latched to a set of 3 of identical capacitors in parallel, unlatches from the source and then latches the three capacitors in series connected to the load. This triples output voltage with no transformer and limits current and duration.
> 
> How would I convert this from analog to a solid state circuit? Mosfets I don't believe can work as a true switch, not sure how to make this simple circuit. I'm not dealing with AC here and the purpose is totally different.
> 
> ...


Depending on how much current you need at the higher voltage, a diode-capacitor ladder may work:
http://en.wikipedia.org/wiki/Voltage_multiplier
If you use an oscillator (a 555 or a micro output) to provide a square wave to the input it should boost just fine, but the current will be limited to whatever the micro can provide, divided by the number of stages. You can use the micro or 555 to drive a push-pull power FET pair, and get higher drive levels.


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## rmay635703 (Oct 23, 2008)

The source is pure DC in the 12v-20v area and the load is in the 48-62v area. I was hoping for around 2 amps or so but maybe up to 10amps and 80v later on if I get a prototype working at the lower voltage.

So apparently the only way of multiplying voltage is to first convert to AC? Bleh

Cheers
Ryan


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## Coulomb (Apr 22, 2009)

rmay635703 said:


> The source is pure DC in the 12v-20v area and the load is in the 48-62v area. I was hoping for around 2 amps or so but maybe up to 10amps and 80v later on if I get a prototype working at the lower voltage.


That's getting slightly beyond the power level where a capacitor multiplier circuit makes sense. The capacitors would have to start getting towards soup can size, and the capacitors and the circuit charging them would have a hard time with the spikes of current.

For this 100-800 watt level, you are far better off with a purpose built high frequency switching DC-DC converter. You might be able to start with a pair of 12 V to 24 V converters; these are cheap and 100 W versions are readily available. You'd need isolated output on one of them. It may be possible to use just one, and change the turns ratio on the transformer, or number of turns on the main inductor, to make it say 15 V to 60 V. Then you'd need to modify the voltage feedback (often just a voltage divider, two resistors) to make the output adjustable and in the range you are interested in. Most likely, the switching devices will handle up to 60 V with no problem, and even 80 V is likely. You'll need to change some filter capacitors to higher voltage versions.

Another easy route is a 12 V to 110 V (or 220 V) inverter, connected to a power supply (perhaps as simple as a transformer, bridge, and possibly filter capacitor). Look for one with a wide input voltage range; most 12 V inverters won't be expecting 20 V input. Inverters are cheap and readily available. Again, it may be possible to modify one to produce a lower voltage, avoiding the bulky low frequency power transformer.

If designing from scratch (but it's much harder than it looks), what you want is a boost converter.



> So apparently the only way of multiplying voltage is to first convert to AC? Bleh


Yes, pretty much. Unless you happen to have multiple isolated sources (like batteries or the isolated output of a 12 V to 24 V converter) that you can simply wire in series.

Switching DC/DC converters are also converting to AC, just higher frequency. The contactors were also converting to square wave AC.


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## TigerNut (Dec 18, 2009)

rmay635703 said:


> The source is pure DC in the 12v-20v area and the load is in the 48-62v area. I was hoping for around 2 amps or so but maybe up to 10amps and 80v later on if I get a prototype working at the lower voltage.
> 
> So apparently the only way of multiplying voltage is to first convert to AC? Bleh
> 
> ...


When you did your latch - charge - unlatch - stack - discharge operation, you were doing the same thing, just in a more explicit way. 

Pulling 2 amps is a fair bit of current, and 10 amps at 80 volts is serious power. 800 watts is a lot of heat ^H^H^H^H^H smoke if it piles up in the wrong component 

A better way to go might be to make an inductive booster. Charge a coil to a high current level, then disconnect it from the source and use a diode to dump the charge on your high-side capacitor. Repeat as required to transfer the right amount of power. Moving 800 watts this way will require some decently sized parts. Make sure you put a bleed resistor on the high-side cap so that charge doesn't hang around while you're working on it...


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