Please help me understand voltage multipliers

Discussion in 'General Electronics Chat' started by wannaBinventor, Aug 11, 2010.

  1. wannaBinventor

    Thread Starter Member

    Apr 8, 2010
    I'll admit that I'm much weaker than I should be in ohm's law at this stage of my learning. Despite this, I'm hoping you guys could clear up something that perhaps should be more obvious.

    Long story short, I'd like to make something that recharges a battery (say, 2 AA batteries) from a mini (and I mean mini) wind turbine. Like using a small hobby motor, attaching a mini-pinwheel to the shaft, and exposing it to something like a 5 MPH wind (that's all I'll be able to get).

    Now, I'm not suer about the math behind figuring up how many RPM I can get the motor to turn based on a 5mph wind, but I'm going to assume with the right combination of DC motor and pinwheel I could get maybe 1.5-1.6 volts on the output.

    So, if I were to make a DC to DC voltage multiplier, what should I use to figure the current capabilities of the multiplier's output?

    It doesn't seem that I could multiply current from say 1.5 volts to 3 volts and then apply ohm's law to figure output current capabilities because it's only being fed 1.5 volts at the start.

    Thanks for any help you can give on clearing this up.
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    To get some idea of capability, try some experimentation using the motor and the pinwheel. You will need a meter, too.

    Set the miniturbine up with a resistor across the output. Use the meter to see the voltage developed, and Ohm's law will let you get the current. A load will place a drag on the motor. There will be some resistance small enough that the motor will not produce a useful output. The idea is to find some load that gives maximum power out - the product of voltage and current.

    You will probably need to start with a resistance of around 30 ohms and go smaller. A 2 watt 50 ohm pot would be perfect.
  3. marshallf3

    Well-Known Member

    Jul 26, 2010
    Don't underestimate the amount of voltage a small motor can produce with the proper propellor and a light breeze.

    Hit up a thrift store or an old electronics repair shop and scarf up some broken home, auto or computer CD &/or DVD players. These usually have at least two motors in them designed to run at 3, 6 or 12V. has a selection of propellors as do other places like but remember you're trying to catch air, not create it. (design of the blades is a bit different) --- or of course play around with making your own.

    Not really going to be any definitive math on this type small scale project so experimentation comes into play.

    BTW: has some rather interesting "surprise boxes" in three sizes for very good prices. I love it when I find a place that sells decent grab bags and this is one of them. As with all of these (from anywhere) there will be a ton of stuff you'll probably never have a use for (or even know what it is) but I've always come out ahead with theirs by a long shot.
  4. radiohead

    Active Member

    May 28, 2009
    If you can pre-test a model, try stepping up the voltage using a miniature audio output transformer, a telephone transformer or a small power transformer. Then you can rectify it to DC. Then your experiment begs the question, what are you going to power with this device? Are you going to power a grain-of-wheat bulb, LED or something with more current draw? Look up Bill Marsden's blogs on 555 circuits (this web site). I'm sure he has something in there for voltage inverters and multipliers using 555 chips.
  5. wannaBinventor

    Thread Starter Member

    Apr 8, 2010
    Thanks for all the insights guys. Much appreciated.