[Noisebridge-discuss] N00b question - changing power supply voltage

[davidfine]

The practical thing to do is to buy a big 12V supply for $5 on ebay. But 
that's less fun to think about so-
It's actually easier to transform AC voltages (with a transformer) and 
then put the output through a DC rectifier. Then you could use something 
like http://en.wikipedia.org/wiki/Autotransformer  Using a transformer 
to convert DC involves more circuitry.

For a high current operation like this (12v x 4.5a = 54 watt), you don't 
really want to use a linear converter (something that dissipates excess 
energy as heat) You *could* use a battery of linear rectifiers, or you 
can buy a dc-to-dc converter on an IC which is actually a different beast.
The potentiometer or voltage divider solution is not so good because 
you'd need beefy parts and output varies based on load.

A dc-to-dc converter is a switching converter as opposed to a linear 
regulator:
http://en.wikipedia.org/wiki/DC-to-DC_converter
A switching converter is the best choice for high current, here's the 
relevant paragraph from wikipedia:

    In these DC to DC converters, energy is periodically stored into and
    released from a magnetic field
    <http://en.wikipedia.org/wiki/Magnetic_field> in an inductor
    <http://en.wikipedia.org/wiki/Inductor> or a transformer
    <http://en.wikipedia.org/wiki/Transformer>, typically in the range
    from 300 kHz to 10 MHz. By adjusting the duty cycle
    <http://en.wikipedia.org/wiki/Duty_cycle> of the charging voltage
    (that is, the ratio of on/off time), the amount of power transferred
    can be controlled. Usually, this is done to control the output
    voltage, though it could be done to control the input current, the
    output current, or maintain a constant power. Transformer-based
    converters may provide isolation between the input and the output.
    In general, the term "DC to DC converter" refers to one of these
    switching converters. These circuits are the heart of a
    switched-mode power supply
    <http://en.wikipedia.org/wiki/Switched-mode_power_supply>. Many
    topologies exist. This table shows the most common.

So yah, you need a circuit to control the duty cycle of the input 
voltage, a transformer, and perhaps some circuitry to smooth the output.
--D


On 7/1/10 1:57 PM, Sean Cusack wrote:
> Hey kids -
>
> So, I've got a relatively simple dilemma that I sure pops up all the 
> time for those of you that are way better at electronics than me. I've 
> got a power supply providing a fixed 24V @ 4.5A, but I only want to 
> use 12V of that for my circuit. I originally thought I could lower the 
> voltage using a potentiometer, but because the power is so high, they 
> all cost some serious bank. Is there another (not necessarily analog) 
> solution to changing voltage and/or amperage through a circuit that is 
> cheap to build?
>
> Sorry if this one is likely obvious - but everyone can blame Mitch for 
> getting me way to into electronics for my own good :).
>
> Sean
>
>
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