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I often hear about changing voltage. Like someone would say "I bumped 12V to 38V" or like "I converted these 100V to 10V". Can you change any voltage to any voltage? What's the exact mechanism for that?

I'm a complete newbie to electronics. I only know Ohm's law V=IR.

To go from 12V to 38V as in my example, does that mean I simply find a resistor that is aproximately 3x the current one?

And to go from 100V to 10V, I find a resistor that is 10x smaller?

Is there anything else to this?

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For AC, you use a transformer which is, basically, two coils of wire wound on a common core. The voltage step-up or step-down is determined by the ratio of turns in each coil. A transformer passes power (Voltage times Current), so if the output voltage is higher than the input, the output currrent will be lower than the input current.

Transformers depend on the varying magnetic field induced by the varying AC voltage to induce a voltage in the output (secondary) coil. No parts other than the transformer itself are required to do the voltage conversion.

For DC things are more complicated since DC, by itself, does not produce a varying magnetic field, so a transformer alone won't work. Instead, we have to use some electronics or other means to chop the DC input into pulses, which a transformer will "see" as AC. A transformer can then be used to step this chopped DC voltage up or down as required.

A DC-DC converter (also called a Switch Mode Power Supply, or various other terms depending on the exact application) can be used to step a DC voltage up or down - like the transformer, it passes power, so if the output voltage is greater than the input, the output current will be lower than the input.

Linear voltage regulators reduce voltage by acting like a resistor, so the output current will always be the same (or slightly less) than the input currrent. The "resistance" of the regulator will be automatically adjusted to keep the output voltage at the desired value.

Series resistors and resistor voltage dividers can also be used to reduce voltage, but the voltage reduction will vary depending on the load current.

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  • \$\begingroup\$ could yoy elaborate on why dc-dc is more complicated? \$\endgroup\$ – jotadepicas Nov 22 '15 at 15:15
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You cannot increase voltage by using resistors. Voltage will drop across each resistor in series. You can however, use a voltage divider to decrease the voltage based on this principal.

Note that you also need to take into account the power consumption of your circuit. Since P=IV, if you take a large current across your voltage divider, you can burn out your resistors; this is not a very efficient solution in some cases.

If you wanted to increase an AC voltage, you would need to have a transformer with the appropriate turns ratio.

If you wanted to increase DC voltage, you would have to use a DC-DC converter topology such as a boost converter https://en.wikipedia.org/wiki/Boost_converter or a charge pump https://en.wikipedia.org/wiki/Charge_pump.

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