Hardware Hacking 111- The primary voltage, i.e., 120 (US) or 240 (Europe) volts AC (or VAC.)
We need a 120 volt primary voltage. - The secondary voltage, i.e., something in the range of 3-24 volts. We
need a secondary voltage in the range of 12-18 volts. - Whether it has an AC or DC output. We need DC.
- The amount of power the transformer can provide, usually measured
in watts (W), amps (A or MA) or volt-amps (VA). We want a
transformer that puts out a minimum of 100 milliamps (ma), which
may be indicated as 0.1 amps or 5-10 watts.
For example, a wall-wart might say “120vac input, 12vdc output, 200 ma). “ You
can find wall-warts everywhere, with a zillion slightly different types of
connectors, often very cheaply. Some offer switchable output voltages and/or
interchangeable connectors.
When adapting a battery-powered circuit to a wall-wart you must observe two
critical factors:
- The voltage must be within two limits (i.e., greater than 12 volts but
less than 18 volts), but the current can be anything higher than the
minimum need to power the circuit (i.e., a circuit requiring 20 ma can
be powered by a supply producing 20 ma, 100 ma or 1000 ma.) - Note the proper polarity when connecting the wall-wart to the circuit
board – i.e., the + to where the battery’s + was, - to the -.
Sometimes the wall-wart will indicate which wire or which part of the connector
is + and which is -, but it is always safer to test using a multi-meter. Set the
meter to measure “DC Voltage.” One probe plugs into the meter’s “ground” or “-
“ input, while the other connects to something probably marked “voltage” in
red. Touch one probe to the end of one of the power supply’s wires or one part
of the connector, and the other to the other. If the meter reads out a voltage with
no prefix (i.e., “13.6”), then the wire/connector touching the ground probe is the
- output, and the other is the +. But if the meter puts a “-” before the number (“-
13.6”) you know that the connections are reversed, that the wire touching the
minus probe of the meter is actually the + output and the other is -. Confused?
Try this test on a battery.
Once you figure out which wire is which, mark them carefully!
To hook up the wall-wart to your circuit you can either find a matching
connector to whatever is attached to the wall-wart’s cord, or you can cut off the
connector and solder the wires directly to the board. Double check the polarity
before you plug it in! As a safety precaution against frying your circuit with a
backwards power supply you can connect a diode across the power supply as
shown: