The need for a power supply
When a ham
buys (or makes) a radio he usually needs a power supply for it.
A cheap alternative to buy one is to adapt a computer power supply.
I just said the magic word, cheap. Hams like cheap things because
that way they can buy more radios, or wire for antennas, or CW
keys, or any other things they like. And a computer power supply
can be bought in many places of the world, where one cannot find
other radio related items. But a computer power supply is very
different from a radio power supply, so we need to check the differences
and see what can be done.
and the modified PSU
A typical PC
compatible power supply is usually a switch-mode power supply
with at least one fan. So one can expect RF noise and some acoustic
noise. The RF noise can be ignored if the objective is to feed
a VHF/UHF FM rig, and (some) modern PC power supplies have reasonably
There are several
documents on the internet that give information about transforming
one PC PSU in one radio PSU. They have several shortcomings, they
are for older AT style PSUs, they usually need changes in the
internal circuit of power supply, or in alternative they can supply
only small currents.
But, in the
recent times there is a new kind of PC power supplies called ATX12V
and they are interesting, very interesting. In a simple way, old
style AT PSUs had a strong +5V output, and a reasonable +12V output.
The ATX standard brought different connectors and a +3.3V output.
The ATX12V standard comes from the need of low voltages and strong
currents for the actual processors. The motherboard manufacturers
place voltage converters on the motherboard and feed the voltage
converters at +12V. So its easy to find
PSUs with a capacity of between 15 and 25 Ampéres on the
+12V line (without circuit changes).
is not the maximum current, but the minimum current because switch-mode
PSUs have a minimum current limit, if their load is lower than
the minimum, they can be damaged (or they shutdown if they are
the resolution of this problem is simple,
we just need to place some resistors on the outputs that have
a minimum current requirement. Usually the +3.3v, +5V and +12V
outputs have a minimum current requirement. The resistors must
have the correct wattage, preferably with a safe margin. Some
of the more recent power supplies only have a minimum current
requirement on the +12V line, so they are very nice for our needs.
How to proceed
1) Get the
ATX12V power supply design guide.
2) Choose the
ATX power supply to buy.
The PSU must
have a power switch.
has a page on the Internet?
Does the page
has the full
Is the PSU
a "big fan" (120 mm) type PSU?
What are the
What are the
What are the
outputs with a minimum current requirements?