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ANTENTOP- 01- 2005, # 007

The hAmTX Power Supply




by Paulo Ferreira CT2ILQ




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.



The Normal 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 quiet fans.


Moving Closer


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).


The minimum


The trouble 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 protected).



But 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.

Items to check:


The PSU must have a power switch.

The manufacturer 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 maximum currents?


What are the minimum currents?


What are the outputs with a minimum current requirements?

Page 82


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Last Updated:

February 23, 2020 14:47

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