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Monday, October 18, 2010

Regulated Power Supply 20A

A heavy duty 13.8V regulated power supply is a fine thing to have in the shack, but unless you acquire one secondhand, is an expensive little beastie to buy. This means building one should be considered, not only for the cost savings, but also because you can brag about it on air to your mates. Of course, careful consideration must be given to the properties of the completed supply, and after talking to a few of my friends who have built their own and fallen into all the traps, here are the printable ones : RF proof, easy to make, commonly available parts used, but above all CHEAP.

The details published provide a transformer and rectifier structure capable of providing 8 amps DC continuously (and short current peaks up to 20 amps). This is sufficient to adequately power SSB transceivers with 100W PEP outputs, BUT WILL NOT power a transceiver providing a continuous carrier of greater than about 40 watts.e.g. AM, FM, continuous key down morse, single tone SSB testing etc. Demanding a continuous output of more than 8 amps will result in the transformer secondary overheating, with a possible fire risk. The reason we can get away with a supply with an 8 amp continuous rating is simply that speech is very "peaky" data, and so SSB has the odd high power peak but a very low average power level (usually about 20 -30% of peak value). It is on this basis that transformer and heatsink sizes are usually selected for domestic hi-fi equipment.

The winding resistances of the transformer have been very carefully chosen to avoid excessive current peaks which will cause failure of the 35 amp bridge rectifier. If you wish to use a transformer with higher current ratings to make a continuously rated 20 amp supply, you MUST use more heavily rated diodes with peak repetitive surge current ratings of around 800 amp.The second part of the circuit (filter caps and regulator) will supply 20 amps continuously and can be used unchanged in a heavier duty supply. The transformer, rectifiers, and regulator circuit all generate large amounts of heat, and fan cooling MUST BE PROVIDED for safe and satisfactory operation.