Amplifier AC Power Supply Requirements

So today I learned a thing. Like why linear amp manufacturers spec their AC requirements so high. Like the Elecraft amp calls for 20A at 240v and a bunch of others say 15A etc. The reason why I am thinking about this is because I have been thinking about upgrading my license and then doing some sums to work out the costs of adding an amp to the line up.

If these things really did require ALL THE AC AMPS, I would have to pull a dedicated 20A line from the sub panel in the garage. Lucky there is room on the board to do this, but, its going to cost about $1500 to have a elechicken come and take half the roof off the house to pull a cable to where it needs to go and that is on top of the costs of an amp and the like.

My radio room already is on a 20A circuit that it shares with the water pump, bathroom and laundry, so leaving some overhead for the freezer and washing machine, there is room to spare, but not 15A, maybe 10A and then you would not want the wife to dry her hair in the bathroom. LOL

So why do these things need so much current? Inefficient power supplies. If they are using linear power supplies, because hams are scared of switch mode supplies, they might only be at best 60% efficient compared to say 80% efficient for even the worst switcher from MeanWell. And there is where I had my eureka moment. Math will give me a rough idea of the actual AC current requirements for any particular amp.

AC AMPS == DC Power (watts) / (Power Factor * AC Volts * Efficiency%)

So for your typical LDmos amp these days with 50V at 30A a maximum of 1500 watts, with linear power supplies 1500/(0.8*240*60%) you are looking at 13 Amps AC (guessing the power factor here). Compare that to a fairly efficient switch mode supply with power factor correction and your total AC current draw is going to be around 8 Amps or better.

This got me thinking, legal limit here is 400w and those numbers are for something closer to US power limits. So in theory, without knowing what the actual DC Power requirements are, but lets say 50V at 10A, thats 500 DC Watts and with a 90% efficient switch mode power supply we are looking at only around 2.5A AC.

That is not going to stress the existing household circuitry at all, in fact there is more than enough capacity to not even stress things even a tiny little bit. So maybe this upgrade and build and amp, yeah build one not buy it because I have the skills to design the boards and write the code to control it all, could be done for a reasonable cost, not in the 5 to 10K kind of mark.

So maybe this stupidity is actually within reach and I should give a little more thought to upgrading the license. Currently I am really close to finishing off 40m and 10m dxcc which will give me 4 bands and for the 5th band we are talking 80m and for that having 400w on tap with crappy dipole antennas might make dxcc a lot less painful.

So who knows, but watch this space. 🙂


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