A 300 Watt DC Power Supply for a QRO AM Transmitter

This post describes construction of a 300 watt DC power supply for use in a 150 watt Class D AM transmitter. The transmitter design calls for about 150 volts DC at 2 amps continuous and up to 4 amps peak. This post describes the 240v mains DC supply. The switch mode regulator is described in another post. The other modules in this QRO transmitter project are to follow.


I used a large surplus Autotransformer sourced from Rockby disposals a year or two back. This transformer had two secondaries at 8 volts and 19 volts and a multi-tapped primary (with taps at 0, 204, 210, 216, 220, 240. to 240v AC). An additional secondary would need to be hand wound on.

This transformer would make a decent boat anchor, weighing in on the bathroom scales at 5.7kg. All the primary flying leads are HEAVY duty wire, terminated in big bolt-able lugs. It was sold as 240V 20A Autotransformer. That’s 5kW! But the absence of a tap lower down on the primary suggests that the source and load were both on the 200 to 240v taps. So I think it was used in a mains conditioning unit, that could cycle thru the primary taps to correct for mains fluctuations, providing a smoothed 240V source to a load. The secondaries would have powered some low voltage monitoring and control circuits.

Winding on a 110 volt secondary

I bought a 35m spool of 1.29mm Enameled Copper Wire from Wiltronics. Using a timber bobbin of about 40cm, it took a few hours to get it all on. When I powered up the transformer via a 2 amp mains fuse … bang! Tried a bigger fuse… bang also! With a 7.5A fuse, the next size up I could find, the fuse survived power up, and the hand-wound secondary delivered 102 VAC. Soon I would read about the potentially large in-rush currents in toroidal mains transformers.

Using a bobbin to wind a new secondary onto a toroidal transformer.

Rectifier, filter bank

If you exclude the regulator, there’s not much to a DC power supply. It’s really just a matter of making sure everything is conservatively rated for the expected power output, and that both primary and secondary power circuits are fused. I chose a 400 volt 35 amp rectifier bridge from Jaycar. For filter caps I used 5 x 1000uF 250v and 4 x 1000uF 200v, both sets branded Jamicon, both ex Rockby disposals and specials.

Soft start

A big transformer needs a soft starter. That got me reading up on these circuits and when to use them. This excellent article by Rod Elliott from Elliot Sound in Sydney explains it all.

Amazingly, a 500VA toroidal transformer may draw tens of amps on switch-on, depending on the mains phase at the time. Reading this left no doubt of the need for a soft starter. The operation of this module is simple. When the mains is switched on, the small 12v transformer, bridge and 220uF filter capacitor produce about 15VDC which close a relay after a few hundred milliseconds. The relay contacts short out a 50 ohm 10 watt resistor in series with the Active 240VAC line to the main power transformer, which therefore sees about half mains for a fraction of a second. The two LEDs indicate that DC is available to the delay circuit (green), and that the relay has power (red).


The DC power supply supplies a healthy 145 volts under no load. Completion of the associated switching regulator module will allow for some power to be delivered into a suitable load. That’s the next stage of this project.

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2 thoughts on “A 300 Watt DC Power Supply for a QRO AM Transmitter

  1. […] a 150v HT had to wait until the mains power supply was built. This uses a partially rewound 500VA toroidal mains transformer, rectifier bridge and filter bank. […]


  2. […] run cool due the use of switching designs.Previous posts describe the switching regulator, 300 watt DC power supply and dummy loads for this […]


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