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.

Transformer

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).

Performance

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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One thought 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. […]

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