Choosing your power supply unit for your radio

Choosing you power supply unit for your radio

First a quick analogue electronics lesson, a moving coil ammeter will only measure the 1^st time constant of its measurement.

Why so:-

The ammeter circuit has a shunt resistor, but the moving coil has an inductance, and an internal resistance, which equates to a time constant.

The shunt resistor develops the current drain into a voltage drop across the moving coil ammeter, upon which the moving coil then measures the current drain as a relative voltage.

Unfortunately, if the ammeter was designed to read a top end scale of 30 Amps, then the ammeter measurement scale reading would have to take into account the time constant of the moving coils internal circuit.

If a current drain of 22 Amps is measured, then this would be the 1^st time constant, equates to 66% of measurement, upon which the actual current drain would thus perhaps be :-

( 1 / 66% ) * 22 amps = 33 Amps, at full tilt.

A 30amp Power supply unit would perhaps struggle with a 22amp radio on Tx mode, so a 40 Amp PSU would perhaps be more suitable.

Now there is reason for this, as I just watched a YouTube video relating to a new QRP radio. The power lead was connected to a digital power measurement unit, where the voltage, current, watts, was measured.

I noticed that on CW mode, the radio used around 24 watts, but 7 watts RF was measured by the RF power meter. On other bands the radio gave 6watts RF power.

The significant point being, that the radio on CW transmit, was essentially 25% to 30% efficient.

With a 100 watt radio, if the same CW Tx mode test is conducted, the then 100 watt RF, perhaps could relate to four or three times that upon the DC power demand requirements.

To use a 100watts radio on CW Tx mode, a 300 Watt or 400 Watt DC supply may well be required.

At a rough calculation :-

300watt / 12Volts = 25 Amps,

400watts / 12 volts = 33 Amps

From this, it can be thus seen, that a 40Amp DC supply is essentially required just to give a bit of margin to the power supply design.

It is quite possible to link in parallel two power supply units, say two 40 Amp unit to create a 80 Amp supply. Each 40 Amp psu would need essentially require an out going rectifier diode, whereby the diode is forward biased when the psu current supply to running. The rectifier diodes are there to prevent back flowing into a power supply unit that is lacking in its supply voltage.

To align two or more parallel linked power supply units, assume that we require a 66 Amp supply for a 200 watt radio. The two 40 Amp psu's would each in reality supply 33 Amps. To set the over psu supply, set the radio to say a ten Amp current drain from one psu.

Switch on the second psu, and adjust the output supply voltage until the current drain meter reads 5 Amp on each power supply unit. The combined current supply would still 10 Amps, but spread over two power supply units.

When the 200 watt radio sparks up into life, the 66 Amp current drain would be spilt into two 33 Amp loads from each of the two power supply units.

Bear in mind, if the 200 watt radio is 25%, then a 800 watt power supply unit is required. The 800 watts at 12 volts, equates to the 66 Amps spread into two 33 Amp power supply loads.