Radio Friendly Power Supply
Circuit :  Andy Collinson
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Description
Power supplies should not create any unwanted interference across the radio spectrum. Switch mode power supplies are one of the strongest sources of interference, and harmonics can be heard throughout long, medium and short wave bands. This power supply, is extremely noise free, and therefore radio friendly. In addition, it has a very high rejection of line frequency and is suitable for powering receivers and small power transmitters.

Notes
T1 is the transformer, its primary voltage is chosen to suit the local electricity line voltage, e.g 120V for North America or 240V for UK. The secondary voltage and current depends on the load requirements. A common supply voltage is 12V so a secondary of 12V, 1A would be typical for a small receiver or transmitter. Sometimes secondary values are also quoted in VA units. A 12V secondary of 12 VA rating is designed to supply a load of 12 Volt at 1 Amp.

A working transformer generates a magnetic field at line frequency (50Hz for UK 60Hz for North America). The magnetic field extends for at least the diameter of the transformer core, although a sensitive amplifier with any inductive components may "hear" this field and produce a hum in its output at the line frequency.

A toroidal core transformer (shown left) has a smaller magnetic field than a conventional transformer and they are often used in high quality audio amplifiers. T1 can be a toroidal transformer or standard chassis mounting transformer.


Next a standard bridge rectifier or separate rectifier diodes are used to provide full wave rectification. In parallel with each rectifier is a small 100nF capacitor. Please also note that all individual 100n caps across each full-wave rectifier diode are to suppress diode conduction spikes, which collectively occur in this case at twice the line frequency 100Hz (for UK and Europe).

C1 is the filtering capacitor. The impedance at high frequencies of large electrolytics can be high so C1 may be paralleled with a 10uF electrolytic capacitor and a 100nF ceramic capacitor, if desired.

A standard regulator IC is used to stabilize the supply voltage. A LM7812 is shown in my circuit, but the regulator voltage should be chosen to match the supply voltage of the circuit. Also, the LM7812 (TO220 style) is rated at 1.5A max. It would also be beneficial to apply a little heat transfer compound and then push-clip a suitable heat sink on, such as Maplin code KU50E to dissipate heat away quicker from the device, to reduce the possibility of thermal runaway.

Regulators are also available with different output currents, the "M" series, e.g. 78M12 is rated at 0.5 Amp and "L" is rated at 100mA. For example 78L05 is a 5V regulator rated at output currents up to 100mA. In choosing the regulator the current requirement of the circuit to be powered must be known.

For added protection a back EMF diode is wired with reverse polarity across regulator input and output. This is the same in principle as a back EMF diode wired across a relay, and prevents any high voltage spikes from damaging the regulator.

Toroid Cores

The final components are an LC filter made with RFC1 and RFC2 and three 100nF capacitors. The radio frequency chokes have no effect at DC audio frequencies but present a high impedance to RF frequencies. With transmitter circuits any hum from the power supply must not reach the transmitter or oscillator stages. If hum did get through it would be broadcasted and heard in the received signal, RFC1 and RFc2 are designed to prevent this.

Notice that the chassis terminal is taken from one end of RFC2. Both RFC1 and RFC2 should be wound in the same direction. For circuits up to 1 amp try 20 turns of insulated 18 S.W.G. wire. For circuits requiring more current use larger diameter wire. The RFC are wound on Amidon FT 82-43 toroid rings. These are about 1 inch diameter, shown on the right.

An alternative to RFC1 and RFC2 is to take one ferrite core and wind turns in parallel. This is known as bifiliar wiring and can be seen below (the white wires over the yellow core).

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