Power supplies.

Types.
There are two main types of power supply, the standard linear and switched mode types. The linear regulated types can be either series or shunt regulated.
Switched mode power supplies are more efficient and run cooler but, as they run at high frequencies, they may radiate RF signals that can interfere with radio equipment.

Legal requirements.
As the supply connects to the mains supply, there will be legal and safety issues associated with such devices. If working on any device that plugs into the mains supply, it is recommended that you read up on the legal and safety requirements at your location.

Radio requirements.
A standard, legal mobile UK CB radio requires a voltage of between around 11.4 to 14.2 volts and a current of up to 2 Amps. The voltage should be smoothed and regulated - it should not have too much ripple and should not drift when the current used changes.
If it does not meet the requirements the CB radio may make humming noises or, even worse, the radio and the supply may be permanently damaged.

Ideal supply.
For equipment to last, it should not be run at its limits. Running at the limit will cause damage over time. The ideal supply will be capable of delivering slightly more current than is required by all the equipment which will be connected to it.
For a standard, legal UK CB radio, this would be around 13,5 volts and 3 Amps.

Linear power supplies.

Basic power supply.
A basic power supply, as shown on the right, uses a transformer to reduce the voltage, a rectifier to change the AC to DC and a smoothing circuit to reduce the ripple on the output.
This is not really good enough for CB use, as any changes in mains voltage or current used by the equipment may cause changes in the supply output voltage. This could damage a CB radio.

Regulated supplies.
To ensure that the output remains constant, regardless of changes in mains voltage and current taken from the supply, a regulator circuit is added.
As a CB radio uses more current on transmit than receive, it is important that the supply has this circuit.

Current issues.
The problem with the above supply is current. A basic regulator circuit cannot supply much current before it struggles to maintain a constant voltage at the output.
The answer to this is to add a current amplifier circuit. This has to allow current to bypass the regulator without allowing the voltage to increase.

Switched mode power supplies.

Switched mode supplies.
A switched mode power supply works by switching a DC supply on and off.
As any active components are always either on or off, rather than any level between the two, less energy is wasted as heat. Also, as these supplies are generally run at higher much frequencies than the mains supply, smoothing is much easier and there will be no mains 'hum' present.



Input rectifier and smoothing.
The rectifier converts the mains AC input to DC. The output from the rectifier will be rising and falling at either mains frequency or twice mains frequency, depending upon the type of rectifier circuit used.
The smoothing reduces the ripple from the rectifier stage, to produce a more steady voltage output to the chopper circuit.

Oscillator and chopper.
The oscillator creates a square wave to drive the chopper circuit, which then switches the power to the transformer on and off. These circuits run at a higher frequency than the mains supply, and the timing of the pulses will affect the final supply output voltage.

Transformer.
This converts the input voltage to the required output voltage and isolates the input and output circuits. For supplies with a high voltage input, this isolation can be important for safety reasons.

Output rectifier and smoothing.
As the DC has, once again, been converted to AC, this must be changed back to DC and smoothed as before. However, this time the frequency is higher, so smoothing the output is much easier than for the mains frequency input.

Feedback.
To regulate the output voltage, a feedback circuit controls the oscillator. To ensure isolation between input and output, this may come from another winding on the transformer or use some other component to isolate the two circuits.