Bredhurst Receiving and Transmitting Society


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Power supply unit or PSU

TRANSFORMERS

A simple transformers has one primary winding and one secondary winding but other combinations are possible as transformers can have one or more PRIMARY windings (two times 115 Volt used in series for 230AC and in parallel for 115AC in America. Transformers may have one or more secondary windings.

The voltage ratio of a transformer is the input voltage compared to the output voltage, ie. 240volts/12volts a ratio of 20:1 or 1:20 depending on which way you are comparing.

The number of turns of wire on the primary winding or input of the transformer can be directly compared to the secondary winding or output of the transformer, the ratio of the turns is the same as the ratio of the voltages.

Transformers are usually referred to as either "step up" or "step down" voltage transformers where the secondary voltage is either greater (step up) or smaller (step down) than the primary voltage.

Whilst the secondary voltage may step down, the current that may be drawn does the opposite, it can increase by the same ratio as the voltage. ie. Primary winding 240v 1amp or(240va) secondary winding 12v 20amp (also 240va) the voltage & turns ratio is 20:1. The size of the wire on the secondary must be capable of carrying the current to be used. On a step down (voltage) transformer the secondary wire will be much thicker if it is going to carry 20 amps, a 240va will be quite large in physical dimensions.

A step up transformer will have a wire size either the same or smaller than the primary winding. To step up the voltage more turns of wire are required on the secondary winding, so to try and keep the transformer to a practical size some trade off on wire size may be required.

Some transceivers of oriental origin have quite small transformers for their power output, they rely on the duty cycle being low. ie SSB use. This is one of the reasons transceivers are derated for AM and FM use.

A screen is inserted between the primary and secondary windings of a transformer and it is connected to earth.

The screen is a safety measure should the insulation between the primary and secondary windings break down. The screen however is usually a single layer of windings with one end connected to earth and the other left disconnected. If the other end were also connected to earth it would act as a shorted turn and greatly impair the transformer output.

Transformer power losses cause the temperature of the core and windings to increase, if too much power is drawn for a great length of time the transformer will burn out.

Auto transformer

If an auto transformer (nothing to do with cars or other vehicles!!) is to be used on the mains, care must be taken as there is no isolation at all from the mains, and is ALWAYS a dangerous thing to use if there is any chance of coming into contact with the output.

Under fault conditions lethal voltages will be around. Take for instance a 110V output auto-transformer will have an output of 110V with respect to earth or neutral, when working correctly BUT if the neutral INPUT goes open, the output will be at 230V AC with respect to earth - DEATH could loom!!!


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