Electrical Transformers Tutorial

DC Isolation in Transformers

• A simple transformer is basically a pair of insulated wires wrapped around a common iron core. AC (alternating current, like from your wall electrical outlet) voltage is induced in the secondary winding as magnetic flux (magnetic fields surrounding wire when current flows) changes with voltage variations in the primary winding.

  There is no DC (direct current with no changes, like from a battery) connection between the primary and secondary windings; only AC passes from primary to secondary.

  One way to quickly check if a transformer is still good is to use an ohm meter on a transformer disconnected from the rest of the circuit. The reading with one ohm meter lead on a transformer primary terminal and the other lead on a secondary terminal should be open (meter display flashes or reads OL for over limit).

  If the ohm meter reads a resistance under 100,000 ohms, there is a short or leak between the windings and the transformer is damaged.

Turns Ratio

• The number of primary turns (windings) compared to secondary turns is the turns ratio. This ratio determines how many volts will be induced in the secondary by the primary. For example, for a 500 turn primary and 200 turn secondary, 100 VAC (volts-AC) on the primary will induce 40 VAC on the secondary.

  This is an example of a voltage step down. A step down is used at the receiving end of a power transmission line or as the first step for a power supply (like in a radio or battery charger) converting 110 VAC at your wall outlet to power the equipment can use.

Power is Constant

• In an ideal transformer, total power on the primary equals total power in the secondary. For the example above, 100 VAC at 50 mA (50 milliamps or 0.05 amps), giving a total of 5 watts (100 V times 0.05 A = 5 W) on the primary will produce 40 VAC at 125 mA ( 40 V times 0.125 A = 5 W) on the secondary.

  Inefficiencies in a transformer result in waste heat at the transformer (requiring air or other cooling) and higher operating expenses. More efficient transformers are also heavier and more expensive to make, forcing a trade off for designers.