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How does a Power Transfomer Work
Written By: John R. Sewell

In this discussion we will cover basic information on 120/240V 47-63Hz Line operated transformers and their uses and is not meant as a definitive design guide. Power Transformers are rated several different ways; by VA (Volt-Amps), VAR (Volt-Amp-Reactive) and W (watts). The most common rating is the nameplate VA with the Output Voltage. VAR is mainly used in conjunction with distribution systems and motor applications. In Simple resistive circuits the VAR is near zero the W and VA will be almost equal. So back to the basic Power equation P=E times I; solving for current; I=P/E therefore a 12V/24VA transformer can be approximately rated 12V at a current of 2Amps. The smaller the transformer, the more the internal resistance and efficiency play a role. A transformer under 30VA can have a 20-30% loss, no load to full load.

Most common hobbyist circuits uses a straight forward linear type circuit where you have a AC Line input. First, the line connects to the transformer where the voltage will either be increased to a higher value or more commonly, to a lower the voltage value like 12V. 12V, 1A Center Tapped (6-0-6) Transformer The Line is said to be connected to the “Input Side” or the Primary. The output is then called the Secondary. Transformers can have multiple windings. A common type is the Dual Primary where you have 2 windings allowing you to connect them in Parallel for 110-120V use or in series for 220-240. The Primary can also be a “Tapped” where the winding is a single with multiple connections available to adjust for a High/Low line or 220V. Also, a common type is the multi-winding secondary that provides isolated windings for multiple circuits like a + 12V & a -24V supply. The windings can be identical so that they could be connected in parallel for 2 times the current or in series for 2 times the voltage. Specifications usually listed as: 12/24V@4/2A meaning 12V@4A Parallel or 24V@ 2A Series. The junction of the Series connection can be used as a Center Tap (CT). In some transformers the “junction” is internal so the windings cannot be paralleled. These are listed a 12-0-12 meaning 24V across whole winding or 12 between end & Center tap (CT).

At this point we can connect to a “rectifier” circuit to provide us with a pulsing DC output. This raw DC voltage is dependant on the secondary rating and the type of “Filtering” and rectifying circuit used. The rectifier/filter circuit will affect secondary & output current and they can be very different.

Some examples are:
- Full wave center tap choke input filter: Transformer secondary amps are .7 X required output amps.
- Full wave center tap cap input filter: Transformer secondary amps are 1.2 X required output amps.
- For Full wave bridge choke input filter: Transformer secondary amps are = to required output amps
- For Full wave bridge cap input filter: Transformer secondary amps are 1.8 X required output amps.
- A 2A supply would require a transformer rating 2 X 1.8= 3.6A minimum secondary rating.

Volts & Current listed for small power transformers are nominal RMS (Root-Mean-Square). The Volt value is the voltage at the terminals with the listed Amp. load. Open circuit and loaded values are different as the lower the VA rating, the lower the inherent regulation. A 12V secondary will yield a Peak Value of 1.4 x 12V or 16.8V. Enter the rectifier and filter; the most common is a Full Wave Bridge Capacitor input type. So the theory here is to use 2000uf (microfarad) of filtering per 1A of load. So you would use 4000uf or more. In the bridge rectifier there are 4 diodes and with 2 diodes always in play. Each ½ line cycle so you have a loss of 1.2V/diode that equals 2.4 volts. If you add a regulator circuit, this will cause a loss of another 2-3Volts. Now putting all this information together; you have a raw DC supply of approximately 16.8V – 2.4Vdiode-3Vreg= 11.4V NOT 12V. So for a 12V 2A supply, you would need a Transformer with output around 15V @ 3.6A Minimum. There are other concerns that are too complicated to cover here but this will give you the basics.

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