Transformers, Power


The Power Transformers listed on our website are those you find and use in electronic equipment as opposed those used by the local power company. A Power Transformer is two or more coils of wire wrapped around a common steel core and used to change a higher voltage to a lower voltage, a lower to higher or both and provide isolation. For Encased unit with built in AC Plug and Low Voltage terminal strips used in alarm systems etc see our Plug Supply Department. Our Power Transformers are 120/240V 47-63Hz (AKA 110/220V) Line operated. Power Transformers are rated several different ways; by VA (Volt-Amps), VAR (Volt-Amp-Reactive) and W (watts). The most common rating is VA. In Simple circuits the VAR is near zero the W & VA will be almost equal; therefore a 12V/24Va transformer can be approximately rated 24 Watts or 12V at a current of 2Amps. The smaller the transformer the more the internal resistance & efficiency play a role. A Power transformer under 30VA can have a 20-30% loss no load to full load so size you transformer accordingly and never use one at its max output. The Line is said to be connected to the “Input Side” or the Primary. The output is then called the Secondary. Power Transformers can have multiple windings. Common is the Dual Primary where you have 2 windings, this allows you to connect them in Parallel for 110-120V use or in series for 220-240VAC. The Primary can also be a “Tapped” winding where the winding is a single winding with extra connections available to adjust for High/Low line or for 110/220V. Also common 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 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 as 12-0-12 meaning 24V across whole winding or 12 between either end & Center tap (CT). The Power Transformer secondary can be connected to a “rectifier” circuit to provide us with a pulsing DC output. By adding a capacitor, the pulsing can be smoothed. This raw DC voltage is dependant on the secondary rating and the type of “Filtering” & rectifying circuit used. The rectifier/filter circuit will affect secondary & output current and they can be very different. Full wave center tap, cap input filter: The Power Transformer secondary amps are 1.2 X required output amps. Requires 2 rectifiers. This is one of the most common circuit used. For Full wave bridge cap input filter: The Power Transformer secondary amps are 1.8 X required output amps. Requires 4 rectifiers. This is also one of the most common used. Full wave center tap choke input filter: The Power Transformer secondary amps are .7 X required output amps. For Full wave bridge choke input filter: The Power Transformer secondary amps are = to required output amps A 2A supply would require a transformer rating 2 X 1.8= 3.6A minimum secondary rating. Power Transformers, Variable Transformers Our Power Transformers are priced right! Every Benchtop needs a Variable Power Transformer when you need a variable source voltage. Power Transformers, Variable Transformers

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