Step Down Transformer

What is a step down transformer: is one whose secondary voltage is less than its primary voltage. It is designed to reduce the voltage from the primary winding to the secondary winding. This kind of transformer “steps down” the voltage applied to it.

As a step-down unit, the transformer converts high-voltage, low-current power into low-voltage, high-current power. The larger-gauge wire used in the secondary winding is necessary due to the increase in current. The primary winding, which doesn’t have to conduct as much current, may be made of smaller-gauge wire.

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Step-Down Tranformer Considerations

step down transformerIt is possible to operate either of these transformer types backwards (powering the secondary winding with an AC source and letting the primary winding power a load) to perform the opposite function: a step-up can function as a step-down and visa-versa. One convention used in the electric power industry is the use of “H” designations for the higher-voltage winding (the primary winding in a step-down unit; the secondary winding in a step-up) and “X” designations for the lower-voltage winding.

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One of the most important considerations to increase transformer efficiency and reduce heat is choosing the metal type of the windings. Copper windings are much more efficient than aluminum and many other winding metal choices, but it also costs more. Transformers with copper windings cost more to purchase initially, but save on electrical cost over time as the efficiency more than makes up for the initial cost.

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Step-down transformers are commonly used to convert the 220 volt electricity found in most parts of the world to the 110 volts required by North American equipment.

How to Wire a Step Down Transformer

  1. Observe and identify the schematic and rating of the step down transformer to be installed. Remove the terminal connection box cover placed at the lower side of the transformer. Only the high amperage types will have this enclosure, while lower powered transformers will have an exposed screw terminal.
  2. Know termination identification follows for all step down transformers: H1, H2, H3 and H4 signify the high voltage side or power feed end of the transformer. This holds true regardless of the size of the transformer. Interconnection of the transformer will vary depending on the manufacturer and voltage used for feeding the transformer.
  3. Terminate the feed power wires first by cutting the wires to length. If you are using large wire lugs be sure to take into consideration the length of the lug and the amount of wire that can be inserted into the female crimp area.
  4. Strip back the outer insulating of the wires with the pocketknife or wire strippers. Insert the eye ring or wire lug over the bare copper wire and crimp the connection device, using the appropriate-size crimper, permanently to the wire.
  5. Terminate the high side, high voltage of the step down transformer. If the high side terminals are bolts, be sure to follow any torque requirements that are listed by the manufacturer.
  6. Terminate the low side, low voltage of the transformer. Note these terminals will be identified by X1, X2, X3 and X4. Again follow the manufacturer’s individual schematics for that particular type of transformer. Note that on small control transformers there will only be an X1 and X2. X1 is the power or “hot” side and X2 is generally the grounding and neutral portion of the low voltage.
  7. Terminate the small control transformer for X1 and X2. X1 will go directly to the control circuit after passing through a small fuse that is rated for the circuit. X2 will be terminated not only to the neutral side of the control circuit, but the grounding safety as well. In other words, the X2 side of the small control transformer must be tied to the grounding system of the electrical circuit.
  8. Replace all covers on the transformer and any enclosures that protect you from electricity. Apply the high voltage to the transformer by switching on the feeder power circuit. Turn on the low side safety circuit control.
  9. Use a volt meter to test for proper voltage on the step down side of the transformer. It should be the same that is listed on the specs tag provided by the manufacturer.

How to Check a Step Down Transformer

  1. Remove all wires from the transformer terminals using the screwdriver. Identify the wires if they are not already identified. Use a clear tape and pen. Write the terminal that the wires are attached to and place the identified tape on the wire’s end.
  2. Turn the volt ohmmeter to the “Ohms” position and place the red lead into the connector identified as “Ohms.” Touch the black lead to the metal frame of the transformer.
  3. Touch the red lead to the transformer’s terminals in the following order: H1, H2, X1 and then X2. The meter should read infinite ohms or wide open. Infinite ohms on a digital meter will be identified as a blank screen or a wide open will have the word “Open” displayed. If the meter registers any form of resistance, there is an internal problem with the windings. The copper coils may be shorted to the metal frame of the transformer. The transformer will have to be replaced.
  4. Check the continuity of each separate coil using the ohmmeter. Touch the black lead to H1 and the red lead to H2. The meter should give a resistance reading. Generally, it should read in the range of 3 to 100 ohms, depending on the style and type of transformer. Perform the same test to the X1 and X2 terminals. You should receive the same results. If the meter reads infinite ohms or a wide open when checking between the terminals of the same coil, the wires are broken. Replace the transformer.
  5. Use the ohmmeter to conduct the transformers isolation circuit. Touch the red lead to H1 and the black lead to X1. The meter should read infinite ohms or a wide-open circuit. Perform the same test, but to H2 and X2 respectively. If any resistance at all is read on the meter other than a wide-open circuit, the isolation of the transformer has been compromised and must be replaced.

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Join the conversation!

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  • Raghu

    hello sir,
    pls can you help me in knowing in depth details of transformers.
    how to measure the inductance at each pin.
    i confused about primary,s1,s2 and how they made and work?
    how to count turns for each section

  • Innocent Abhishek

    What is the apparatus Required to construct a 3 Phase step down transformer..

  • bhuwan sapkota

    what is increased in step down transformer,current,power,voltage or frequency

  • Potential

    Pls I’m a new comer here, I need ur help. I wants to design core type step down transformer of 220v/110v and Frequency of 50Hz, I wants to know the (1) size of copper wire to use for primary winding and number of turns then. (2) size of copper wire of secondary winding and number of turns (3) The iron core size. Pls help

  • b.k.vadehra


  • rakesh pandey

    Sir pls tell me ….15-20 ampere three phase step down transformer how to make …I am using this transformer in small electric arc furnace to melt iron chips …through graphite rod …and tell me can melt???

  • okpala michael

    sir pls tell me how to calculate for
    transformer turns and the size of wire
    to be used for 2kva inverter.
    specifications are:
    input voltage: 24vdc
    output voltage: 240v

  • kevin

    Have voltage on neutral line what is the posible cause.. please help.. thnx

  • Neil

    hi, I’m planning to build a transformer to be use for power tools came from US. I have a step down transformer with 240v input, 120v output and 1000va. what’s the maximum amps I can get for this transformer on my output ? my input power would be 220v. what amps of fuse do I need to put in on the input and output. thanks in advance.

  • jims

    how many tuns of wire at primary and secondary for input 220 VAC Output 12 vac and out put 2A

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