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  • 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.

    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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    46 Responses to "Step Down Transformer"

    1. State how transformer can be re coiled both step down and step up

    2. Capacitor says: on August 11, 2012 at 10:31 pm

      Plz sir how to figure out the maximum wire guage in stepdown tranx plz help me

      • Jim Keith Jim Keith says: on August 12, 2012 at 2:22 pm

        The wire size (gauge) is determined by the current that it must conduct. To provide an acceptable temperature rise in transformers, the rule of thumb is 500 to 1000 circular mils per amp–the larger provides a lower temperature rise. A circular mil is simply the diameter of the wire (in thousandths of inches) squared. For instance, AWG #18 wire has a diameter of 0.0403″ or 40.3 thousandths of inches–squaring this equals 1624 circular mils. The current rating of this wire size in a transformer = 1624/500 to 1624/1000 or 3.2 to 1.6A. Rating it at 1.6A provides 0.25% the temperature rise of 3.2A because P=I²R, but it makes for a larger, more expensive design.

        Download this valuable reference guide:

    3. Plz sir can u giv me d fomular…2 calulate d input(primary) and output(secondary) current of a transfomer..to select d maximum AWG ..plz help becoz electricity is dangerous it can kill help,help,h…,..e……l……p oooooooooooo

    4. Can some1 helpp me plz hi nid more details ..on step down …giv 4 details how to calculate d wattage,current,voltage,KVA..both input and output of a transformer.hurry and reply plz ….author contributor of electroshematics.com plz help..hi nid ur assitant over me

    5. Jim Keith Jim Keith says: on August 15, 2012 at 2:46 pm

      I assume that you are attempting to estimate the power rating of an unmarked surplus transformer.

      Estimated VA ranges from 43VA /kG (for 0.6kG) to 100VA /kG (for 10kG)

      Wattage = VA (for resistive load)
      Wattage < VA (for inductive load or non-sinusoidal current waveform)

      Max Input current = VA /input V

      Max Output current = VA /output V

      Select wiring ampacity per electrical code.

    6. Yehhhh!???????? K..how to select AWG for primary and secondary of xtrans….plz help hi am yet to complete my inverter project becoz of dis problem …can som1 rise up and giv me a full explanation…….for how to calculate d current pri/sec of a xtrans and giv me correct fomular…@least 2 example for better understanding and …once again fomular to calculate d wattage of a xtrans is require for any electronics project……..as u help me may God help u too plzzzzzzzzzzzzzzzzzzzzy reply very….faster like..hi can understand jim keith reply post …….. ..DMOHANKUMAR……P.MARIAN…..also som of my frnds at schematics have mercy on me dis is my post comment watever ideal u may reply….plzzz ELECTRICITY IS DANGROUS….hi av 2 b carefull…so help…. ….,.me

    7. Cry!!!!!!! Jim keith give me a full explanation …….FOMULAR to calculate current of primary/secondary of a xtrans and some relevant example for better understanding by knowing dis it will b easy for me to select maximum AWG dat hi can for both pri/sec of xtrans…. .hi am expecting ur reply

      • Jim Keith Jim Keith says: on August 15, 2012 at 8:56 pm

        Will help as much as I can, but you must provide particulars–please answer the following:
        ? KVA of transformer in question, or KVA of inverter
        ? Primary voltage
        ? Secondary voltage
        ? Isolation transformer or autotransformer
        ? Single phase or 3 phase

    8. (1) electricity is dangerous (2)it can kill (3)everyone is aware d advantage/disadvange of electricity (5)plz reply me very fast and don,t seek my down fall

    9. Actually (1)KVA of inverter=1KVA (2)primary voltage=220VAC (3)secondary voltage=10vdc (4)3phase which means 3anode xtrans ..fact don,t tell me directly give d fomular 2 calculate it……..and some relevant example

    10. Jim Keith Jim Keith says: on August 15, 2012 at 10:59 pm

      I assume that you are mainly interested in the heavy secondary wiring.

      DC rectifier output: Idc = kW /Eo
      AC rectifier input (3 phase transformer secondary) Iac = Idc * 2/3
      3 phase transformer primary:
      Iac = kW /Ein /1.732

      For wiring ampacities, refer to:


      • Jim Keith Jim Keith says: on August 16, 2012 at 2:35 am

        It is assumed that you wish to convert all of your 1kW (1000W) inverter power output to 10VDC.

        DC current (10v) = P/Edc

        From transformer to bridge (3 phase AC)(I secondary RMS) = Idc * 0.81 (this is due to RMS value of rectangular waveform–correction from previous post)

        From inverter to transformer, I primary = P/V/1.732 (standard 3 phase convention)

        This assumes that transformer has proper turns ratio–there are many more details, but they are unlikely to affect wire size unless your DC current requirement is actually less than maximum capacity

    11. Thank

    12. Nonsco1023 says: on September 5, 2012 at 3:30 am

      Plsss sir, i’m new comer here help me how to determine effective area of iron core nd how to know the size i used. I’m awaitin reply sir

      • Jim Keith Jim Keith says: on September 5, 2012 at 6:43 am

        There are two common transformer area details- -the first is the cross section area of the center leg of a standard EI lamination- -the 2nd being the area of the window where the windings go. For lamination dimensions and identification, refer to:

    13. What switch is used to change voltage from higher to lower voltage?

      • Jim Keith Jim Keith says: on October 17, 2012 at 4:19 pm

        The ratio of the number of primary turns to the number of secondary turns is called the turns ratio. The voltage ratio is identical to the turns ratio.

    14. Answer my question please sir..

    15. hello great men,
      I want you to help me with the name of the device that can be use in place of transformer.I so one inverter about 2kva to 5kva without transformer.

    16. suresh kumar aran says: on November 28, 2012 at 1:31 pm

      how will we Step-Down Tranformer protection?

    17. Hi,
      I’m very bad at the maths n hope someone can help me
      i have been using this step up/down transformer, i can choose 110v/220v as input and come from a place of 220V.
      i have been putting it at 220V input and have been using many products with no problems. the setting is fixed.

      i just plugged in a product with 120v (60 cycles AC) and saw sparks at the area where the product plug is inserted into the transformer.

      fuse damaged.

      is there some products in the market with 120V output or why am i having this problem? the product is

      Thermal fuse protection
      Overheat- cut off
      Cool down- reconnected.
      150Watt Max

      input AC 110C 50/60Mhz Output AC 220V
      Input AC 220V 50/60mhz Output AC 110V

      did i get the wrong product or 120V isnt going to work on a transformer?

      Pls help. Thank you


      • Jim Keith Jim Keith says: on December 10, 2012 at 9:25 pm

        Assuming that you have a 220VAC line, it appears that is is being used correctly.
        What is the type of load and its power or current rating?

        DUT (device under test) could be defective–replace fuse and connect a different device to see if it is working, or measure output with voltmeter.

        Also note that Isolation transformers behave poorly when connected to a half-wave rectifier. (DC component saturates transformer and blows fuse.)

    18. Nuur Azeerah Baharruddin says: on December 14, 2012 at 12:28 pm

      Hello.. if anyone of friend here can give any sample of calculation related with step-down transformer.?

    19. Nonsco1023 says: on December 17, 2012 at 9:07 pm

      Pls sir, in a step down trafo which its primary wire diameter is 0.32mm or 30swg and secondary wire diameter is 0.62mm or 23 SWG then if i want to change it bigger diameter like 0.62mm(23) for primary and 1.00mm(19) for secondary. How can i achieve that sir.

      • Jim Keith Jim Keith says: on December 17, 2012 at 9:15 pm

        You can do it provided that all the turns fit within the lamination winding window. But changing the lamination size changes many things.

    20. Nonsco1023 says: on December 17, 2012 at 10:59 pm

      Sir if the lamination winding widow is not changed, it remain the same. How can i determine turns of the both while the gauge have been changed to bigger gauge. For instance Np=1714 of gauge 0.32mm and Ns=257 of gauge 0.62 now the gauge is bigger primary 0.62mm and secondary 1.00mm. What should i do to get the turns, if there is any calculation for that tell me sir.

      • Jim Keith Jim Keith says: on December 18, 2012 at 1:54 am

        You must use the same number of turns–and rarely will they fit in the available window. I do not know off hand the conversion to aluminum, but I have heard of others that increase the wire size of aluminum so that the resistance is the same. Rewind with copper–that is my suggestion–better in many ways.

      • Jim Keith Jim Keith says: on December 18, 2012 at 4:14 am

        Correction: Wire size is downsized when replacing aluminum windings with copper. In this case, copper fits easily. I do not know anyone that replaces copper with aluminum.

    21. Nonsco1023 says: on December 17, 2012 at 11:15 pm

      Secondly sir is there any calculation formular for conversion of aluminium wire to copper wire diameter pls sir help me on this.

    22. Nonsco1023 says: on December 18, 2012 at 10:36 am

      Thanks sir but my concern here is to changed aluminium wire diameter gauge with copper wire diameter which i do’nt no how

    23. please help me I have to built a step down transformer.. send me the basic formula of iron core diameter and number of turns of primary and secondary coil and SWG of both coils………

    24. rabiul islam says: on July 23, 2013 at 12:12 pm

      please help me I have to built a step down transformer.. send me the basic formula of iron core diameter and number of turns of primary and secondary coil and SWG of both coils with respect to ampere

    25. Hi there, i found this page where it explains how the transformers work. I am building a machine i need to transformers that do exacly the oposite.

      Transform low voltage in high voltage.

      transformer 1:5 ratio 220v

      Do you know where i can buy one?
      Thank you!

    26. Dear sir,
      Requirement: For my project i need variable voltage constant current step down output.How can i get it.
      With regards,
      Anandaraj M

    27. Muhammad says: on January 3, 2014 at 1:03 pm

      i have to install 1500KVA 11/0.4 KV transfomer , to be installed for a high rise building along with TRM .I got instructions to install control wire 2core2.5mm also ,unfortunately i am new in this field i just dont know where to install these control wires ? can anyone help me regarding this ? how to connect control wire with transformer and what does that mean ?

    28. Sir can you help me to my problem ? How did the primary winding and then secondary winding connect ? Because tomorrow i need to present the diagram of halfwave rectifier circuit :( please can you help me? Reply asap

      • Note that it is not a good practice to use a half-wave rectifier with a transformer –it causes a DC offset current that causes the transformer to saturate in one polarity of the AC waveform thus greatly increasing its magnetizing current in that polarity and also causes substantially increased temperature rise.

        On the other hand, half-wave rectification will function OK for light loads and can be used for demonstration of such.

        One solution is to connect two half-wave rectifiers in opposite polarity so that the DC components cancel –this is how a voltage doubler functions.

        A full-wave bridge or full-wave center-tapped configuration also solves this problem.

      • Corrected transformer numbering

    29. 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

    30. primary coil awg gauge size says: on December 4, 2014 at 4:43 pm

      i want 220v to 12v 5amp transformer primary coil awg gauge size,
      my country support 220v so pls help me find out right gauge size

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

    32. 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.

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

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