I have removed a UPS transformer. It is a Class B Viking transformer. Its primary side has 4 wires coloured black, green, blue and yellow. I found resistance as follows- 1. Black to Green - 17 ohm 2. Black to blue - 14 ohm 3. Black to yellow - 15 ohm The secondary side have 2 thick wires and it's resistance is 0.5 ohm This is the UPS transformer

Now my question is if I hook up 230 V between black and green will it draw 13.5 Amperes? I don't think that much current will be drawn. Where I am wrong? Actually, I want to calculate the secondary winding's max current output but my multimeter doesn't read Amperes. But by transformer formula, I can find secondary current if the primary current is known. As Vp/Vs = Is/Ip = Np/Ns

My calculation - V=IR I = 230/17 = 13.5 A Seeking help, Thanks.

Edit - For 230V input Secondary gives 6 Volts.

  • 1
    \$\begingroup\$ A transformer is not a resistor! You need to read up on how transformers work. If you don't know this, I would recommend you not to hook it up to anything close to 230 V. \$\endgroup\$
    – winny
    Commented Oct 31, 2018 at 10:30
  • \$\begingroup\$ 230V is a standard supply here so there is no issue with that. Before hooking to 230V I verified that I am connecting to correct wires by checking the terminals of UPS PCB which supplies power to transformer. Can you suggest me some good links about transformer which will clarify how transformer windings not works as a resistor. \$\endgroup\$ Commented Oct 31, 2018 at 15:33
  • \$\begingroup\$ 230V is a standard supply here so there is no issue with that” You are missing the entire point! You ask basic questions which someone qualified would not need to ask, at which point you are running a high risk of electrocuting yourself or someone else, if not causing a fire hazard first. 230 V is lethal! \$\endgroup\$
    – winny
    Commented Oct 31, 2018 at 17:41
  • \$\begingroup\$ Yes 230V is dangerous but I take full precaution. At first time I had connected a 100 watt incandescent bulb in series with mains supply during finding out correct primary winding so there is zero risk of short circuit or burning of winding. Also I keep myself isolated from ground so even if I accidentally touch live wire, I will not get electrocuted. That's an extremely basic thing. Even an illiterate person knows that. I think it is useless to talk about this things here. I don't know much about transformers so I expected some knowledgeable persons will teach me. \$\endgroup\$ Commented Nov 1, 2018 at 0:40

2 Answers 2


The resistance of transformer windings can be a useful hint to people who know what they're doing when trying to figure out how a transformer is arranged internally. Unfortunately, it tells you nothing about rated or expected currents, or the voltages that are expected to be present on the windings.

Find a data sheet for it, or a piece of equipment that uses it and measure some voltages, before you apply any voltage to this, and certainly don't simply plug it into mains and see what it does.

The best way to start investigating an unknown transformer is to get another transformer that can deliver a safe low voltage, say 12v AC, and connect that to the highest resistance winding pair on the unknown transformer. Now make some voltage measurements to establish turns ratios.

Even after this, you still won't know whether a single high voltage winding is meant for 120 or 240 volts. Two identical high voltage windings are probably 120v each, intended to be used series or parallel. Several very low voltage windings connected to a high voltage winding are probably tap-change windings.


The 17ohm is most likely the DC resistance of the primary winding.

Assuming that the secondaries are not connected (no load): For AC, which you would apply, the much larger main impedance Zm=2*pifLm of the transformer would dominate. Therefore, the AC primary current I=230/Zm would be much lower than the 13.5A.

Assuming that the secondaries are short: The current would be limited mainly by the primary DC resistance of 17ohm, and the resulting current would be 13.5A (probably not a good idea).

With a load connected, the currents are load-dependent. So you have to specify the load.

There is a poor-quality picture of the transformer schematics e.g. here. You should try to find a better one before working with 230V.

  • \$\begingroup\$ I live in India and 230V is a standard supply and all equipment are designed for 230V. If I am correct, What I understood from your reply is that transformer winding don't work like pure resistors rather they have impedance also so combined effect gives higher resistance. I can calculate current if load is connected but what will be the current if I short circuit the secondary? Theoretically it should be infinite but it isn't, why? \$\endgroup\$ Commented Oct 31, 2018 at 16:00
  • \$\begingroup\$ Note that the short-circuit limit of 13.5 amps applies to the primary. In principle the secondary winding would be greater by the ratio of the primary and secondary voltages. In reality, the core would saturate at these high levels (unless the transformer is designed to handle it, as in an arc welder), and the actual current would probably be a good deal lower. \$\endgroup\$ Commented Oct 31, 2018 at 16:11
  • \$\begingroup\$ If you short the secondary, the stray inductivity governs the behaviour of the circuit. That's actually an important figure for any transformer. Short the secondary, then measure at which primary voltage the rated secondary current is flowing. That voltage in relation to the rated primary voltage is given as short circuit impedance in % and tells how much the secondary voltage is dropping according to the secondary current. \$\endgroup\$
    – Janka
    Commented Oct 31, 2018 at 16:11

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