transformer questions.

Discussion in 'General Electronics Chat' started by DougB, Dec 22, 2010.

  1. DougB

    Thread Starter New Member

    Sep 11, 2010
    I have an old transformer. I know it is 117 vac in, 16vac out. What I cant identify are the primary and secondary leads. There are a pair of leads that are solid copper. On the other side are wire stranded leads. I suspect the solid copper leads are the primaries but am not sure.

    I also have a transformer that is labelled and wired (0-117, 0-117) in and (0-6.3, 0-6.3) out. Both outputs are at 5 amps. I would like to make make a 12.6 volt out but am unsure on how to jumper and or connect the leads on the transformer
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    The heavier gauge leads are more likely to be the secondary connections.
  3. tyblu


    Nov 29, 2010
    In a step-down transformer, the smaller wires are the primary. This is because they carry less current. Ideally your current will step up the same amount that voltage steps down. Practically it is a bit less than that due to losses.

    Just to double check -- the second Xfmr has 8 wires, right?
  4. DougB

    Thread Starter New Member

    Sep 11, 2010
    Just so I am clear. The first transformer has 2 leads that are solid copper. About 14 gauge I would guess, same as household wiring. Other set of leads are not solid but stranded. I would guess that these are maybe an 18 or 20 gauge. So the solid copper leads are the secondary's???

    The second transformer is not yet wired but does have 4 connections per side, with inputs and outputs labeled as per the original post. Need 12.6 volts out of if I can.
  5. debe

    Well-Known Member

    Sep 21, 2010
    Check the resistance of windings, 117v should be higher resistance than 16vwindings.
  6. #12


    Nov 30, 2010
    The fatter wires are the output.

    as for the second transformer, the secondaries are to be connected in series. If you get a total of zero volts, reverse the polarity of one of the secondaries.

    The primaries will be connected in series if you are using 220V mains, and in parallel if you are using 115 volt mains. It is smart to do tests at low voltages to confirm the polarities of the windings.
  7. someonesdad

    Senior Member

    Jul 7, 2009
    If you have access to a function generator, then you can have it output a 1 Vrms sine wave and apply it to one of the pairs of windings. Measure the output voltage on the other winding and the voltage ratio will tell you which is the primary winding and which is the second.

    However, you need to be careful; if you apply the voltage to the secondary, the primary may have a hazardous voltage on it if the step-up ratio is large. So it's something that has to be done with care. If you've not done this before, ask for help and someone will give you some instructions. Note that a typical function generator can output 10 Vrms into 50 ohms, which means a 200 mA current. If that 1 volt were stepped up by 100 times, the current level would drop to 2 mA. Thus, it's unlikely there's any real hazard; still, it's good practice to make the measurements in a safe manner.

    You can also use a filament transformer for the excitation voltage instead of a function generator. The second transformer you mentioned could serve this duty, although now the current supply is much higher than a function generator and you could be dealing with hazardous potential currents with a large step-up ratio.

    Another technique is to estimate the number of windings by measuring the wire diameter, average winding radius, and measuring the DC resistance. You can make a rough estimate (repeat, rough!) of the number of turns of a winding by using the resistance to calculate the linear distance of wiring; then use the average winding circumference to estimate number of turns. Of course, it's a crude estimate and depends on the other windings there too.