I have a Transformer with the following writing on top of it :

Z150H E154515
LEI-4 8B25

it has three thick wires on one end : Blue Brown Red ; the resistance reading is almost Zero between any pair of them

the other end has 4 thinner wires Blue Yellow Brown and Red the resistance reading is ( in ohms ) :

  • Blue <-> Yellow : 1.2
  • Yellow <-> Brown : 12.6
  • Brown <-> Red : 1.9
  • Red <-> Blue : 13.4
  • Blue <-> Brown : 12.4
  • Yellow <-> Red : 13.9

Note :

  • the Digital miltimeter reads 0.8 ohms when in short circuit
  • the Transformer has been used before

I'm using this to replace a transformer of car battery charger which should take 220V AC in and output a 12 or 24 V AC

Q: which is the primary and how is the correct wiring of this transformer ?

  • \$\begingroup\$ How can you be sure it has a 220V primary rating? \$\endgroup\$
    – Andy aka
    May 24, 2015 at 18:37
  • \$\begingroup\$ @Andyaka I'm not sure , how can I confirm this ? \$\endgroup\$
    – Chebhou
    May 24, 2015 at 18:39
  • \$\begingroup\$ @Chebhou: by reading the datasheet \$\endgroup\$
    – PlasmaHH
    May 24, 2015 at 19:50
  • \$\begingroup\$ @PlasmaHH if i had a datasheet i wouldn't be asking \$\endgroup\$
    – Chebhou
    May 24, 2015 at 19:51
  • \$\begingroup\$ The secondary is most probably the one with thicker wires, but (as @Andyaka pointed out) you must be sure its primary is rated for mains operation. If it is not, you put your life at risk by connecting it to mains! You said it was used. Can you retrieve the equipment from which it was removed and verify if it was connected to mains? \$\endgroup\$ May 24, 2015 at 23:35

1 Answer 1



First, make a table with the wire colors shown and the measured resistances between them arranged so that there are no crossovers, otherwise it's confusing - for me anyway - to see what's what.

enter image description here

Next, draw the transformer and plug in the resistances you found according to the relative lengths of the winding they'd occupy.

Since you have continuity between all the taps there's only one winding and, assuming it's all made with the same wire, the resistances you've given indicate that the highest resistance, 13.9 ohms from yellow to red, would appear across the entire winding.

Then, two taps would be made in from the two ends, one from yellow to blue and the other from red to brown, with the remainder of the winding appearing as 12.4 ohms between blue and brown.

There seems to be a discrepancy between the sums of resistances, and I suspect that may be due to measurement error but, in any case, that's how you you can find out what's what with an ohmmeter.

enter image description here

With thin wires on on one side of the transformer, three thick wires on the other, and a very low resistance between the thick wires, the thin wires appear to be connected to a high-voltage low-current primary, with the thick wires connected to a low-voltage high-current secondary, probably center-tapped.


Since yellow/red appears to be the entire primary, what I'd do would be to use a VARIAC to SLOWLY increase the voltage across yellow/red while monitoring the transformer's input voltage and current, and unloaded output voltage.

That would pretty much allow me to basically characterize the transformer, and later on, with a load, nail it.

Failing that, I'd connect about a 40 watt incandescent in series with the primary and the mains, and measure the transformer's input and output voltages to get the turns ratio, for starters, and to determine whether the thing was any good.

  • \$\begingroup\$ Thanks for the logical detailed answer I'll try it when I'm around the transformer and comment back \$\endgroup\$
    – Chebhou
    May 25, 2015 at 23:05

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