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Could use some help identifying transformer leads.

I've got myself an old Hammond 270X transformer for a project that I'm going to be working on, I looked up the datasheet for it (http://www.hammondmfg.com/pdf/EDB270X.pdf)however mine is really old and has cloth covered leads that have faded the same color now. I've hooked up my ohm meter and found four wire groups that have continuity. There are three sets of 2 wires and there's this 3 wire bunch. In the datasheet it shows the 480 volt winding being center-tapped, however it also shows a 115-125 volt center-ish tapped selectable primary I'm assuming since mine is really old that it doesn't have the selectable 115-125 primary and that the three wire bunch is the 480 volt secondary. How could I find the primary winding. (I really don't want to smoke another transformer lol.)

How would I go about finding which winding is which.

Thanks in advance -Ray, KD2JID


Active Member
Sort out the thickest wires to a matched pair ie continuity, they will be either the 5V or 6.3V windings.

Feed 5v AC (or 6.3V AC) in to one of the pair and measure the others.

You should measure close to what the other windings are supposed to deliver so the mains windings will be those delivering somewhere in the region of 115-120V (could be marginally higher or lower).

If the leads are all terminated with stranded wire leads, the 5V or 6.3V windings will be the lowest resistance ones


Well-Known Member
Most Helpful Member
According to the specification, the 480 V winding will be around 500 Ohms. The mains winding will be around 15 Ohms. You should be able to identify those on the basis of their resistance. The low voltage windings are really low resistance so could be easily confused.

If you avoid the low-resistance windings, and you have a 1 or 2 A slow-blow fuse in circuit, it will be hard to damage the transformer. If you power the 480 V winding with 120 V mains, you will just get around 30 V from the mains winding, and 1 - 1.5 V from the others.

Once the transformer is powered, you should be able to find which is the 6.3 V winding and which is the 5 V winding from the voltages they produce. With no load I would expect them to be 10 - 20% higher, so you might get around 7 V from the 6.3 V winding and around 6 V from the 5 V winding.

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