Identifying The Various Windings

Note:These resistance's are approximate. Readings may vary by several ohms from one transformer to the next. The idea is not to be concerned with the exact resistance of the windings, but to identify the various windings by the difference in the resistance of the windings. (Refer to the resistance readings on the schematic diagram above for resistance's of an actual transformer )

Determining the Secondary Voltages

Now that we have the various windings identified, what voltages does our transformer produce? To identify the voltages we must apply voltage to the primary and measure the secondary voltages. We could apply 120 vac to the primary and start measuring the secondary windings, however, our previous resistance readings tell us that the windings have continuity, but it is difficult to determine from these readings if a winding has shorted turns. Applying full input voltage may spell disaster in the form of a smoking transformer. The best way is to use a Variac (variable auto-transformer) which will allow us to vary the input voltage to the transformer, and bring the voltage up slowly (see note below).
Connect a line cord to the primary winding of the transformer (be sure to insulate the connections to avoid an accidental shock). Insulate the exposed ends of all but one of the secondary windings so they will not short together or come in contact with you. Electrician's wire-nuts are good for this, or use electrical tape. Attach an ac voltmeter to the leads of the secondary to be tested (for safety, use test leads with insulated clips instead of probes; this allows your hands to remain free of the leads). If the secondary being measure is a filament winding, set the voltmeter on a scale that will read 10 to 12 volts AC. Plug the primary of the transformer into the Variac (make sure the voltage is set to zero), and then adjust the Variac to slowly increase the primary voltage (some Variacs have a built-in voltmeter to monitor the output voltage, if yours does not, hook up a second voltmeter to do this). Note the reading on the secondary winding. If it doesn't begin to rise as the input voltage is increased, the transformer may have some shorted windings and should not be used. If the meter reading does increase, bring the voltage up to the normal input voltage (115-120 vac) and note the secondary reading. A filament winding should produce one of the typical filament voltages (1.5, 2.5, 5.0, 6.3 ect). On a center-tapped filament winding it will be difficult to tell which is the center tap lead from the resistance readings, as these values are very low. If you are checking a filament winding with a center-tap you can tell the two outer windings as they will give the highest voltage reading, with the center-tap giving half the voltage reading of the two outer windings.
Example: A 5 volt, center-tapped winding will show 5 volts ac between outer windings, and 2.5 volts from center-tap to each outer winding (since the transformer we are testing is not under load, the voltage readings may be slightly higher than if the transformer were under actual operating conditions).
After taking this reading, check the other secondary windings in the same manner taking care to reduce the input voltage to zero before each measurement. Make sure all the bare ends of the windings, except the one being measured, are insulated. Be very cautions when measuring the h-v winding as voltages may be as high as 800 vac. It is suggested that you take measurements between the center-tap and each outer winding. By doing this you will only be measuring one-half of the high voltage. Be sure and set the voltmeter on the proper scale to read this higher voltage. Both measurements from the center-tap to each outer winding should yield the same reading within a few volts. If the voltages are quite different, it may mean that one side of the h-v winding has some shorted turns.
Now that you have the windings identified, label each one and place the transformer in stock for that next set that needs a new one.
Refer to the transformer schematic above for typical voltage readings.

Note: An auto-transformer does not offer isolation from the ac line. For safety reasons, it is recommended that a 1:1 isolation transformer be used in conjunction with the auto-transformer.

Replacement Size

Note: The number of filament windings can vary, and may or may not be center-tapped. Windings that are center-tapped, the center-tap will carry the same color as the winding plus a yellow tracer. The exception is the five-volt rectifier winding where the center-tap will carry some other color tracer such as black or blue.