TRANSFORMERS

SECTION NINE

                               TRANSFORMERS

All transformers and coils are tested the same way. This includes chokes, coils, inductors, yokes, power transformers, EHT transformers (flyback transformers), switch mode transformers, isolation transformers, IF transformers, baluns, and any device that has turns of wire around a former. All these devices can go faulty.

 

The coating on the wire is called insulation or "enamel" and this can crack or become overheated or damaged due to vibration or movement. When two turns touch each other, a very interesting thing happens.

The winding becomes two separate windings

We will take the case of a single winding such as a coil. This is shown in the first diagram above and the winding is wound across a former and back again, making two layers. The bottom and top layers touch at the point shown in the diagram and the current that originally passed though A, B, C, D now passes though A & D.

Winding B C becomes a separate winding as shown in the second diagram.

In other words the coil becomes a TRANSFORMER with a SHORT CIRCUIT on the secondary winding as shown in the third diagram.

When the output wires of a transformer are shorted together, it delivers a very high current because you have created a SHORT-CIRCUIT. This short-circuit causes the transformer to get very hot.

That’s exactly what happens when any coil or transformer gets a “shorted turn.”

The shorted turns can be a single turn or many turns.

It is not possible to measure a fault like this with a multimeter as you don’t know the exact resistance of a working coil or winding and the resistance of a faulty winding may be only 0.001 ohms less.

However when a transformer or coil is measured with an inductance meter, an oscillating voltage (or spike) is delivered into the core as magnetic flux, then the magnetic flux collapses and passes the energy into the winding to produce a waveform. The inductance meter reads this and produces a value of inductance in

Henry (milliHenry or microHenry.)

This is done with the transformer removed from the circuit and this can be a very difficult thing to do, as most transformers have a number of connections.

If the coil or transformer has a shorted turn, the energy from the magnetic flux will pass into the turns that are shorted and produce a current. Almost no voltage will be detected from winding.

The reading from the inductance meter will be low or very low and you have to work out if it is correct.

However there is one major problem with measuring a faulty transformer or coil.

It may only become faulty when power is applied.

The voltage between the turns may be sparking or jumping a gap and creating a problem. A tester is not going to find this fault.

Secondly, an inductance meter may produce a reading but you do not know if the reading is correct. An improved tester is a RING TESTER.

The circuit for a ring tester can be found here:

It sends a pulse to the coil and counts the number of returning pulses or "rings."  A faulty coil (or winding) may return one pulse but nearly all the energy will be passed to the shorted turns and you will be able to see this on the scale. You will only get one or two return pulses, whereas a good winding will return more pulses.

One way to detect a faulty power transformer is to connect it to the supply and feel the temperature-rise (when nothing is connected to the secondary).

It should NOT get hot.

Detecting shorted turns i s not easy to diagnose as you really need another identical component to compare the results.

Most transformers get very hot when a shorted turn has developed. It may deliver a voltage but the heat generated and a smell from the transformer will indicate a fault

ISOLATION TRANSFORMER

An isolation transformer is a piece of Test Equipment that provides "Mains Voltage" but the voltage is "floating." You will still get a shock if you touch the two output leads, but it has a special use when testing unknown equipment.

Many electrical appliances are fully insulated and only have two leads connected to the mains.

When you take these appliances apart, you do not know which end of say a heating element is connected to the "live" (active) side of the mains and which end connects to the neutral.

I am not suggesting you carry out the following tests, but they are described to show how an isolation transformer works.

If you touch a soldering iron on the "live" (active) end of the heating element it will cause a short-circuit.

However when the appliance is connected to the main via an isolation transformer, you can touch an earthed soldering iron on either end of the heater as both leads from the isolation transformer are "floating."

Note: As soon as you earth one lead of the output an isolation transformer, the other lead becomes "active." You can make your own Isolation Transformer by connecting two identical transformers "back-to-back."

The following diagram shows how this is done:

You can use any transformers providing the primary and secondary voltages are the same. The current capability of the secondary winding does not matter. However if you want a supply that has almost the same voltage as your "Mains," you need two transformers with the same voltages.

This handy isolation transformer will provide you with "Mains Voltage" but with a limited current.

In other words it will have a limited capability to supply "wattage." If you are using two 15VA transformers, you will only be able to test an appliance rated at 15 watts.

This has some advantages and some disadvantages.

If you are working on a project, and a short-circuit occurs, the damage will be limited to 15 watts.

If you are using two transformers with different VA ratings, the lower rating will be the capability of the combination.

If the secondaries are not equal, you will get a higher or lower "Mains Voltage."

If you get two old TV's or Monitors with a rating on the compliance plate of 45 watts, or 90 watts, you can assume the transformers are capable of delivering this wattage and making an isolation transformer will enable you to test similar items with the safety of being isolated from the mains.

Colin Mitchell designs a lot of "LED lighting lamps" that are connected directly to the mains. He always works with an isolating transformer, just to be safe.  Working on exposed "mains" devices is extremely nerve-wracking and you have to very careful.

DETERMINING THE SPECS OF A TRANSFORMER

Suppose you have a "mains transformer" with unknown output voltages and unknown current capability. You must be sure it is a mains transformer designed for operation on 50Hz or 60Hz.

Switch-Mode transformers operate at frequencies 40kHz and higher and are not covered in this discussion.

To be on the safe-side, connect the unknown transformer to the output of your isolating transformer.

Since the transformer will take almost no current when not loaded, the output voltages it produces will be fairly accurate. Measure the input AC voltage and output AC voltage.

If the transformer has loaded your isolating transformer it will be faulty.

Mains transformers are approx 15 VA for 500gm, 30VA for 1kgm   50VA for 2kgm and 100VA for 2.5kgm.

VA stands for Volts-Amps and is similar to saying watts. Watts is used for DC circuits, while VA refers to AC circuits.

Once you have the weight of the transformer and the output voltage, you can work out the current capability of the secondary.

For transformers up to 30vA, the output voltage on no-load is 30% higher than the final "loaded voltage."

This is due to the poor regulation of these small devices.

If the transformer is 15VA and the output voltage will be 15v AC, the current will be 1 amp AC.

You can check the "quality" of the transformer, (the regulation) by fully loading the output and measuring the final voltage. If the transformer has a number of

secondaries, the VA rating must be divided between all the windings.

OPTO ISOLATORS and OPTO COUPLERS

Opto Isolators and Opto Couplers are the same thing. A common opto-coupler is 4N35. It is used to allow two circuits to exchange signals yet remain electrically isolated. The signal is applied to the LED, which shines on a silicon NPN phot o-transistor in the IC.

The light is proportional to the signal, so the signal is transferred to the photo transistor to turn it on a  proportional amount. Opto-couplers can hav e Light Activated

SCR's, photodiodes, TRIAC's and other semiconductor device s as an output. The 4N35 opto-coupler schematic is shown below:

An opto-Coupler using a TRIAC

TESTING AN OPTO COUPLER

Most multimeters cannot test the LED on the input of an opto-coupler because the ohms range does not have a voltage high enough to activate the LED with at least

2mA.

You need to set-up the test-circuit shown above with a 1k resistor on the input and 1k5 on the output. When the 1k is connected to 12v, the output LED will illuminate.

The opto-coupler should be removed from circuit to perform this test.