Transformers
| Transformers | |
|---|---|
| What it does | Transfers AC power between coils by magnetic coupling, changing voltage, current or impedance |
| Voltage ratio | Equal to the turns ratio |
| Impedance ratio | Equal to the turns ratio squared |
| Works on | AC only; DC does not pass from one winding to the other |
| Types | |
| Power transformers | Laminated iron cores at the 60 Hz mains frequency |
| Switching-supply transformers | Small ferrite cores at much higher frequencies |
| RF transformers | Ferrite or powdered-iron toroids; tuned IF transformers |
| Antenna transformers | Baluns and ununs, such as 1:1, 4:1, 9:1 and 49:1 |
Transformers are devices that transfer alternating-current power from one coil of wire to another through a shared magnetic field. With different numbers of turns on each coil, a transformer steps voltage up or down, and in doing so it also transforms impedance. Ham stations use transformers at both ends of the scale: heavy iron-core units in linear power supplies and tube amplifiers, and small ferrite units inside radios, in matching networks and at antenna feed points.
How a transformer works
A transformer is two or more coils sharing a core. Alternating current in the input winding, the primary, creates a changing magnetic field, and that field induces a voltage in the output winding, the secondary. The effect is called mutual inductance. Only a changing current produces it, so a transformer passes AC but not DC, and its secondary is electrically isolated from its primary. That isolation is an important safety feature of mains power transformers.
The voltages follow the turns ratio:
- Es / Ep = Ns / Np
where E is voltage and N the number of turns. A transformer with 2,250 turns on its primary and 500 on its secondary, fed with 120 volts, delivers 120 × 500 / 2,250, about 26.7 volts. A transformer does not create power, so the current changes the opposite way: stepping voltage down by a factor of ten steps current up by ten. Real transformers lose a little power as heat, both in the resistance of the windings (copper loss) and in the core (core loss).
Impedance transformation
Because voltage and current change in opposite directions, impedance changes by the square of the turns ratio:
- Zp / Zs = (Np / Ns) squared
To find the turns ratio for a given match, take the square root of the impedance ratio. Matching a 600-ohm audio output to a 4-ohm speaker is an impedance ratio of 150, so it needs a turns ratio of about 12.2 to 1.
| Turns ratio | Impedance ratio | Ham example |
|---|---|---|
| 2:1 | 4:1 | 200 ohms to 50 ohms, as in a 4:1 balun |
| 3:1 | 9:1 | 450 ohms to 50 ohms, the 9:1 unun used with random wires |
| 7:1 | 49:1 | About 2,450 ohms to 50 ohms, for an end-fed half-wave antenna |
| 8:1 | 64:1 | About 3,200 ohms to 50 ohms, also for end-fed half-wave antennas |
Power transformers
Mains transformers have cores built from thin, insulated sheets of iron called laminations. A solid core would act like a shorted turn, with eddy currents circulating inside it and wasting power as heat; laminating the core breaks up those currents. Power transformers are rated in volt-amperes (VA) as well as voltage.
A traditional linear 13.8-volt supply uses a heavy transformer to step 120 volts down, then diodes to rectify it and capacitors to smooth it. A tube amplifier's plate transformer does the opposite, stepping mains voltage up to the thousands of volts its tubes need. Switching power supplies rectify the mains first and then chop it at a much higher frequency, which lets a far smaller ferrite transformer do the same job. That is why switching supplies are so much lighter, and why a poorly designed one can produce RF interference.
An autotransformer uses a single winding with taps instead of separate windings, so it gives no isolation. The variable autotransformer, often called by the trade name Variac, is used to bring old equipment up to full voltage slowly, a common step when restoring boat anchors.
RF transformers
At radio frequencies, transformers are usually wound on small ferrite or powdered-iron toroids.
- Broadband transformers in solid-state amplifiers match the very low impedance of power transistors to 50 ohms, and couple one stage to the next.
- IF transformers, the small shielded "cans" in a superheterodyne receiver, are tuned with a capacitor and an adjustable core to the receiver's intermediate frequency.
- Baluns and ununs at the antenna match the feed line to the antenna and, in a balun, convert between balanced and unbalanced lines. See Balun. Many are transmission-line transformers, wound with paired wires or coax so that the windings behave as short transmission lines, which gives them a wide frequency range.
The 49:1 transformer for an end-fed half-wave antenna is often wound as an autotransformer, with a few primary turns forming the start of a winding seven times as long. Like any ferrite device, an RF transformer heats up at high power and on long transmissions, so it should be rated well above the power in use.