10-12-2016, 10:34 PM
The impedance ratio is the square of the voltage ratios. The voltage ratio, current ratio and turns ratio are identical, though current is inverse.. I.e a 3.16 to 1 turns ratio is 10:1 impedance. 316V AC primary is 10V secondary, 10mA primary is 316mA secondary. Assuming no power loss, this has to be true. As power is unchanged, the impedance ratio has to be a square of the current or voltage ratio.
V= IR
Power = I x I x R
Power = (V x V) / R
However there are several other nasty things about transformers:
V= IR
Power = I x I x R
Power = (V x V) / R
However there are several other nasty things about transformers:
- The saturation DC current of the core. One trick is to fed an opposing current in another winding, that's why push pull transformers don't have the problem. But if turns ratio is say 4:1, and the Anode current is 5mA, then you can "cancel" that with 4 x 5 = 20mA on the secondary, in antiphase. Then load has to be capacitively coupled.
- The Inductance. This and the core limits the lower frequency response.
- AC core saturation. The AC power handling. 1000KHz needs a tiny core compared to 27kHz, which is tiny compared to 50Hz. 50Hz needs a slightly bigger core for same power as 60Hz. Hence SMPSU, originally at 15KHz to 30KHz to use a tiny cheap transformer, now 1MHz is common.
- Capacitance. All those wires with thin insulation.
- Coupling factor, how well the primary and secondary share all the magnetic flux. Hence Pot cores and toroidal transformers.
- Insulation. Any transformer with more than 48V must have extra insulation between primary and secondary. Transformers in mains gear (audio out or mains transformer) or connecting to phone line have to provide usually 2kV isolation.
- Other factors
