Measuring a transformer with a regular ohm meter or multimeter is very difficult and provides inaccurate results. This is because an ohm meter or multimeter uses DC to measure the resistance.
Consider a transformer, which is basically a coil of wire, the DC resistance will be fairly low. If an AC signal is passed through the winding the impedance will be much higher, unless the winding is shorted. In this case the reading will be very low.
In the case of a short circuit the inductance of the transformer tries to impede the current flow through the winding. The nature of transformers will result in the impedance being reflected into other windings on the transformer being tested. This means a shorted turn on any winding will show up on the other windings.
An impedance meter is also critical for accurately measuring low impedance loudspeaker systems, incorporating multiple speakers and crossovers, and for measuring commercial 100V line speaker systems, which may include many speakers with line transformers and several hundred meters of cable. Trying to measure this type of load is virtually impossible with a standard multimeter.
For example a short circuit on several hundred metres of cable will not show up as a short circuit on a standard multimeter. You will actually measure the DC resistance of the cable which will be around 22 ohms for 500 meters of 24/0.2 cable.
Using an impedance meter, measuring the AC resistance will read a short circuit.
A short circuit located remotely in a speaker line is the major cause for amplifiers overheating and blowing DC fuses. Shorted turns on mains transformers primary windings can also be measured with this meter. If this meter displays a value of less than 100 ohms, this will indicate that the transformer has a shorted turn.
Altronics’ new ‘impedance meter for dummies’ is designed and manufactured in Perth and calculates and displays speaker loads in Watts. Most impedance meters on the market display the impedance in ohms, leaving the installer to calculate the system.
The Q 2003 is available now from Altronics and authorised resellers.
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