Electrical – Step down transformer voltage between neutral and ground

mainsstep-downtransformer

I just decided to test the voltages in my old tube guitar amp against the schematic. I live in Australia and it's a US amp, so I use a separate 240-110v step down transformer to run the amp. Upon testing the amp, I realised there was around 240v between the chassis of the amp (grounded) and the neutral line. I went back to the step down transformer and got the following readings

~110v between hot and neutral
~130v between hot and earth
~244v between neutral and earth

My wall plug is fine, with ~244v between hot and neutral, ~244v between hot and ground, and 0v between neutral and ground.

Does anyone know what's going on here? I've left the amp in bits and I'm not playing it until I've worked this out, as it seems I might end up playing with 240v on the guitar strings!
Thanks in advance

Best Answer

You don't specify whether the transformer is isolating or "auto-transformer". The auto-transformer isn't isolating and one of the output terminals is connected through to one of the input terminals.

schematic

simulate this circuit – Schematic created using CircuitLab

Important: the link between N and earth / ground is shown for reference only. This is done by the utility company at their transformer and possibly where the supply comes into the building. Do not link N and E anywhere in your circuit.

If you swap the L and N on your wall plug you may find the problem is solved.

schematic

simulate this circuit

Figure 2. Figure 1a configuration with the amplifier added.

The amplifier will, most likely, have an isolating transformer on the mains input. This means the output is isolated from both mains and neutral wiring.

Note the connections of the earth wire and the three symbols used.

  • The leftmost indicates a connection to ground or earth. This is usually done at the supply transformer or building incoming connection depending on local regulations. The neutral is so called because the link between it and ground "neutralises" the voltage on that line.
  • The central symbol is a chassis connection - usually a crimped ring terminal bolted onto the chassis somewhere. This is done to keep the chassis at ground potential.
  • The third symbol is the signal - audio in this case - ground. Typically there will be one connection between signal ground and the chassis. More than one runs the risk of ground or earth loops which can cause hum in audio circuits.
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