Output voltage of shaver socket, seems high

That was my understanding from a thread some time ago. Something to do with the output floating and nominal voltage being produced only when the correct load is applied. Someone also suggested that a multi-voltage shaver should always be plugged into the 115V socket to remove the risk of over-voltage.

It would be interesting to know

a) the voltage of the mains at a 3-pin socket nearby

b) the voltage at the shaver socket when a typical shaver is plugged in

It is quite possible that the meter is mis-reading. I've got a Kasa mains-monitoring plug in a socket by my desk and I glance at the voltage from time to time. I've seen readings of around 230-250 V, varying over the course of the day. But never anything as high as 280V.

I've just plugged our washing machine into another of these, to record cumulative energy over the course of the day to see how much energy it uses typically. Various other similar devices are recording voltages of around

Real RMS AC voltages depend on the shape of the wave. If something is distorting the wave form it could well affect the reading

Dave

If BIL really IS an electrician and is satisfied his meter isn't telling lies he should have telephoned his supplier emergency number and reported the situation. 280volts will be regarded as a dangerous situation and an urgent response team visit. Is there a preponderance of solar panels on roofs around the site? Of course solar inverters SHOULD shut down under over voltage conditions.

The meter may be assuming a sine wave output.

My /guess/ is that at very low secondary currents - almost none really - the transformer core hysteresis is significant. As the very small primary current changes polarity, there's a period when it's not magnetising the core, then the hysteresis is overcome, so the magnetic field 'jumps' as it changes direction.

This will affect the wave shape which may affect the measured secondary voltage and will affect the actual voltage howsoever defined in some way.

Basically, no load isn't a fair test.

The shaver socket is likely the type with its own isolation transformer. I get the impression from reading the OP that thus voltage is present on its secondary, not on its input. i.e. this is not the mains supply voltage that is high.

Yes, I'd like to see what it is under load (even though of course the load is tiny). If it had been me, I'd have done that, and measured the voltage across the primary/input.

What provoked the Sparkie to measure the voltage in the first place ?

Exactly correct! :-)

On further testing, this particular Kasa is under-reading the supply voltage, even under no-load conditions, in a variety of sockets. Other Kasas and a digital multimeter all give similar voltage readings, so the one I happened to use for the washing machine is dodgy. The question is: is it under-reading by a constant factor? Time to test it with known loads (eg 1x and 2x 60 W tungsten bulb, compared with another Kasa that is giving sensible voltage readings) and see if the power readings are linear. If so, I can at least work out a conversion factor by which I multiply any power/energy readings.

I'm surprised that a shaver outlet has an output with such high impedance that it only reads true when it has a shaver as a load, and has to be designed to over-read on no-load, so that it reads true with the load it is designed for.

Most small transformers have poor regulation. 10-20% is not to be unexpected. 280V still sounds high but without knowing the spec and working practice associated with these shavers sockets it is difficult to comment.

The field is driven by magnetising current but the magnitude of the field is dependent on the applied voltage.

Any 'jump' in field will therefore be effectively clamped by the applied AC mains voltage. Any field leakage will also be of the same order of

Cheap DVMs tend to fudge rms measurement of AC voltages and make assumptions about the waveform. I might suggest 'spikes' could have a disproportionate effect on a measurement.

Agreed.

Open circuit voltage measurements with a transformer are pretty pointless. Measure the voltage under load.

checking here,I find the shaver socket shows 270v ac while an ordinary socket shows 241v on the same meter,

Not quite sure how design influences it, but every transformer I've come across gives more volts off load than on. And a low cost DVM could also have quite a high tollenace on AC. Assuming even the waveform is good.

We look after a large communal TV system that works using 44VAC line power. 4:1 transformers produce the nominal 44V from the mains supply. With no load the output is about 55V (which is not a quarter of mains voltage!). With a normal load the line voltage is about 44V. At the repeaters we use 5:1 transformers to step the power up to about 220V when there's a load. That's so we can use mains equipment.

It all works pretty well; keeps slogging on! It is virtually immune to EMP from lightning, unlike DC systems, which get splatted every time.

Bill

So what BIL is seeing isn't unusual/unreasonable, you learn something every day! :-)

For example, a no-load voltage of 275 V and an output load voltage of 230 V would lead to percentage variance approaching 20 %. The output voltamperes must be not less than 20 VA and not more than 50 VA.

The transformer of a shaver supply unit is an isolating transformer, intended to provide protection against electric shock by ?electrical separation?.

The transformer is of the short-circuit proof type (either inherently or noninherently).

A thermal cut-out is provided in a shaver supply unit to limit the temperature of the transformer under conditions of short-circuit or overload.

The cut-out may be of either the self-resetting type or the manually resetting type. "

In other words, a box of squirrels rather than a regular take-no-prisoners transformer. They may have played with the Core Saturation characteristic or something.

Examination with a battery-operated oscilloscope may indicate just how screwed-up the waveform is. Normally, you would not want harmonic content in the output waveform, because other motors or transformers would not like that. Who knows what the shaver looks like as a load... Maybe another box of squirrels is involved.

Paul

They are designed for loads between 20 and 50 VA - so not huge loads.