Engineering tips and 'work-arounds'....

"Buy a cheap shaver socket outlet"

It is not obvious to this American what kind of device you are referring to. I totally agree with your idea of isolating your experiments from the mains. The only way I know of isolating AC is via a transformer. What I have done is go to the local surplus electronics store and find a power transformer which is intended to connect to either 120 or 240V by placing the two primaries in either parallel or series. I tape over the low voltage windings and ignore them. One of the primaries is used as input at 120, then the other 'primary' becomes the secondary, providing 120V at half the rated total power output. By choosing an appropriate size transformer, you can easily get a lot more than 20 Watts for about the same cost (or if you only need 20W, you should be able to find something for around 10$). I always use one of these on my oscilloscope for safety.

Dick

The shaver socket ourlets I was referring to are self contained powered from the mains and have the necessary isolating transformer inside together with a power switch which is actuated whenever a shaver plug id inserted into the socket...

This also saves the need for an on off switch.

Not sure what you use in the USA... But for bathroom use in the UK a shaver socket HAS to be from an isolated supply...

To make one would cost quite a bit not just for the transformer but box, switch, socket, thermal fuse, 115 and 230 Volt switching etc... I'm not sure about your wiring of two ex-surplus transformers, that sounds a tacky none portable and of dubious safety solution.

John.

I suspected as much...

In the USA, the bathrooms in modern homes are required to have either Ground Fault Interruptors (outlets with a built-in protection circuit) or Ground Fault Circuit Interruptors (where a single controller can protect several outlets), but to my knowledge, there is no requirement for isolation.

I don't use an electric shaver, but the one I used to own and all the ones I've paid any attention to run on very low voltage, supplied either by a battery charger or a 'wall wart', which include a transformer and therefore are isolated.

You didn't read my post carefully. My suggestion was a single transformer which has two separate windings that are normally connected in series for 240V input or in parallel for 120V input, and have one or more secondary windings for output of other voltages. I tape over the connections for the normal secondaries to prevent accidental contact and don't use them at all. I use one of the primaries as input for 120V, and use the other winding, which was originally intended as a 120V input, as a 120V secondary. The transformers that supply the controller power on large machines are commonly wired this way.

For anyone who might read this, note that THIS IS NOT A CENTER-TAPPED 240V winding, but two totally separate windings of the same number of turns of the same size wire. There must be 4 contacts, with no connection whatsoever between one pair and the other pair (measure with an Ohmmeter to be sure).

Your post wasn't very clear on that....

I was hoping to have others show their inventiveness in using equipment or appliances or just ideas to make not only a safer working environment but a cheaper way of working for others to learn from...

Okay so it hasn't worked, so far, but I would be interested in your ideas for avoiding paying for a specialised and safe isolation supply source... I'm not really interested in home brew transformers with no box, switch or 115 / 230 volt or fuse protection and not designed for the job either....

Sorry dkwarner, your idea of using one primary to supply another primary isn't a good and safe method... there is no guarantee of good isolation, together with a bare transformer sat on the bench, you are just asking for trouble...

John.

Good call.

When working on controlled rectifiers eg. field supply for an MG set or armature control DC drive, looking at the voltage waveform can be confusing with all the noise. Trying to spot the actual trigger point is often hard with the trace rocking back and forth. I like to connect a Hall effect current transducer to the CRO and view the current pulses. If a pulse is missing it is obvious and the phase control is just as obvious.

This is just an extension of using current to fault find. For instance many of the telemetry systems I've worked on use a current loop and measuring the voltage doesn't always show what is wrong, especially if leakage or ground faults are causing spurious signals.

In AC drives the technique is even more useful, even for understanding the mode of operation. I typically view input current on one phase, DC link charge current, Inverter current and output current on one phase. The harmonics are sometimes quite interesting especially if the power source is a traction alternator excited by its own tertairy winding.