Mains voltage tester proving unit

[Chrispy57]

Whilst reading this thread
https://www.vintage-radio.net/forum/...d.php?t=158535
a year or so ago, I resolved to follow the consensus and acquire a mains voltage tester proving unit, before I really regretted not having access to one. More recently, I decided to try to design one using parts that I already had on the shelf, and this project is the result of those musings.

The ULN2068B is one of a range of Darlington Switch ICs available in different configurations, this being a quad version with an output capability of 50V at 1.5A per switch. I wondered if it could be pressed into service to make a useful 50Hz astable multivibrator and pair of buffers, to drive a "backwards-connected" low voltage mains transformer, resulting in a 240V AC output. This might be an unusual application for Darlington Switches, but the prospect of a single chip instrument powered from a 9V battery was appealing, so I began experimenting with a 6-0-6V 6VA transformer that I had. It did work quite nicely, but was un-necessarily current-hungry due to the 1 Ohm resistance of the low voltage windings.

I remembered that I had a batch of 11V 0.8VA PCB mounting encapsulated transformers that I picked up for peanuts on EBay in 2005 (I also found the invoice!) and decided to try a pair of these, since I was using a push-pull configuration to achieve an approximation to a sinusoidal output waveform and there was no centre-tap, but they had a resistance of 45 Ohms, which was much more sensible in this application. I did however need to use a 15V supply to obtain a 240V output at a few mA, and was able to obtain a decent looking sine-wave using a suitable capacitor across the series-connected output windings of both transformers. A couple of 33V Zener diodes were used to tame the 70V transient at the leading edge of the transformer input waveforms to avoid issues with the quoted 50V rating of the Darlington Switches.

The twin transformer version required the use of two PP3 9V batteries in series, activated by a push-button switch between them, and a simple 7815 voltage regulator as my portable power source, but they fitted nicely into an available plastic housing with dual PP3 battery drawer!

I have incorporated a neon bulb and resistor across the output to confirm activation when the power push-button is operated. The DC current draw of 125mA for a few seconds of activation shouldn't over-burden the batteries, so I don't lose any sleep over the woeful inefficiency of this circuit in the intended application. The output current in use is about 3.5mA at 230V measured with an AVO 8, (220V on an AVO 7 and 250V on a DVM), and the output short-circuit current was about 8mA.

I have satisfactorily tested each of my neon screwdrivers and LED voltage indicating screwdrivers, and now hope to rest more easily, rather than Rest In Peace, on the next occasion that I am called upon to do any work on a domestic installation.

The attached photos and a hopefully legible page from my working notes showing the circuit may be of interest to other homebrewers.

[Chrispy57]

Last few pictures