Repairing Analog AC Voltmeters is Fun and Easy

Isolation Transformer: the AC Voltmeter in on the left

 I find isolation transformers really handy. They are simple to make and greatly improves safety when repairing switch-mode power supplies, which is nearly ubiquitous these days. A 230Vac-230Vac is just 2 transformers connected back-to-back.  

A 48VA Isolation Transformer is just 2 230V/24V transformers back-to-back

It takes in mains 230V(or 220V, 110V, etc) and works by limiting the output VA to that of each individual transformer. The energy transfer is via magnetic coupling and you can actually short-circuit the output and still only draw 48VA (in this case). Live-Neutral shorts occur a lot in SMPS failures and an isolation transformer reduces the drama (ie smoke, flames) when they do.

When the output of an isolation transformer is being shorted, it is handy to have some visual indication, perhaps a neon light, and especially an AC voltmeter. If you turn the faulty SMPS quickly enough it is possible to prevent multiple component failures.

But make no mistake, 48VA (or 209mA at 230Vac) can still be lethal so all the usual safety precautions on mains voltage still apply! The isolation protects your repair item, not you.

Another use is to limit power to an electric power drill. Maybe you want to use it as a screwdriver- it does not take long to discharge a battery-powered drill and can take hours to recharge, a real project buzzkill. Or maybe you did not want your work piece to spin loose when the drill gets stuck. I work with tropical hardwood sometimes and often the drill at full mains power burns the wood as much as cut it. Or maybe you are using it with a holesaw and do not want your wrist broken when it gets stuck ...

If you use transtormers with multiple outputs you can actually produce multiple outputs by switching the secondary coils. Here I have used two 12-0-12V 2A transformers to produce 230V or 115V at the output. Note your VA will vary with the secondary coil tapping you select and must be recalculated.  In my case the VA at 230Vac is 48 but at 115Vac is just  24VA.

So when the AC voltmeter failed after many years of being left on continously, I popped it open, curious to see if it can be fixed.

AC Voltmeter, disassembled. Faulty resistor on top right.

It was just  a coil moving a needle against a spring. There were 2 precision wirewound resistors at each end, the left one to set the zero offset and the right one to set  the full scale deflection of the needle.

There is an excellent website with the details.

Basic AC Voltmeter

The correct  way seems to be:

The coil measures about 5K and  the zero offset resistor 10K and the full scale resistor 16K. This resulted in the meter reading 150V when it is 200V. And often the coil EMF was insufficient to move the needle from zero without a vigorous knock. This often resulted in the meter always reading zero - a short circuit condition false alarm!

The full scale resistor was black with heat and actually melted the meter coil wire insulation next to it. That looked like a likely suspect. When I replaced it with a 5K1 resistor it shows a much more reasonable 210V.

Now I should have set the correct reading using a decade counter (ie precision variable resistor) and bought the correct wirewound resistor precision to replace it. I only had that one 5K1 5W ceramic resistor, and it was Chinese New Year season (even online shops are on holiday) and 10Vac error did not look so bad plus I only needed it to indicate a short-circuit ...

I also drilled a couple of tiny holes in the meter casing to let out all that heat, and it was back in service. A more modern digital one would have required poking at a switch-mode power supply (typically a buck DC-DC converter) powering a microprocessor which read the volatage using and analog-to-digital converter and drives an LED display.

Repair of an old-school analog AC meters was fun and easy in comparison. Happy Trails.