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.  

Electronics Without Semiconductors: The Strange Case of the VW Beetle (Bug) Fuel Gauge Regulator

 

Volkswagen Beetle (Bug) Type 1. Photo by Vwexport1300 

The fuel gauge in my 1969 VW Beetle (Bug) Type 1 failed. The needle kept indicating 'Full' no matter how much petrol I have in the tank. And the workshop said that a new meter assembly will need weeks to arrive and I really did not fancy driving with a jerrycan of fuel in case I ran out.

Now I know very little about cars; my thing is electronics and software. However, from 1968, the Beetle fuel gauge system is electrical (aha!), and armed with Speedy Jim's excellent webpage on Beetle/Bug fuel gauges, I got to work.

VW Beetle Type 1 fuel gauge schematic by Speedy Jim

The schematic shows a voltage regulator (quaintly called a 'vibrator') powering the meter in series with a potentiometer controlled by a float. The potentiometer and float ('sender') lived inside the fuel tank

VW  Beetle electrical type fuel sender 

The fuel tank is easily accessible for testing - just pop the front bonnet.

Beetle/Bug fuel tank. Sender is in front middle of the fuel tank

The output of the potentiometer (sorry - sender) connects directly to the gauge and is easily disconnected.

Fuel Gauge Sender Connector 

The fuel gauge is built into the speedometer, at the top middle. This explains the high replacement cost.

VW Beetle Speedometer: fuel gauge is top middle 

The cabling is at  the back, and luckily the wires are also easily accessible. 

Speedometer, back view. The regulator is riding on the meter's shoulder, on the left

Top view of mounted regulator. Photo by wagohn

Speedy Jim has a cut-out view of the fuel gauge; the current heats up a bimetallic strip and directly drives the needle. Far out! No magnet, no electrical coil. This is so cool. 

Picture by Speedy Jim

The regulator when dismounted looks like this, and is a little reminiscent of a 3-terminal regulator:

VW  Beetle/Bug Fuel Gauge Regulator

3-terminal regulator: LM7812 in TO-220 package

It is time to test. A quick check with the multimeter showed that my sender terminal is reading 5V with the ignition on. With the sensor disconnected the wire from the gauge reads 10.8V. With the ignition off the sender reads 18 Ohms. Speedy Jim has it as 73 Ohms empty and 10 Ohms full. After a couple of days running the sender read 28 Ohms. So the sender seems to be working. This is further confirmed by parking the Beetle uphill and then downhill to move the needle some more. 

Next is the test for the fuel gauge.  Speedy Jim (thanks, Jim!) has detailed instructions. With the wire disconnected (do not let it short to the VW body!), the gauge read empty. Short the wire to the car body and now it reads 'Full' as before.

This leaves the voltage regulator as the prime suspect. My guess was it shorted out its input to its output, and is applying the full 12V input voltage to the gauge bimetallic strip. Hence the constant 'Full' reading. Happens often enough in 3-terminal regulators. 

Regulator pinout: photo by Speedy Jim

Now rather than fumble around with my gauge, Speedy Jim has very handy pictures of a disassembled regulator.

Photo by Speedy Jim

There are no semiconductors in the regulator! It is not even solid-state: it is all metallic. The redoubtable Speedy Jim is worth quoting in full:

"... 12V from the battery heats up the heater element and warms the strip.  The thermal mass is small and the strip responds very rapidly.  As soon as it begins to move, the strip causes the contact points to open.  This breaks the circuit and the current ceases.  Now, the strip begins to cool off and bends back to its original shape, closing the contact.  This repeats, over and over.  The result is a series of pulses, each with a voltage of 12V.  When the pulses are fed to the gauge, the heater element in the gauge averages the pulses out.

The closer the pulses are together or of longer duration, the hotter the heater in the gauge will get.  By accurately controlling the pulses, the stabilizer has the effect of regulating the voltage (here, we're talking about RMS or "effective" voltage).  Suppose that battery voltage goes up (as when the generator increases output).  The heater in the stabilizer will heat up more rapidly and open the contact points sooner.  The result will be shorter pulses of 12V sent to the gauge.  The opposite happens when the battery voltage goes lower."

Diagram by Speedy Jim

The diagram leaves no doubt. Far from a humble LM7812, this is a switched-mode power supply. The diagram show a pulse-width modulated (PWM) output at a frequency of 3Hz and duty cycle of 33%. And all this using just metal. It is as if I decapped a SMPS controller IC like the MC34060 and all I found was solder and wire! Compare this to a typical SMPS:

Solid-state SMPS

And you cannot argue with the reliability: I got the Beetle in 1992 and that regulator must have lasted 30 years of steady use, in a harsh environment with lots of vibration and shock. I would have been pleased if a solid-state regulator lasted half as long. Since it is a 1969 model there is a good chance it might have lasted over 50 years.

Now since the switching rate is only 3Hz, it should be visible if connected to a light. Speedy Jim used a light bulb, but these days there are very cheap 12V LED modules, especially if you cut one off a strip light.

12V LED module: just connect directly to the regulator

I unhooked the sender wire and connected it to the LED module. It lit up but did not blink, so there is no switching by the bimetallic strip in the regulator. I ordered a cheap China part for RM28 (USD4) . This would just be a rudimentary solid-state regulator with just a zener diode and a limiting resistor. 

Now I do not recommend connecting anything electrical to the fuel tank much less 12V from a car battery that can potentially deliver 200A, so extreme care is necessary, in particular when you connect up the wires. Note that the current from the regulator is from the 12V car battery and is being limited by the coil in the fuel gauge so we will use that as the power source. Still the following section delivers a 12V PVM signal into the fuel tank potentiometer (as well as the fuel gauge) and there is always a risk of sparks, especially if you move the fuel tank.

But it was still a good 10 days before it arrived, and in the meantime it would be nice to actually produce those PWM pulses if only to see history in action once more ...

L293D Motor Shield for NodeMCU ESP-12E V2

 

I had been working on a WiFi-controlled L293D Motor Shield for ESP-12E NodeMCU and it produces 12V PWM pulses suitable for driving DC motors as well as LED lighting. It runs an Arduino sketch, and you can get a copy from github. 

The key change is to the PWM frequency:

analogWriteFreq(3); /* Arduino v1.8.5 only */

Note the current Arduino reference says that the minimum value is now 100Hz. However I am on an old version, 1.8.5 and I could dial down the PWM frequency right down to 3Hz, so your mileage may vary.

I disconnected the regulator from the sender, and wired up the motor shield thus:

Pinout for VW Fuel Gauge Regulator PWM

The LED module is used to observe the PWM blink rate. You enter your WiFi access point SSID and password, recompile and download the program into the ESP-12E V2, usually via the microUSB port.

You will want to test youe setup driving 12V LEDs instead of the sender. Since you want to connect to the fuel tank at the last possible moment. I set up everything, including the phone browser before I did so. In particular you do not want to accidentally reverse the polarity to the sender, either by miswiring or by using the program's motor reverse command. An LED indicator is better than a filament bulb here.

 Once the  program starts your 12V led module will start blinking at 3Hz. To send a signal to the fuel gauge, you use a browser (I used Google Chrome on my Android smartphone) and type in:

http://12.34.56.78:8080/pwm1/33

And the fuel gauge immediately started registering the petrol level in the tank. This is because the L293D produces a 3Hz pulsetrain at 33% duty cycle, just like Speedy Jim said.

And you can produce a zero fuel reading by:

http://12.34.56.78:8080/pwm1/0

A full tank reading is

http://12.34.56.78:8080/pwm1/100

And not wanting to push my luck, I took down the setup as soon as I could. 10 days after, the new regulator arrived and the VW fuel gauge was fixed. I kept the faulty regulator: it is a reminder that SMPS is a lot older than solid-state electronics.

Root cause: the heater element appears to have disintegrated so the regulator is stuck in the 'On' position and failing to switch off

Happy Trails.