Variac Current Meter

[alfa88]

I'm having a Senior moment. I want to add a current meter to a 10A Variac. My thought is to add a series/shunt resistor and then a rectifier/cap. to the meter. The meter draws 104.5uA @ .05Vdc full scale.The Variac puts out 1 to 140 Volts. What I initially come up with is .01 Ohm @ 1 Watt but I know that doesn't factor in the voltage drop across the rectifier.

 

[duke37]

I presume that the meter is a DC type.

Voltage
At 140V, the average voltage is 140*0.9 = 126V
To get 104.5uA you need a resistor of 1.2M, also a bridge rectifier. A capacitor should not be necessary unless the meter vibrates.
If you rectify first and fit a capacitor, the voltage will be 140*1.414 = 198 and a resistor of 1.89M would be needed.

Current
If a bridge rectifier (10A) is placed in series with the output and a resistor (0.1 ohm, 10W) is placed across the bridge you will get a voltage of 1V or 0.9 V mean. A resistor in series with the meter would need to be 0.9/104.5u = 8.6k. Subtract the meter resistance from this.

I think you will need a two pole changover switch or two meters!

Duke

 

[alfa88]

Reply to Duke

You, sir are Godlike! And yes you presume right that it is a DC meter.

 

[duke37]

If you are using a single meter, it would have a cleaner display wih one scale. If you chose 0 to 15, the resistors will need to be changed to suit.

 

[alfa88]

Meter

What I plan on doing is using a single meter with a dpdt switch. I'll make a scan of the present meter face and clean it up with graphics program. I'll compare the accuracy with a digital meter and perhaps tweak component values then add numbers to the meter face with said graphics program and finally decoupage the printout to the meter face.

 

[alfa88]

Voltmeter/ Ammeter Schematics

I'm about to wrap things up so just to clarify are these schem's what you had in mind?

 

[duke37]

The voltage measurement is good. Chose a high voltage low current bridge.

The current measurement is wrong. As drawn, there would not be enough voltage across the resistor to turn the rectifier on. You must put the resistor across the + and - terminals of the rectifier. All the current goes through the rectifier so chose a 25A (low voltage) bridge.

When you get it working, please smile!

Duke

 

[alfa88]

While I await the arrival of the shunt resistor via Ebay I am contemplating what would happen if I powered my box up without that resistor in place. Me thinks there would be a lot of smoke. To tell you the truth I've never seen a bridge wired up this way but the concept is brilliant. BTW I have the Voltmeter section wired up and with the addition of a variable resistor the results are outstanding.

 

[duke37]

Yes indeed, without the resistor there will be full voltage across the rectifier (which only needs to be low voltage) and meter. Two possible smoke sources.