View Full Version : measuring high AC voltage

01-28-2006, 12:07 PM
I'm trying to connect a sewing machine pedal to BS2. The pedal sends out various AC voltage 0 to 120 AC, and I want· BS2 to transform to digital output, so that I can use the numbers in my computer.
Is there a way to convert the AC voltage into small DC voltage?

01-28-2006, 08:01 PM
Most sewing machine speed controls are somewhat specialized and a bit on the crude side. They depend on the specific current drawn by the motor to give acceptable control. In practical terms, if you measure the output without the motor connected, the variation in voltage will be very small. It will likely look like its at 120 volts all the time.

But, if it really does give an AC variable voltage, put a 120/6 volt transformer in place of the motor. On the 6 volt side put a full-wave rectifier with a bit of capacitance (maybe 50 uF) with a 15K resistor in parallel with the capacitor. Now you'll have a DC voltage which will vary with the pedal. Feed it into an A/D converter and get the Stamp to measure it.


Bruce Bates
01-28-2006, 09:12 PM
EJ -

Are you sure, and have you checked the actual voltage at the plug which is connected to the sewing machine to ensure that it's getting AC voltage? The reason I ask is that many (earlier) sewing machines used universal brush motors. Universal brush motors work equally well on AC or DC. If you have DC already there, you won't be able to adopt the circuit suggested by Stamptrol, although it is the right way to go, if it IS AC.


Bruce Bates

01-28-2006, 10:48 PM
One small point, if it IS an AC system, is that a ordinary 6V transformer will produce on the order of 8.4 volts with a full wave rectifier and cap. Multiply a transformers output by 1.4 to calculate the max volatge. If it is a wallwart type, check and see how high the floating volatge is, it is never as rated on the back, always higher. Don't forget a back EMF diode, and you might want to consider a zener diode to clamp the the voltage in case of a spike. Also, use a series resistor of at least 220 ohms and prefferably higher on the Stamp pin.

You could then run it through a voltage divider with a 2:1 ratio, giving 0 to 4.4V.

However, there is no way that the motor will run properly at less than 50% voltage and give proper torque. I suspect that Stamptrol is right in that the pedal is doing a little more than just cutting the voltage. I have never really played with one, but they don't typically have a variac in them. If you have a variac, you might try and see what voltage ranges it works OK with.

One thing you could try is to replace the guts of the pedal with a simple potentiometer. Then use an AC motor contoller run by the Stamp.

Can you measure the voltages and current coming from the pedal? That might give us a clue as to what is going on.


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