# Help needed for simple circuit!

Discussion in 'Electronic Basics' started by skk, Sep 9, 2003.

1. ### skkGuest

Can someone help me build a simple device which basically is an
ammeter with TORQUE?

The current from each phase of the household current has to be summed,
averaged and displayed through a pointer that rotates through 75
degrees. The device shall take the output of three current
transformers, find the average and create an output that shall drive a
servo motor between zero degrees and 75 degrees.

When the current being drawn is zero, the rotor position shall be at
zero degrees and when the current is 5 amps, the rotor shall be at 75
degrees. Any intermediate voltage should correspond to an intermediate
degree position.

There should be a potentiometer to control the current at which the
rotor moves out of the zero degree position.
There should also be a test circuit that enables checking of the
movement of the rotor by simulating 0-5 amp input.
All parts should be rugged and durable ... able to work continuously
day in and day out.

Hope that someone can assist.

Regards

Skk

2. ### petrus bitbyterGuest

Skk,

I'm sure I saw this posting before and there were some replies already, but
can't find it anymore.

The circuit in itself may be simple, hardest part of the job is finding the
right components. First, you will need current transformers. They are pretty
expensive and may be hard to find. Farnell has some. You can also make them
yourself. Another possibility is finding some Hall devices.

Using the devices mentioned, you will get three ACvoltages representing the
current in the three phases. you can rectify them, add them and
amplify/attenuate the result to the voltage required to control the servo.

You cannot use a simple diode or bridge to do the rectification. Low
currents/voltages become invisible when the measured voltage lowers below
the diodes knee voltage. Some Googling through various opamp circuit
collections on the internet will bring in a useful rectifier circuit.

Servos are often used in model building. Remote controlled planes for
instance need several off them. Conrad sells them. Most of them come with a
build in servo amplifier so you only have to provide the correct control
voltage to become the required deflection.

pieter

3. ### skkGuest

Dear Pieter
Thanks a lot for the advice. Appreciate it and your time. Skk