# One Multimeter, Four Wires

• posted

Let's say I have a device that contains a 24v AC motor. The motor operates a butterfly flap, spinning on its motor-driven axis. When the flap goes around, it triggers a switch that will either be open or closed, depending on a position of the flap. There are four wires coming from this unit. I need to identify the function of each using a simple multimeter.

I am going to assume that the default, "parked" position of the flap causes the contact switch to be open.

My approach would be to first try and identify the two switch wires; they would have no continuity between them. If I find only two, I will be able to conclude that they belong to the switch.

Before I test this out, I want to understand: will/should there always be continuity in the 24v AC motor wires or is there a position when the continuity is lost? If continuity is always present, then I will easily identify the two pairs (two from the motor, two from the switch).

Another approach is to test the resistance. Again, if the switch is open, I can determine that without ambiguity. As for the 24v motor, what should I expect for the resistance reading there? Is there a way to determine that the two wires being tested, by resistance readings, are definitely from the motor?

I can make the assumption that the switch is in the closed position. in that case, I would imagine its resitance readings to be far higher than those of the motor.

Am I going in the right direction with the logic and methodology on all this?

What else can I get from a simple mulimeter? Is it possible to determine the polarity of the motor?

• posted

I would expect the motor to always show resistance - perhaps in the few hundred ohms range (wild guess)

If the switch is closed, it will measure very close to zero ohms. When open, it will read open circuit.

You said it is an AC motor - so no polarity.

```--
Peter Bennett VE7CEI
email: peterbb4 (at) interchange.ubc.ca        ```
• posted

There will always be a reading on the motor wires. It will be a low resistance as you are only measuring the resistance of a coil of wire.

The wire from the switch will be either a short circuit or an open circuit. You can probably tell the difference between the closed switch and the motor using your ohm meter.

Dan

```--
Dan Hollands
1120 S Creek Dr```

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.