# Position Transmitter Equivalent Circuit

I’m working on a project that requires us to read the output from the position transmitter in the attached picture. My intention is to replace the “indicator” side with an Arduino which I’m assuming will need an analogue input for each tap.

I know 28VDC driving voltage is too high for Arduino but I don’t think that is too relevant because, intuitively, it looks like it is essentially just a three way voltage divider. The trouble is I’m having a bit of a blonde moment and struggling to come up with an equivalent circuit for the output.

Could anyone please provide some assistance with this?

Can you disconnect the 28volt supply.
And connect Arduino’s 5volt and ground to that feed point.
That would make things easier (no voltage dividers for the three outputs).
Leo…

Yep we can replace that supply with anything but I thought the three outputs would still end up being a particular ratio of that 5V input?

It's rather like the old selsyn system...

whether you can do it from the arduino's 5v supply depends on the resistance of the sender - the arduino will only supply about 50mA....

Could you measure the value?

regards

Allan.

Ok sure I'll measure it once we get a hold of one. I'm not too worried if we have to connect it to an external supply of some sort, whether that's 5, 12, 24, etc. It's more just that I'm having trouble working out the relationship between the position of the transmitter arm and the voltages at each of the three taps.

Well, it's just simple trig.....

in your picture, let's call the top right ouput A, next anticlock B, veritcally down C - and the starting angle as shown 0 degrees......

as you rotate the coupled wipers anticlock thru 360 degrees..

A has starting angle +30, B +150 ,and C + 270 degrees

And the voltage you get on A,B,C is Vin * ( abs ( 180 + P - 30 ,150,270 )/180 )

regards

Allan.

allanhurst:
whether you can do it from the arduino's 5v supply depends on the resistance of the sender

• the arduino will only supply about 50mA....
1. I doubt 5volt will be a problem if it's normally 28volt.

2. The 5volt pin can supply way more than that. Max current depends on the raw supply on V-in ofcourse.

5volt on the "transmitter" will make the tap voltages ratiometric (is good) when measured with default Aref.
Leo..

Thanks very much for your help fellas. The winding resistance is ~600Ω as is the resistance between each tap. So it should only be about 8mA at 5V.

Allan, that equation is so simple, thank you. It also occurs to me, after seeing that, that I only need to read one or two of the outputs to get the position

two

regards

Allan.

Curious to see the full code to calculate the angle from two (or three for higher accuracy) 0-1023 A/D values.
Leo..