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Topic: Reading current instead of voltage confusion... (Read 630 times) previous topic - next topic

bluephaux

When I measure the input voltage vs ground, I get 5V. There might be a resistor or something because the voltage of the cell output vs ground is 3.2 volts.  Therefore, (if R1 = 0 ohms), the analog input is approximately 650 regardless if the cell is at high or low state -- pretty disappointing.

I'll have to try manipulating the circuit more to try to still figure it out. Essentially since voltage of the cell output is always the same, the voltage drop measured across a shunt resistor is always the same.. :-/  Not sure..

James C4S

You don't need an R1 unless you want to make the math more complicated.

Now that I've studied the device a litte bit more, it is a constant current device.  In the low state it will always allow up to 6mA to flow through it and in the high state it will always allow up 15mA. (both of these are nominal values.)  Remove R1.  I will refer to R2 just in name.

So you pick a value of R2 that makes sense for 5V.

Ohm's Law:  V = I * R

5 = 15mA * R
R = 5 / 15mA = 333.33333 = 330ohm

Knowing there is a 330ohm resistor there when your device is outputting a "low" your voltage will be:
V = 5mA * 330 = 1.5V which should give an A/D reading around 300.


Quote
Essentially since voltage of the cell output is always the same, the voltage drop measured across a shunt resistor is always the same.. :-/  Not sure..

The current through the device is always the same.  The voltage drop is relative to whatever resistor it is in series with.  With the 330ohm resistor, the voltage across your device should be nearly 0V when it is HIGH.  It'll be about 4.5V when it is LOW.
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DVDdoug

#7
Jan 05, 2012, 02:43 am Last Edit: Jan 05, 2012, 02:47 am by DVDdoug Reason: 1
Quote
Essentially since voltage of the cell output is always the same, the voltage drop measured across a shunt resistor is always the same..

- Your resistor might be the wrong value.
- The device might be wired backwards or otherwise connected incorrectly.
- The device might be defective
- Your magnetic field may be too weak to trigger the device.

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