Maker Pro
Maker Pro

Voltage drop in a wire

P

Peter

Jan 1, 1970
0
Recently a question came up for my group's project.


We're using a temp sensor chip that has micro-amps of max output current.
We're sending this down a long piece of wire (maybe 20awg).

I suggested a unity gain buffer because the long length of wire will create
noise but more importantly: the chip can't drive a signal through the long
wire.


Unfortunatly, I only know a buffer should be used, hoewver, I don't know
how to mathematically calculate (in theory) how long the wire can be before
the chip can't drive it.


If the wire is 2ohm for 100ft and the chip has 100uA max output current; is
the voltage across the wire 200uV (2ohm x 100uA)?

I found a chart for voltage drop per 100ft of wire, but it only listed
specific current values. Is there a better chart or an explaination on how
to calculate everything?


thanks in advance!
 
P

Peter

Jan 1, 1970
0
I suggest using shielded twisted pair such as microphone cable.

you may want to add a ferrite bead and small capacitor to help choke
RF.


the sensor must want a high Z load. if you need to drive an meter
movement the buffer is appropriate, however the gain may need to be
more then 1 to compensate for transmission line loss (your 100 ft
wire).


yes, it's generally referred to as the IR drop.


type of wire makes a difference. i.e. aluminum has higher resistance
then copper.
use a chart from the manufacturer of the exact make of transmission
line (wire) that you have, if possible. if not find a way to
accurately measure it.

you will probably be much happier if you can use your buffer convert
your current source into a voltage source that operates with an output
of 2 volts or greater. just add a series resistor to the meter for
calibration.

The chip is an LM70 and it's driving a microprocessor. I didn't re-
review the datasheet, however, current or voltage output isn't as
important. If it's outputting current, then I can use a transimpedance
circuit and if it's a voltage output, I can use a unity gain buffer.

How did you know my made up numbers would be for 23 gauge wire? usually
the wire lists go by 2s.
 
Top