These are electrically in parallel not mechanically. I built two
Wheatstone
Bridge circuits out of resisters. After applying voltage to one of these,
I
had 15 MV output. I then soldiered the second bridge on top of the first
bridge and I got a 31 MV output. I then soldiered 4 resisters between the
bridges to make a box like circuit. I was looking for some kind of
strange
recirculating curcuit. I came up blank.
I know the two bridges in parallel double the output voltage or summed as
the scale books say but I have no ideal why.
---
Please bottom post.
So what you're saying is that when you do this: (View in Courier)
+V>-----------+-------+
| |
[R1] [R3]
| |
+-------|------>E1
| |
| +------>E2
| |
[R2] [R4]
| |
GND>----------+-------+
E1-E2 = 15mV,
and when you do this:
+V>-----------+-------+-----------+-------+
| | | |
[R1] [R3] [R5] [R7]
| | | |
+-------|-----------+-------|----->E1
| | | |
| +-----------|-------+----->E2
| | | |
[R2] [R4] [R6] [R8]
| | | |
GND>----------+-------+-----------+-------+
E1-E2 = 31mV?
If that's true, the only way that can happen is if the resistors in
the bridges aren't proportional. That is,
R1 R5
---- =/= ----
R2 R6
or
R3 R7
---- =/= ----
R4 R8
or both.
If we set up a single bridge to give us a 15mV differential across
the outputs, one solution would be:
+1V>----------+-------+
E1 |R1 |R3
[493] [507]
| | E2
+-------|------>0.507V
| |
| +------>0.493V
| | E3
[507] [493]
|R2 |R4
GND>----------+-------+
Where:
E1 R2 1V * 507R
E2 = ------- = ------------- 0.507 volts
R1+R2 493R + 507R
Similarly,
E1 R4 1V * 493R
E3 = ------- = ------------- 0.493 volts,
R3+R4 507R + 493R
and
E2 - E3 = 0.507V - 0.493V = 0.014V
Now, if you connect two identical bridges in parallel:
+1V>----------+-------+-----------+-------+
|R1 |R3 |R5 |R7
[493] [507] [493] [507]
| | | |
+-------|-----------+-------|----->E1
| | | |
| +-----------|-------+----->E2
| | | |
[507] [493] [507] [493]
|R2 |R4 |R6 |R8
GND>----------+-------+-----------+-------+
R1 will be in parallel with R5, R2||R6, R3||R7, and R4||R8,
so the total resistance two 493 ohm resistors in parallel will be:
R1 * R5 493R * 493R
Rt = ---------- = ------------- = 246.5 ohms
R1 + R5 493R + 493R
similarly, the total resistance of two 507 ohm resistors in parallel
will be 253.5 ohms.
Since the two bridges in parallel can be simplified to a single
bridge with the corresponding resitors in the legs in parallel, we
wind up with:
+1V>----------+-------+
E1 |R1 |R3
[246.5] [253.5]
| | E2
+-------|------>0.507V
| |
| +------>0.493V
| | E3
[253.5] [246.5]
|R2 |R4
GND>----------+-------+
Solving for E2, we have:
E1 R2 1V * 253.5R
E2 = ------- = ----------------- = 0.507 volts
R1+R2 246.5R + 253.5R
and for E3:
E1 R4 1V * 246.5R
E2 = ------- = ----------------- = 0.493 volts
R3+R4 253.5R * 246.5R
Therefore, the voltage across the outputs of the bridge, as earlier,
is:
E2 - E3 = 0.507V - 0.493V = 0.014V
Consequently, the difference in the readings you're experiencing
must be due to the tolerances of the resistors you're using, or it
may have something to do with the extra resistors you _soldered_
into the circuit. There's no telling unless you post a schematic
and specify what you're using for resistors and what you're using to
measure the output voltage from the bridge.
