DSGarcia said:
I would like to build a load cell simulator to use with various scale indicators for software testing. I know a strain gauge is basically a resistor bridge, but what value resistors should I use and across which terminals is the potentiometer?
Thanks,
Dale
hi,
The range of bridge resistors I have used are from 100R thru 500R.
ie: the individual resistor elements are 100R thru 500R, giving a source impedance of 100R thru 500R.
The bridge excitation is either dc or ac, I use a Vx of 5Vdc, 10Vdc or 12Vdc.
The excitation voltage used should be as specified by the cell manufacturer.
[ its best if you use the cell indicator unit Vx, that normally excites the cell]
eg: 10kG cell, excitation 5Vdc, Vout 10mv/10kG
I would recommend that you do not use a variable resistor to set the simulated bridge offset, it will drive you nuts!!.
Get a good quality 12 pole rotary switch, buy some high precision resistors.
I use the 0.1% 249R version, use 4 to make a bridge [you may have to select a matching 4 from a batch],
leave the 'top' connection of the bridge open.
Thats where you connect 12, very low value precision resistors wired to the 12 pole switch.
The reason for the 12 poles is position 1 will give a -Vsig [-1mV] output and position 1 is Vsig [0mV],
the remaining positions have low value resistor giving +1mV signal increments upto +10mV.
The switch wiper is connected to the +Vx supply.
Use screened cables and a metal enclosure for the bridge and switch assembly, connect the case and screen to the analog ground,
Does this give you the general idea??
EDIT:
Attached a rough working drawing I used a couple of years ago.
The important point to remember is the accuracy of the DVM that is used to confirm the mVolt steps for each switch position.
Make a careful note of these values and they can be used to create a calibration graph/chart.
You will find it very difficult to get each switch position milli-voltage to within better then 0.1mV,
thats where the chart will be useful when the simulator is used to test an actual indicator.
Eric