RS Strain gauge amplifier problem
Posted by kikk0 at
Hi,
I've got a strain gauge amplifier board from RS number 435-692 with corresponding parts in the datasheet.
The power source is 6V dc.
I've soldered it all together, but it's not working properly.
The supply voltage to the bridge is not measurable with my multimeter
The output is (after 1001 gain) 1.55 volt. It does not change with change resistance change in the bridge circuti..
What can be wrong?
Thank
Frode
Replies
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What strain gauges have you used. From experience people often connect them up the wrong way.
In RS document http://docs-europe.electrocomponents.com/webdocs/0099/0900766b80099349.pdf
Figs 3 and 4 show typical configurations, have you remembered to connect the compensation pin to Bridge +ve? Not sure if this will cause zero or maximum supply voltage if missing.
If all is OK what is your bridge voltage with the strain gauge replaced by some fixed resistors Use Fig 5 as a quick test configuration?
Just noticed you mention the supply voltage is 6V, is it actually + and - 6Volts? You do require the negative rail.
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Also check the voltage across D1 and D2, this should be + and - 5Volts.
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Hi,
Thanks for answering.
I've planned to use this with my arduino, so I've now hooked it up the the arduino's power supply 5V now...
According to the datasheet this is how I've connected:
Vs+ is connected to the 5V pin on the arduino
Vs- is connected to the ground pin. (you mention the negative rail, should this have -5V? Any suggestion to how I do that?)
The compensation is connected to bridge ve+
I use 5 pot trimmers like VR1 & VR2 for the strain gauges and try to vary the resistance to get a reding. The connection is like figure 3, the full bridge.
Figure 4 & 5 confuses me as the input line crosses the R3 with a dot. Is it supposed to be connected there ? (In my head it would reed 0V between the input and output line..
The bridge supply i have is adujstable from 1.6 to 7mV using VR1. (very low interwall on the turns, when turned further in both directions, nothing happens)
The voltage between D1 & D2 is -3,73 & +0,5
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look above for reply...
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I'm sure your supplies are the problem. You do need dual supplies i.e. +-6V. The diodes are there to provide a +-5V range for the circuit, one controls the reference pot that selects the bridge voltage.
Have a look at some DC to DC converters 5V in to + and - outputs, I would choose a 5V to something at dual 6V output or higher. Check the circuit voltage and power range to choose someting. Traco pcb modules or similar should be OK.
You might be able to checkout the circuit with two 9V batteries as a quick test.
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Hi
Boss must be right: the difficulties arise from your power supply. The circuit requires a symmetrical one, with a 0V connection. Did you get a result with two 9V batteries?
According to the datasheet, the minimum power supply range is + & - 2V, the 5V of the Arduino is just above these limits... It looks to be a bit small, but OK.
You need a ground point (0V) for the circuit to operate properly; you can achieve this easily by creating a false ground with an opamp connected as a voltage-follower, the – input connected at the centre point of a divide-by-two voltage divider across the +5V and 0V of the Arduino (two 10K resistors, paralleled with two 10 uF 10V tantalum capacitors will do the job). Note that more sophisticated circuits are available to achieve better ground simulation and avoid possible unstabilities,
But first of all, I would suggest you to carefully check the placement of the components on your PCB: The measurements across the diodes D1 and D2 show that one of the zener diodes is reverted; you should measure a symetrical voltage at these points with respect to ground. If connected to the Arduino, with the false ground installed, the measurements should indicate + & - 2.5V.
When this will be done, disconnect the bridge, shorten the + & - inputs of the IC, place a resistor of 1K across the feeding pins of the bridge and rotate VR1. You should be able to vary the voltage at these points from zero to nearly + & - 2,5V, if the IC was not damaged!
Then try again - and good luck.
B.B.
P.S. : the dots in figures 4 & 5 are erroneous...
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Hi BB,
I just had the pdf files up again when you posted! Good idea regarding the virtual ground as long as Arduno (I haven't tried them yet as I like MBED at the moment) accepts this on A/Ds.
Kikk0 had used -VS as the Arduno ground so not sure where he connected the real 0V?, but that's why I sugessted using 9V batteries as I'm not sure what the diodes would do (might not actually be the wrong way round) in such an arrangement.
Although speced to + and - 2V the supply typ is + and - 12V so I'm a little cautious of trying to get it working splitting the 5V supply (but was a good idea).
Yes those dotted lines had me confused, I assumed they represented what was local to the PCB and what was remote at the sensing point, which worked for Fig 3, but not Fig 4 or 5 as the compenastion was not carried to the measuring point.
Also regarding Fig 4 the =Ve input is erroneously shown connected to R3 and R4. The connection dot below R3 should be ignored and +ve input goes directly to G1, G2 only. I believe the compensation wire should go direct to the top of G2 as well, but lets get Kikk0 circuit working first!
Am I wrong? But I seem to remember this circuit from about 1985 when RS produced huge ring binders of data sheets! Thank goodness for pdf's and the internet.....
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Hi Boss and kikk0
I went a bit deeper into the schematic diagram. Diodes D1 and D2 are temperature-compensated zener diodes, delivering 6.2V @ 7.5mA. I confused them with sharp zener's, sorry. The power supply MUST then be + and -12V for the circuit to operate properly, and the gain adjusted for the output voltage not to exceed the input voltage range of the Arduino.
If operated with lower (symmetrical!) power supplies, the circuit will be at risk of (maybe) unacceptable drifts; it depends upon kikk0's requirements. The solution 5V/virtual ground is still valid, but the PCB will work at the very lowest side of its abilities. In this configuration, a 2.5V output voltage will represent the quiescent state of the bridge, and the gain must be subsequently lowered to protect the Arduino's input.
Figures 4 and 5 are rather ambiguous. R2, R3 and R4 are replacement resistors for G2, G3 and G4 in the half or quarter bridge configurations (on the PCB?). The bridge supply must be applied to the gauges AND the resistors to deliver the - input voltage to the IC. In Fig 4, the “compensation” must, of course -as Boss wrote, be connected directly to the top of G2 to... compensate for the length of the wire up to the gauge. In Fig 5, this “compensation” has no sense, I presume it is connected only for the IC to function properly.
The dot below R3 must be ignored, and in Fig 7, the top of R1 must be connected to the top of R2 and pin 18 (gain adjustment).
The huge ring binders of data sheets!
Power supply - once again: 7812/7912, four caps, a bridge rectifier and a transfo (2 x 15V / 200mA to supply the bridge at full voltage and therefore maximum sensitivity), or a Traco module?
Let's wait for kikk0's answer, now...
B.B.
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Yep, D1 and D2 clamp the volts at 6.2V or 5V if other part used.
This stops bridge voltage and offset changing if the supply voltage changes or is unregulated.
Dont use 5 v split for 2.5 v rails, use a few volts more than D1 and D2 voltage to get good results with small tsrain gauge signals