Connecting a load cell to a SG-24

[drog]

I need some help in resolving a problem.

I am using a SCC-SG24 in a SC-2350 platform. I have bought a couple of load cells from Transducer Techniques with the TED option (9 pin connector). I can read the TEDs data. Plugging the sensor in drives the signal up to the rail and it won't budge. If I unplug the sensor and 'tickle' the input connector, I see a 'waveform'. This is all through Max.

Looking at the wiring for the sensor, I see the manufacturer has tied the TEDs return to the Sig- and the cable shield to Vex-. Could this be the cause of my problems?

I have verified my wiring and an ohms test of the sensor looks okay. If I supply the 10v excitation voltage I measure 5v across the Signal lines.

Thanks for your help,
David Rogerson

[Logan_K]

Hello David,

You are correct that the signal is railing specifically because the TEDS return is tied to sig-. This configuration is not compatible with our SCC strain gage modules, because the Excitation- (Vex-) terminal is internal connected to ground, and the TEDS ChnRtn terminals are also internally connect to ground. When your sensor ties the TEDS return to Sig-, it is essentially shorting both AI- and Vex- to ground. This is a problem because your sensor is bridge-based, so both legs of the bridge are intended to be at roughly the same potential of Vex/2 (or 5V) when resting, so when you short one of the legs to ground, then the difference between the two legs suddenly becomes very large (5V), and since there is a gain of 100 on this module, that signal will obviously rail. The way to fix this problem is to remove the path to ground from Sig- by disconnecting it from the TEDS return line. I would recommend contacting Transducer Techniques about this issue so that they can recommend a solution for you (i.e. use wire cutters to disconnect Sig- and TEDS return).

Logan Kunitz

[drog]

Thank you Logan,

I contacted Transducer Techniques and they confirmed connecting Sig- and TEDs rtn was the problem. They did this for their own products and are changing their manufacturing to prevent this problem in the future. Once I cut the connection, everything was fine.

On another note, are external resistors required for the four LED panellete (185376-01)? I didn't find anything in the datasheets.

Thanks again for your assistance.

[e-Cash-Money]

Hello drog,

Those LEDs all require ~15mA at 5VDC.

I hope this helps!
E.Lee