Connecting an amplified 3-wire transducer to STGM

I'm trying to use an amplified string potentiometer with an STGM input. The string pot is not a voltage divider, but provides a 0-10 V amplified output signal. It has 3 wires: positive excitation, signal, and ground. Both excitation and signal are referenced to the same ground. I don't see an appropriate connection diagram in Dewesoft X3 software or manual. I tried excitation to Exc+ (pin 1), signal to In+ (pin 2), and ground to both Exc- (pin 8) and In- (pin 7), but this doesn't seem to work. What's the recommended connection? By the way, I have other examples of 3-wire transducers in which the excitation and signal reference the same ground conductor.

Dear Joseph,

we would need some additional information about the sensor that is beiing used. Could you please provide a datasheet or a web link of the sensor?

Witch device are you using?

Best regards,

Rok

@Rok Kmetič

Here's a datasheet. This device is connected for 0-10 v output (not 4-20 mA).

Joe

Dear Joseph,

the STGM amplifier has excitation max 15V and 45mA. So we suspect that the reason for the sensor problems is insufficient power supply.

If it's possible please try to connect the sensor to an external power supply as marked in the picture below:

For better assistance please tell us which device are you using so we could have a better look into this issue.

Best regards,

Rok

@Rok Kmetič

I've found that the STGM can somewhat work with the SP3 string pot, using the setup shown (attached). The odd thing is that the full scale output advertised by the manufacturer is 10 V, but I only get 3.15 V at full scale. I don't know whether it's OK to connect the Exc- (8) and In- (7) inputs.

I measured the current draw on the Exc+ line, and found it to be 17 mA, so it doesn't exceed the STGM's specs. A photo of the current measurement setup is also attached. A Verivolt Encore 5C current shunt was put into the Exc+ line.

So, I would like you to answer:

1) Is there a problem connecting the Exc- (8) and In- (7) inputs? and

2) Why do I see 3.15 V full scale instead of 10 V?

Dear Joseph,

The 3-wire sensor has to be connected in single ended mode.

In DEWESoft this cannot be switched because the excitation has to be used and the amplifier goes into differential mode.

A short circuit between the pin7 with pin4 will be needed, this will manually connect the input as a 3-wire single ended mode with the power sensor.

Please note that the scaling of the sensor will then need to be corrected since the reference will not be GND but EXC-.

Best regards,

Rok

Hello Joseph and Rok,

Curious if you could elaborate on the outcome of this case as we are considering purchase of multiple of the same model string pots.

Thanks,

Glenn.

Glenn P. Siddens, PE

Senior Mechanical Engineer

Direct: 1.419.720.5878

Main: 1.419.866.6301

Email: gsiddens@tronair.com

@Glenn Siddens, we implemented the wiring diagram shown on the last post (posted 08.11.2019), and it works fine. The missing piece was connecting pins 4 & 7. We've come up with a universal cable design to connect the STG 9-pin to any transducer, and didn't want to change that cabling, so we use a short "pigtail" cable to attach the string pot to the universal cable. Since the 4-7 jumper is unique to the string pot, that connection is made in the string pot pigtail. The overall end to end wiring schematic (universal cable + pigtail) is shown below.