Using BX-08THM inputs for QPSL-AN-42 Digital Pressure Sensor

[ZoNiE]

Hi folks. We are looking to use two of our unused inputs on a BX-08THM for Transducer input from an Automation Direct QPSL-AN-42 Pressure Sensor primarily to read vacuum.
It looks like we can just use a 56ohm resistor across the inputs to get a 0.224V to 1.120V range when using the 4-20mA output of the Sensor.

We'll effectively get 4-12ma range below 0 PSI which I think the range will be enough for us. (0.224V to 0.672V, 0.448v of range).

I just wanted to be sure this is OK before I fry a $270 module.

The alternative is to add an analog module that is currently backorderd and $230, Plus the space and bus current needed for it.

I've converted 4-20 to 0-5V using a 240ohm resistor before, but this was with a different control system (An Arduino, actually).

If this will work, it would be great.

Any help is much appreciated.

[Greg]

The input impedance of the BX-08THM module is >1M-ohms. So, if you select to configure that particular analog input as 0-1.25VDC range, then the voltage drop across the 56-ohm resistor should easily fall within that range (as you pointed out; 0.224 to 1.12 volts).

This should work.

[ZoNiE]

Hi Greg, Thank you for the quick reply.

Do I need to tie (-) to ground or is it already done so internally?

[Greg]

That's a good question. The BX-08THM is a FACTS Engineering design (not ours at Host Engineering). Our hardware engineers said FACTS Engineering's spec sheet for this module says 0 commons. This makes us think there is no channel-to-channel isolation. Common mode range is limited to 0.6VDC. So that basically means that the TCx- nodes all need to stay within 0.5VDC of each other. From their spec sheet, it is not clear if there is any isolation from user supply 0VDC to TCx- nodes.

So, the bottom line is, you probably need to ask FACTS Engineering to be sure. Sorry. 

[ZoNiE]

Ahh. Though all BRX were yours. I see them on their site now. Their modules should remain white.

[BobO]

Ahh. Though all BRX were yours. I see them on their site now. Their modules should remain white.

Host makes the -3 and UT analogs. FACTS make the rest.