Differential voltage switch

Im working on a control device for pneumatics and Im a bit lost. I need to read two pneumatic signals up to 30psig and have a simple switch actuate a relay if the output of the two pressures differentiates by more than 1psi. Ideally I need to be able to adjust the amount of differential pressure required to actuate the switch by a difference of up to about 5psi.

I found this transducer and I think it will do the reading part. Digi-Key - 287-1010-ND (Manufacturer - ASCX30AN)

As for the logic, Im lost. I just need something that can handle the output of the above sensor 0-5v (or recommend a different sensor 0-30psig) that I can adjust. This will be powered by a 12vdc battery. I can make a voltage regulator if need be, the above sensor is 5-16v input. This will switch a 12vdc relay. Thanks for any suggestions.

Do you need to take the absolute value of the difference, and then pull in a relay if the absolute value is > threshold?

Here is a circuit I think does what you need:

U1 takes the DIFFerence of P1 and P2, and adds a Ref voltage of 2.495V, created by the TL431.

U2 and U2a takes the ABSolute value of DIFF and inverts the signal.

U3 (output GT) compares ABS to a fraction of Ref, which is settable using the pot. The comparator has a bit of hysteresis to prevent chattering.

The relay is pulled-in whenever GT is high.

Follow the various signals through the plots. Opamp could be an LM324 quad.

btw- I charge my consulting clients $150 per hour....

albersondh said:

Im working on a control device for pneumatics and Im a bit lost. I need to read two pneumatic signals up to 30psig and have a simple switch actuate a relay if the output of the two pressures differentiates by more than 1psi. Ideally I need to be able to adjust the amount of differential pressure required to actuate the switch by a difference of up to about 5psi.

I found this transducer and I think it will do the reading part. Digi-Key - 287-1010-ND (Manufacturer - ASCX30AN)

As for the logic, Im lost. I just need something that can handle the output of the above sensor 0-5v (or recommend a different sensor 0-30psig) that I can adjust. This will be powered by a 12vdc battery. I can make a voltage regulator if need be, the above sensor is 5-16v input. This will switch a 12vdc relay. Thanks for any suggestions.

Click to expand...

Not to deter from Mikes post or work but I have a question. The gauge (pressure sensor) you linked to is a Honeywell ASCX30AN which is abn absolute gauge. It is not a PSIG but a PSIA, where the pressure is referenced to absolute or PSIG + 14.656 (Atmosphere).

I think a better choice of gauge as you want to compare the pressure in two pneumatic signals up to 30 PSIG would be a delta pressure gauge like the Honeywell ASCX15DN which has two ports and measures a Δp between the ports up to 15 PSID. That gauge also outputs .250 to 4.5 volts full scale as a function or 0 to 15 PSID. One gauge is all you need with a port going to each pressure source if I read things correctly.

Here is the data sheet for the sensor I suggest.

If the pressure remains closer than 15 PSID you could go with maybe a 0 to 5 PSID rather than the 0 to 15 PSID I suggest.

Ron

Ron, can the device you linked be "biased" so that it's output is mid-range with zero differential? It doesn't appear to specify what happens if the input port is at a lower pressure than the reference port?

MikeMl said:

Ron, can the device you linked be "biased" so that it's output is mid-range with zero differential? It doesn't appear to specify what happens if the input port is at a lower pressure than the reference port?

Click to expand...

Hi Mike,

I thought about that after leaving work today and read the data sheet again and came away with the same conclusion you noticed. I generally work with much larger and expensive Rosemount gauges which do have the output offset like -10 to 10 Volts or 4 - 20 mA with 12 mA as the center or zero. I even looked at the unamplified versions of the Honeywell I suggested assuming maybe it was just a bridge and the output would be able to be offset but I regress on that note. I suggest the OP run with your circuit as it is a sure safe bet.

Ron

Yea my gauge is only good to 15.3 and thats no good for me. I need 0-30psig. Thanks for the help guys.