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Voltage sensing switch with deadband ideas?

Perhaps you need to filter, because when pumping, you get pressure waves. Try to look at the sensor output with an osciloscoop, and check whether you need filtering.

Reply to
Sjouke Burry
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Dropping resistor samples 4-20 mA, low side ground.

RC lowpass filter to take out noise

Two comparators, thresholds at your limits. Opamps will work as comparators if done right.

CMOS R-S flipflop; VH comparator drives S, VL comparator drives R.

Flop Q drives mosfet drives relay coil.

Gotta get the comparator polarities right.

Do I get a ride in the boat?

John

Reply to
John Larkin

Instead of an opamp, try a comparitor set up with a hysterisis deadband set for its output to go low when crossing 4.45 increasing and go high at 4.35 decreasing.

There's a brief article that describes the process at

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Rich Webb     Norfolk, VA
Reply to
Rich Webb

I need to maintain pressure on a hydraulic system within a narrow +-3% band for some tests but the commercial precision pressure switches are way out of my budget . I have a 4-20ma pressure transducer that I can drive at 24V with a maximum 700 ohm shunt resistor (Say 680 ohms). I need a switch that will turn the pump on when the voltage drops below 4.35V and off when it reaches 4.45V.

I have been experimenting with an LM1458 dual op amp and some 10K trimmer pots I had on hand and can get a pair of LED's to light as the input voltage goes high or low outside the .1V range but I am having a brain freeze on how to latch a relay on when the low op-amp turns on and off when the high op-amp turns on. I can change the logic by reversing the op-amp inputs but any way I do it the pump will turn off as soon as the pressure gets in the range. There will be no deadband so the pump will cycle rapidly.

Any suggestions?

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Glenn Ashmore

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Reply to
Glenn Ashmore

Can you just use a single Schmitt trigger? For example, see:

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Reply to
Gareth

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