Voltage sensing switch with deadband ideas?

[Glenn Ashmore]

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?

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

 

[Sjouke Burry]

Glenn Ashmore wrote:

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?

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.

 

[John Larkin]

On Wed, 12 Nov 2008 12:00:26 -0500, "Glenn Ashmore" <gashmore@cox.net>
wrote:

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?

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

 

[Rich Webb]

On Wed, 12 Nov 2008 12:00:26 -0500, "Glenn Ashmore" <gashmore@cox.net>
wrote:

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?

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
http://www.edn.com/article/CA84881.html

--
Rich Webb Norfolk, VA

 

[Gareth]

Glenn Ashmore wrote:

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?

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

http://hyperphysics.phy-astr.gsu.edu/Hbase/Electronic/schmitt.html

--
-----------------------------------------------------------------------
To reply to me directly:

Replace privacy.net with: totalise DOT co DOT uk and replace me with
gareth.harris

 

[Jasen Betts]

On 2008-11-12, Glenn Ashmore <gashmore@cox.net> wrote:

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?

with positive feedback and a single op-amp you can get the behavior
you want.


+12V
|\|
in ------|-\
| >-----+----
4.4V-[1K]-+-|+/ |
| |/| |
R1 | -12V |
| |
`--[270K]--'
R2

R2 is chosen so that with the output low the voltage at the
non-inverting input will be the lower limit voltage and with the
output high it will be the upper limit.

you may wish to use different supply voltages. and use a voltage
divider instead of the 4.4V supply.


maybe something like this

+12V
|\|
in ------|-\
| >-----+----
+8-30V -[7805]-[1.2K]-+---+-|+/ |
| | | |/| |
| 8.2K | 0V |
| | | |
0V -+----------+ `--[150K]--'

bye.


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