rjvh

Hi i would ask for some help as this is my idea of making it

i have 1 pressure sensor that have to control 2 VSDs (Variable speed drive)

but 1 VSD i want to have an adjustable separate switching point

questionmark from my side in this shematic is that i want to have some delay in switching off the 2nd VSD (i am not sure if this is the way i schould make it)

Thanks in Advance

Robert-Jan

Attached Files

vsd control.pdf (51.2 KB, 10 views)

ericgibbs

hi,
Looked thru your drawing, what is the value of the time delay required.

Also at what voltage do you need the Comp to switch over.?

Do you want a Delay at the Comp output when the input rises above the Set point and when it falls below.?

Attached Images

VS1.jpg (51.1 KB, 11 views)

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rjvh

Hi eric

the voltage that the comparator have to switch over is between 4 and 6 volt

it is the goal to let the comparator switch direct if the the set treshhold is reach (swiching if the voltage rises over the set point)

if the voltage drops down to the set point i want to have a delay of several seconds (2 til 5 seconds adjustible would be perfect but setting the delay by trial and error is for me also acseptable) to avoid nervous reactions near the switching point

I had the idea to put a feedback resistor over the comparator (thats the reason the diode is still at the inverted input) for hystiresis in the system but that doesnt give me delays
still than hystireses is still someting that is not out of my mind but in this design it would suply the capasitor constantly so the delay would not realy work i think

Robert-Jan

ericgibbs

hi Robert-Jan,
Look at the attached drawing.
For the switching of the Comp, to avoid 'chattering' at the switch over voltage you require some positive feedback, via the Rhysteresis resistor.

The higher the resistance the narrower the 'dead band', I would suggest you start at about 4.7M for Rhys. When you add or change the value of the Rhys you will have to adjust the Set Point to compensate for the small positive feedback voltage, which will affect the Set Point.

OPA's dont make good comparators, you could use a LM393 as the Comp and a LM358 as the amp.

Also note the 4066 ic's dont have a high current capability, what are you driving into.?

Attached Images

VS1.jpg (51.9 KB, 7 views)

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Eric
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rjvh

hi eric thanks for your advice

the input impendance from the VSD is 30 khm: and the voltages will be never higher than 10 V so in my opinion the 4066 could swich that without any problem

about the lm 324
you sugested the lm 393 for a comp and lm 358 for the amplifiers

no doubt that they will do a better job but is the lm 324 absolut not suitable to use ????

I have to look if they have the other lm's here with a bit luck they do stock them

Robert-Jan

rjvh

just another thought as i have still 3 swiches over in the 4066

i am not sure if i can leave the sensor line to the VSD floating so i could short it with another swich(4066) to the 0V still there is the risk (as both swiches are swiched simultanuasly still there could be a time differance) that there is a direct short from the buffer amp to 0V

i could put a resistor in that line (10 khm: ) to the 0V to limit the max curent in a case of a shorting

or is this over worried from my side ??

Robert-Jan

ericgibbs

hi,
The LM324 is low spec device, I would not use it as a comparator.
The other point is how close to 0V do you require the outputs of the OPA and Comp to go.?

Its never a good idea to leave inputs floating, a 10K from the pin to 0V should be OK.

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rjvh

as the specs say for the 4066 the minimum on voltage at Vcc 12V have to be 8.4V and the maximum off voltage is than 2.4 V

so the comparator should give more than 8.5 V for the on signal and lower than 2.3V for off

the amps need to giv 0v as that 0.1V will activate the drive already

i still looking in the manual of the VSD if i can change that in the menu but till now i didn't find that it is possible

Robert-Jan

ericgibbs

hi,
This will be a problem, check the LM324 datasheet for the output voltage range.

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rjvh

Ok minimum low voltage (max tolerance) at a load of 10khm: is 20mV

so that's not 0V i geus that i will have to dive a little more and deeper into the VSD manual

it's a new type of VSD for my but it should be posible to have several setpoints in the curve and curve adjustment

other wise i need more comparators and short the input from the VSD to ground at lower levels or control the VSD start stop points in the manual swich section of the drive than the comparators only need to swich a transitor

i will do some home work on this

Robert-Jan

rjvh

well home work done

it turns out that the VSD drive is pretty high tech and have function menus manual which you can compare to the bible (size wise)

it is capable to have several set points and curve adjustments so the need for the comparator in this circuit is disapeared

even an inverted input option is posible to select which is the first opamp in this shematic

as i still have to drive 2 VSD units with 1 sensor i think i do still need the buffer amps

Is the lm 324 a choice that will be capable to do that or it is still recomended to find an other candidate??

eric thanks for your comments and advices
all the efforts are not waisted as i will build this circuit and maby still have some questions later on

because with this circuit i can control much cheaper ( less advanced VSDs than this model) VSD units which i will use in the future

Thanks
Robert-Jan

ericgibbs

hi,
Good luck with the project.

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Eric
"Good enough is Perfect"

PIC tutorials:
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