Hi,
i need a monoflop circuit that can create an impulse from a positive edge and an impulse with reversed polarity with a negative edge. I would like to run this with 12v.

The context is:
I have a on/off signal from a thermostat. This needs to drive a bistable solenoid to open a latch. The solenoid only needs a short impulse to change positions. And it also need the reversed polarity to return to its original position.

Thanks in advance!

 

The impulse width needs to be speced, 100's of mS for a solenoid ?

I assume you want the switch / contact initiator to be debounced ?

Lastly does the solenoid not need two positive pulses, one setting it, then
the next pulse resetting it. In other words there is a single coil driving
a latching solenoid ? Or a two coil ?

Regards, Dana.

 

around 50ms would be enough, thanks Dana

 

Here is a solution for two coil solenoid -



One chip used. Most of resources on chip unused, room for expansion,
other functionality.

You would need two mosfets to handle the V,I for the solenoid attached to
output pins.

Regards, Dana.

 

What is the resistance of the solenoid (how much current does it need)?

 

Is the thermostat just a single contact which is on or off?

 

Updated schematic, stuff in dashed lines, MOSFETs, Thermostat contacts, Rs off chip.



Regards, Dana.

 

Many thanks Dana,
since i am not so versed with ic's could you recomend the appropriate components? and i guess i would need to program the ubd?
about the current i can't find it on the datasheet of the solenoid. It only sais 12v 30w but no internal resistance.

 

The solenoid will require 30W/12V = 2.5A to change states.
Do you have a 12Vdc supply that can provide that amount of current?

For that current, the likely choice is a MOSFET bridge circuit to drive the single coil solenoid, controlled by one or two one-shot (monostable) circuits.

 

Question, what is the minimum time that would occur between the rising and
falling edge of the Thermostat contacts ?

Are the thermostat contacts floating, isolated ? Or one side ground, the other side
a closure to ground ? If one side ground is that ground sames as house ground ?

Is it two coils on the solenoid you pulse one coil for open, the other from close,
or one coil you pulse twice, one to open, then again to close ?

The 12V is that DC available to power the chip ?

Regards, Dana.

 

About the

Question, what is the minimum time that would occur between the rising and
falling edge of the Thermostat contacts ?

Regards, Dana.

In theory this can be short like a couple of seconds but i use the thermostat in a process that is slow and is measured in minutes, usually 30min or more.

 

The solenoid will require 30W/12V = 2.5A to change states.
Do you have a 12Vdc supply that can provide that amount of current?

For that current, the likely choice is a MOSFET bridge circuit to drive the single coil solenoid, controlled by one or two one-shot (monostable) circuits.

I've got a 60w 5A supply.

 

Here is a screenshot in operation -



The board you would use is -

http://www.cypress.com/documentatio...s/cy8ckit-043-psoc-4-m-series-prototyping-kit



Regards, Dana.

 

Confirm that solenoid is a single coil ?

Also that you have to pulse it once with a polarity, then again with a reverse polarity as crustchow
mentioned ?

Lastly this is a microprocessor based design, sounds like you might not have
any background working with them ? If so maybe an off the shelf approach makes
more sense.

Regards, Dana.

 

If it had a microprocessor there would be no problem. The pulses can be delivered using one single-pole double throw relay controlled by the thermostat. Also a non-polarized capacitor of some fairly big capacitance, possibly a motor starting cap, with a resistor across it to discharge it after a while. The cap would go from the common switch contact to the solenoid, with the return lead from the solenoid to the common between the plus and minus power supplies, and the outputs going to the NO and NC contacts. When the relay closes the supply charges the capacitor and current of one polarity goes through the solenoid coil. When the relay releases the opposite polarity flows through the solenoid until the capacitor charges with the opposite polarity. That is about as simple a circuit as I can come up with, and it should be quite reliable.
I hope that somebody understands my description well enough to produce a drawing, the lack of my cad drawing software is a pain.

 

Below is the LTspice simulation of an H-Bridge circuit that generates opposite polarity ≈100ms pulses through the single solenoid winding on the rising and falling edges of the input transitions from the thermostat (between 0V to 12V and back).
It use two N-MOSETS, two P-MOSFETS, one CD40106 Schmitt Trigger Hex Inverter IC package, plus a few resistors and capacitors.
U2a performs bounce suppression of the thermostat switch contacts (which adds the observed delay between the input transition and the pulse start).
U2b and U2d generate the pulses.
Arrow indicates positive direction of solenoid current.

 

Also a non-polarized capacitor of some fairly big capacitance

For 100ms time-constant and a 4.8Ω solenoid resistance the capacitor would need to be about 21mF.

 

There is one aspect of doing this with a appropriate UP, that
is startup/power down/brownout behavior. In other words for
any power condition no false pulses are created.

If the above is don't care then discrete logic not a problem. But
if there is UP are designed such that those conditions can be
handled. In a design with an H bridge the possibility exists to turn
on two MOSFETs in the a vertical column, shorting the supply to ground.
That wreaks havoc on the system needless to say. UP again can
handle that issue. Some systems will latch up, others blow fuses,
damage MOSFETs, etc..

In modern H Bridge ICs and/or UP some can insure there is "dead band"
timing implemented in the switching. This prevents any overlap of two
vertical MOSFETs on at the same time.

In discrete logic design no large caps should be directly connected to
CMOS logic input pins w/o current limiting.with a simple R in series to
the input pin. In case Vdd is brought to ground rapidly forcing cap to dis-
charge thru internal protection diodes not designed to handle large currents.


Regards, Dana.

 

OK, so my circuit will require both positive and negative voltages. BUT it is far simpler than any of the other suggestions, even though it requires both positive and negative voltages.
What was not mentioned, I don't think, was if this thermostat system is in a home or a car or a boat or someplace else. If power is available then developing both polarities will be simple.