I need to build a circuit to control a latching solenoid. This type of solenoid will open when a short 6VDC pulse (under 100ms) is sent to it and will closed when I reverse the polarity and send a "negative" pulse. I want to control it using a press-button: As long as I keep the button pressed, the solenoid is kept open. When I let go of the button, the solenoid will close.
This means that when I press the button I have to send ONE "positive" pulse and when I left go, I have to send ONE "negative" pulse.
Another requirement I have is that the whole circuit needs to consume no electricity when the button is not pressed. So I guess that when I let go of the switch, I need to keep the power connected for a couple hundreds milliseconds so that the circuit can send the "negative" pulse to close the solenoid.
I know how to program the arduino really well but as for circuit design, I have no idea where to start... any help would be appreciated
The capacitor value depends on the current the solenoid takes - say it took 100mA and you wanted about 300ms
pulse then at 6V you would need about 4700uF (but it would need to a non-polarised electroytic, since normal
electrolytics cannot tolerate reverse charging).
Telecommando, this will work?! I had in mind an Arduino and a couple of MOSFETs to do the switching... I like simple solutions. If this will work, that would be sweet!
But I don't see how this would give a "negative" pulse and I let go of the switch...
have you ever heard of Gal? They make controllers and clocks for irrigation and pilot valves that are bi-stable latching solenoids. The method I used was to charge a 4700 uF cap to 12V and dump it into a solenoid with a DPDT relay and a Mosfet. The relay for polarity reversal and the Mosfet for switching the load. I could drive a solenoid through 300 meters of 16 Ga wire at 10 bars of pressure.
szangvil:
Telecommando, this will work?! I had in mind an Arduino and a couple of MOSFETs to do the switching... I like simple solutions. If this will work, that would be sweet!
But I don't see how this would give a "negative" pulse and I let go of the switch...
Best not to think about negative and positive pulses but rather the direction of current flow. In Telecommando's simple example when you switch to the positive voltage the current will flow in one direction latching the solenoid in one position and current will continue to flow until the capacitor is fully charged to the amount of the voltage source where current will cease to flow. Then when switched to the ground switch position current will flow in the opposite direction latching the solenoid in the opposite direction and current will continue to flow until the capacitor is fully discharged. It's the current direction that the solenoid responds to, not the polarity of the voltage source. The size of the cap does have to be matched to the current requirements of the solenoid's requirement.
Cool circuit... I investigated them all 15 years and discarded them all because they all use too many parts or the quiescent current was too high.. Much too expensive and no protection from electrical discharge or transverse lightning strike. my device operated a latching solenoid with up to 100 meters (tested at 150) of wire. The circuit I finally settled on used 1 DPDT relay, 1 Mosfet, 2 SS transistors, 1 PNP and 1 NPN, a 4700 uF cap and 2 resistors. I replaced ONE relay in 10 years of production of that assembly... and about 5 Mosfets. The same circuit worked well on the 2 coil solenoids as well with one more wire.
Telecommando:
You may need to experiment to find a suitable capacitor value, but this should work:
I would add a flyback diode, anode to ground, cathode to common switch terminal - otherwise you will get arcing in the switch. Make sure you get a break-before-make switch,
The capacitor doesn't need to be non-polarized if its value is high enough to make the circuit overdamped, which it needs to be anyway in order to get a good current reversal.
But the relay will consume power. I need the whole system to be battery operated get good battery life. So I can only tolerate power consumption when pulsing the solenoid.
What voltage are you using to pulse the solenoid? Do you know how much current it takes? is the foot switch a momentary type (in which case, can you tolerate a small current draw while the foot switch is held down), or is it push on/push off?
This is the switch I use now. It is very low profile and is easy to push with your foot.
But currently, I use a normally closed solenoid which needs constant power to be kept open. So I have a large 7000mah battery or a 110/220 -> 12VDC power supply.
The solenoid take about 200-300ma to operate. The switching will be mostly slow (10-30 seconds apart) but sometimes, it should be able to operate at 1-5Hz.
szangvil:
I built a small prototype circuit with a 9V battery and a 10V 3300uF capacitor. It works really well even at 2-3Hz switching.
But as I said, I need a circuit with an on-off switch...
For that simple circuit you need to use either one SPDT switch or two SPST switches. Can't think of any other solution for your manual switching needs.
Telecommando:
Also, I don't think there will be much of an inductive kick in this type of circuit so a flyback diode should be unnecessary.
The switch changeover won't be instant. There will be the usual large inductive kick-back when the upper contact opens, which is likely to cause arcing until the lower contact closes. How serious it is depends on the self-capacitance of the solenoid.
