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My boiler is connected to a sun-water-heating system. After the sun goes down this system start to draw heat from my boiler. Today I'm manually closing the valve connecting my boiler to the system.

I'll like to build system based in Arduino that will operate a solenoid that will open and close the valve.

The solenoid will probable by normally-closed and I'll open it only when sun is up which is a few hours every day.

I remember reading somewhere that solenoid should be operate for short time, is that true? Can I leave the current on for say 6 hours to keep the solenoid open? Is there a better way?

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There are a couple of solutions to this. The first is to use a specially designed latching solenoid. These typically have a permanent magnet in them so that they can hold in position without power. It's probably the simplest way.

However, you may still be able to use a regular solenoid depending on its design. The force the solenoid exerts is basically proportional to the 1/displacement (though this relationship can be adjusted by shaping the magnetic pole pieces). This means that when the solenoid is disengaged it requires a lot more current to provide the same force to the plunger than when it is fully engaged. Depending on your load and the force-displacement characteristic of the solenoid, you may not need the maximum holding force to keep the solenoid engaged. This means you can reduce the current and hence the amount of heat dissipated in the solenoid coil. If this amount of heat is within a range that can be continuously dissipated by the solenoid then there will be no issue doing this for a long time.

It is quite common in solenoid circuits to have this sort of force/current reduction mechanism, and if you have a well defined load, you can simply reduce the current after a fixed timeout.

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  • \$\begingroup\$ You may want to provide some clarification regarding "reducing the current". The DC resistance of the solenoid coil is what determines static current flow and so really what the technique needs to be is to reduce the voltage of the power supply or to apply a high frequency PWM drive to the coil that reduces the duty cycle from 100% to a lesser amount. Often the PWM technique can be the simpler to implement. \$\endgroup\$ – Michael Karas Feb 24 '17 at 12:38
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The time the valve is allowed to be active differs from valve to valve. Mainly the active time is limited by the heat dissipation produced by the coil windings. Thus, check the datasheet for any limitations.

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    \$\begingroup\$ Solenoids tend to be current hungry beasts and hence get very hot if powered for too long. Using a latch type solenoid reduces the power requirements to a minimum (and hence heat produced) - see comment reference above. Feel free to produce an answer around that. \$\endgroup\$ – JIm Dearden Feb 24 '17 at 11:35

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