Werking Solenoid Slot
5 Solenoid Current Characteristics at +450C Figure 5: Solenoid current characteristics at +45°C Time (ms) Solenoid Current (A) 0 10 20 30 40 50 60 70 80 90 100-0.2 0 0.2 0.4 0.6 0.8 1-30 deg C 0 deg C +45 deg C Figure 6: Solenoid current curves at different Fig. 6 Solenoid Current Curves at Different Temperatures temperatures. A twin coil solenoid has two coils with one wound clockwise and another counter clockwise. A coil must be activated to move it to either end. Often these have three wires with one wire being common on the two coils. This type of solenoid will require two output bits and two driver circuits (one for each coil).
The most common use for electric solenoids is to provide lineal actuation for valves and relays in control systems. Although solenoids appear to be relatively simple devices, there are a number of ope...
April 1, 2000By Ted GroveThe most common use for electric solenoids is to provide lineal actuation for valves and relays in control systems. Although solenoids appear to be relatively simple devices, there are a number of operating characteristics which, when understood, will make their application and maintenance considerably easier.
Electricity travelling through a wire creates a magnetic force, or flux, in a circular fashion around that wire. The direction and strength of that flux is relative to the direction and strength of the electric current in the wire and its rotational direction is identified by the “right hand rule.” Wrap your right hand around the wire with your thumb pointing in the direction of the current flow (make sure the wire is well insulated before doing this). Your fingers will then point in the direction of the force field flow. When this wire is wound into a coil, the flux is focused in the centre of the coil with a strength relative to the number of coils of wire.
Although this magnetic force field flows around the coil in a relatively insular atmosphere, it finds the conductivity of ferrous metals much easier to travel through. It is the overpowering tendency for the magnetic flux to find this “easy path” that creates the magnetic attraction that we are all familiar with.
The wire coil is surrounded by a steel case to collect and provide an easy path for the peripheral forces and reduce the electrical energy supporting the flux in this unwanted area. An iron plunger is floated in the centre of the coil and is generally spring loaded so that in a de-energized state, an air gap is created between the plunger and the end of the housing or pole piece (see figure 1). When the coil is energized, the plunger retracts towards the pole piece in an effort to close the air gap. This plunger action is used to push or pull the control member of a valve or relay to change its operating state.
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AC solenoids
An AC solenoid is operated by current alternating from a positive peak through zero to a negative peak and back again at a rate of 60 complete cycles per second. The magnetic field is strongest at the negative and positive peaks but as the current passes through zero, the pulling force on the plunger is decreased and the spring pressure starts to retract the plunger. This in-and-out motion of the plunger creates an unacceptable buzzing or chattering sound.
To correct this, a small ring of copper wire is put into a groove in the pole piece or end block of the solenoid so that the plunger seats against it when it is fully retracted. The copper ring, being very conductive, allows a relatively high level of electric current to be induced or generated in it by the magnetic field. This induced electric field produces its own magnetic field, which lags the primary field by 90 electrical degrees or 1/4 of the AC cycle. As the AC current passes through zero, the shading ring flux bridges the zero gap, and providing the plunger is contacting the shading ring, holds the plunger in position, eliminating the buzz.
Alternating current also creates varying levels of current flow in solenoid coils, depending on the position of the plunger. When the solenoid coil is first energized and the air gap is at its widest point, the magnetic circuit is incomplete, the AC resistance or impedance is low and the electrical current demanded by the coil is high. This high current level is called “inrush current” and only occurs in AC circuits.
As the plunger starts to move towards the pole piece, reducing the air gap, the AC resistance or impedance starts to climb and the resulting coil current starts to decrease until the plunger is fully retracted (see figure 3). The electrical current in the coil stabilizes at this point to the design level of the coil, called the “holding current.” Inrush current can be three to 10 times higher than holding current and can cause extreme overheating conditions, resulting in coil burnout if it is prolonged.
Buzzing coils
A buzzing coil indicates the plunger is not fully seated. This condition causes an inrush current situation which, if allowed to persist, can lead to an overheated coil and eventual burnout. A buzzing AC solenoid coil is “screaming” at you to check the shading ring for damage or the plunger path for dirt or debris without delay. A continuous-duty coil is capable of withstanding the heat generated by a constant holding current but not by a constant inrush current. Too often, the coil is changed without correcting the cause of the buzzing and burnout.
The frequency of operation of an AC solenoid will also affect the heat buildup in the coil. Each time the coil is subjected to the heat-generating levels of inrush current, its temperature rises a little higher. If the cycle frequency goes beyond the capacity of the coil to dissipate this additional heat, the coil will burn out.
DC solenoids
DC solenoids are simpler in construction and are not hampered by inrush currents and the need for shading rings. The heat generated by the resistance to current flow of the coil windings is constant and weaker regardless of the plunger position. Burnout, therefore, is seldom encountered with DC coils.
The flux generated around a coil charged with direct current (electricity travelling in one direction only) induces its own electric current in the coil wire in a reverse direction. This induced current, although weaker than the primary current, impedes the buildup of the magnetic flux to maximum strength (see figure 2). The resulting delay in actuation time will, generally speaking, produce a slower action than a similar-size AC solenoid.
Voltage spikes
Any load that generates a magnetic force, such as a motor or a solenoid, is classified as an inductive load. When the power is shut off, the collapsing magnetic field generates or induces a high voltage pulse of electricity in a reverse direction to that of the main current. This induced pulse is characteristically up to 10 times higher than the line voltage but has a very low flow or current. It happens with AC current but is stronger when using DC. The spark that is seen as a switch opens or the sparks generated around the brushes in your power tools are caused by this induced electric pulse or voltage spike.
Voltage spikes can be very harmful to other components in electrical circuits such as rectifiers and almost all electronic equipment. Spikes can also induce other electric noise or low voltage signals in adjacent lines close to lines carrying voltage spikes. This electrical noise can create false signals in transistor-based electronic control equipment but can usually be controlled by installing inexpensive suppressing devices around the coils or motors causing the problem.
Ted Grove, corporate training manager for Wainbee Limited of Mississauga, Ont., is an experienced fluid power trainer. Visit www.mromagazine.com on the Internet and click on the Past Issues button to view Practical Automation columns from previous issues of Machinery & Equipment MRO.
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An Introduction to Solenoids
Werking Solenoid Slots
A Small 5VDC Solenoid from Sparkfun
Unlike motors, solenoids directly provide linear (and not rotational) movement. Solenoids are found in many devices such as door locks and valves. If you have a car or have a washing machine, they are using solenoids. Note the spring on the sliding cylindrical bar or armature in the image above. The spring holds the bar out to the left, but when the coil is energized the magnetic force pulls it to the right. Whenever the coil is turned off, the spring pulls the bar out to the left again. André-Marie Amphere invented the solenoid in the late 1820s.
Applications of Solenoids
Solenoids are around everywhere in your home and car. Cars use then in starters, fuel injectors, some electric door locks, and shifting gears in automatic transmissions. Most older electric doorbells use a solenoid to strike the metal chime. In the home, solenoid controlled water valves can be found in washing machines, dishwashers, icemakers, central humidifiers, and automatic sprinkler systems for yards. Pinball machines use solenoids for flippers and bumpers.
Fuel injectors in car engines contain a solenoid
A solenoid water valve from Sparkfun
Solenoids shift gears in a car's automatic transmission
Home Doorbells hit the metal chimes using solenoids
A solenoid door lock from Adafruit
This model train track switch has a twin coil solenoid.
Pinball flipper image from: http://findinglincolnillinois.com/gambling/flippermechanism.jpg
A recent CBS Network news story on the comeback of pinball machines
A Solenoid Hello World
If you want to try using a solenoid with mbed, Sparkfun has a small solenoid that uses 5VDC at 1.1A max. It is somewhat small, most solenoids are larger and need higher voltages and more current. The tricky bits are that it will require a driver circuit (i.e., needs more current than an mbed output pin provides) , an external 5V power supply must be used (i.e., it requires more current than the mbed's power supply provides), and it can only be left on for short periods of time (i.e, it can overheat and burn out).
Sparkfun Power MOSFET driver board
For this example, a small MOSFET driver board was used from Sparkfun. This MOSFET has a maximum rating of 60V 30A, so it can also handle larger solenoids that may need a higher voltage and more current. It also has the internal snubber or suppression diode that should always be used on inductive loads. The board's PCB traces and terminal blocks would likely fail well below 30A. Larger gauge wires may also be needed for higher current solenoids. Breadboard jumper wires are only good for a bit more than one amp, but the MOSFET PCB's terminal blocks allow the use of larger wires on high current devices. For higher voltage or AC solenoids, a relay or solid state relay can be used instead for the driver circuit. Sparkfun also has a 5VDC 2A low cost AC wall adapter power supply that was used for this test. Breadboard jumper wires can be attached to the MOSFET driver board screw terminals and they can also be pushed into the connector on the solenoid, so everything will hookup on a breadboard.
Information
When using multiple DC power supplies, don't forget to connect power supply grounds together, but not the V+ outputs!
Wiring
| mbed | MOSFET PCB | External Device | External 5 V supply |
|---|---|---|---|
| gnd | JP2-2 gnd | gnd | |
| JP2-1 RAW | 5V | ||
| P8 | JP2-3 Control | ||
| JP1-1 | Device(neg) | ||
| JP1-2 | Device(pos) |
Demo Code
A basic code demo using the mbed DigitalOut API and the MOSFET driver circuit is shown below. Time delays are needed to ensure that the solenoid is turned on long enough to close, but this solenoid also needs waits for the time delay to cool down to avoid overheating. The on and off cool down times will vary quite a bit depending on the application and solenoid used.
The code is simple enough, but it must always be turned off to avoid overheating. It might make sense in many applications to use a timer to automatically turn it off later in a timer interrupt routine, so that a program can do other things while the solenoid is on. Assuming the RTOS is not used, the TimeOut API can be used. With the RTOS, multiple threads could be used to avoid the issue of blocking while the solenoid is on.
Here is an example with a 'Solenoid' class that sets up a timer to automatically turn off the solenoid. The solenoid works exactly the same as the first example, but the program does not need to stop for the solenoid on-time delay and it turns off automatically later with a timer interrupt.
Warning
This solenoid is rated only for intermittent duty. It may overheat and fail, if left on for longer than a few seconds. The data sheet suggests a 25% duty cycle with a max on-time of 1 sec followed by 3 seconds off. Even running the example code (20% duty cycle), it gets a bit hot after several minutes. Only a larger and more expensive continuous duty rated solenoid can be left on, but most are rated for intermittent duty only.
If an external event triggers the solenoid, it might also make sense to ensure in software that it stays off long enough to cool down. For example, consider a system where a user presses a button to activate the solenoid. If the user kept the button pushed down forever or the button fails in a short condition, the solenoid would burn out. A wait or timer could be used to ensure that the solenoid would remain off long enough to avoid overheating. If needed, this feature could also be added to the solenoid class. This can also be handled by a hardware pulse (one shot) circuit. Once a system has a microprocessor, the software solution is likely to be more economical.
Here is the new class setup that enforces a minimum off time delay with a timer and also still turns off automatically using a timer interrupt. Even through the code turns on the solenoid in an infinite loop without any waits as fast as possible, it functions in a identical way to the two earlier examples as far as the solenoid is concerned. It turns on for 0.5 seconds and then off for 2.0 seconds. So it is not possible to overheat it and burn it out.
It is probably advisable to back off the maximum rated duty cycle a bit. It is likely that this value assumes the solenoid is mounted on a piece of metal that provides a heatsink effect and is also probably for room temperature conditions with some airflow. Mine seems a bit too hot even at 20% after several minutes of operation on the breadboard. Some newer devices with solid state drivers (i.e., not relays) even use a PWM output control bit to vary the power of the solenoid's stroke.
One could still envision a possible scenario where the processor crashes while the solenoid is on and it somehow disables the timer interrupt and the solenoid burns out (possible, but with extremely low probability of ever occurring over the life of a device). The watch dog timer and/or the brown out detection interrupts could also be used to make such a burnout event so unlikely that numerous other failure modes including hardware failures become more likely to occur first.
A solenoid RTOS timer setup
If an application is using the RTOS, a thread can be used to time the solenoid on/off cycles. Since the solenoid timing is 10-100ms or more, the RTOS scheduler with a thread::wait() can do the timing delays and other threads can all run during the delays. Keep in mind that RAM size limits the total number of threads possible with the RTOS to just a few (each thread needs it's own stack and some other RAM). Here is a simple example with a class that sets up the solenoid thread and delays. Main sets a flag, turnon, that the solenoid thread checks. This could also be done with a signal, but it is a little messy doing it in a class for more than one solenoid since the thread object name must be used to get the signal to the correct thread. Once again, it operates the same as the earlier examples as far as the solenoid is concerned (i.e., 0.5s on and 2.0s off).
Some larger and higher voltage solenoids can be found in stock at Sparkfun, Adafruit, and Digikey. The MOSFET will handle almost all of them, but you are going to need a higher voltage DC power supply.
Other Types of Solenoids
There are a couple of additional types of DC solenoids although they are not as common this basic type with one coil and a spring. The other two types do not use a spring to return to the other position. Although they require a bit more complex driver circuit and two control bits, they offer an advantage in that power is only consumed briefly when changing positions.
A twin coil solenoid has two coils with one wound clockwise and another counter clockwise. A coil must be activated to move it to either end. Often these have three wires with one wire being common on the two coils. This type of solenoid will require two output bits and two driver circuits (one for each coil). It is pulsed briefly on one of the coils to move it one way or the other and when the coil is turned off, it stays in the last position.
Another type of DC solenoid is sometimes called a latching solenoid. It requires the current to be reversed though the coil to move it the other direction. An H-bridge driver is needed for this type of solenoid with two output bits (i.e., forward and reverse) to control it. Sometimes a latching solenoid still has a small return spring and a permanent magnet that holds it at the other end in addition to the single coil.
Interesting Solenoid Videos
A Solenoid Rhythm Section
Eight Solenoids used to power a strange V8 style engine for those that hate normal boring DC motors. Note the relays under the solenoids on the right side and crankshaft contacts on the left.
Another V8, but with state-of-the-art mbed-based electronic ignition. Note the mbed and eight power transistors with LEDs near end of video. It uses a shaft encoder, PWM for solenoid control, and a PID control loop for timing.
Brain control for Solenoid Flippers on a Pinball Machine
A Player Pipe Organ using Solenoid Air Flow Valves
Smart yard sprinkler systems are basically IoT controlled solenoid valves
Many hotel room locks with swipe card keys use a solenoid. A hacker with an Arduino and a cable plug can unlock over 4 million of the older ones. A tragic security design flaw for a lock!
An electronic design error is believed to have overheated an intermittent duty solenoid that blew up one of the world’s first military drones in WWII killing Joseph Kennedy, the older brother of U.S. President John Kennedy. In addition to RF remote control, it also had live video feeds. The drone’s mission was to destroy Hitler’s Supergun bunkers in France that were designed to destroy London.
Werking Solenoid Slot Cars
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