The flippers in a pinball machine have a unique design challenge compared to most other devices in the game. They need to kick the pinball at least as hard as any other device whenever the player presses the flipper button, but they also need to last. Most of the strong devices in the pinball machine (pop bumpers, sling shots, magnets, etc.) are driven by high powered solenoids to make the game lively and exciting. But if left on for more than a few seconds these solenoids can overheat, melt the plastic sleeve, grip the steel plunger and probably blow a fuse.
The game designers took precautions in their games to ensure that most of those solenoids would only stay on very briefly. The solenoid in a pop bumper for example is only on long enough to draw the plunger into the solenoid sleeve and kick the ball away. Watch the video in the pop bumper section to see this up close and in slow motion.
Flippers on the other hand are under the player's control and can be kept on indefinitely by holding in the flipper button on the side of the cabinet. In fact part of the game playing strategy is to capture the ball on a raised flipper and hold it there before taking a shot at a specific target.
So the question is, how can a flipper stay on indefinitely without overheating and blowing a fuse? This video from the flipper section gives a quick explanation.
But read on for a more thorough answer.
The Solenoids, Relays and Electromagnetism lesson better describes how electric current activates solenoids. But as a general rule, the amount of electric current passing through a solenoid helps determine the strength of the solenoid and the power it consumes. So a flipper strong enough to kick the ball requires a solenoid that uses lots of electric current and power.
While a stronger flipper solenoid will give the pinball a better kick, it also introduces the potential problem of overheating. Most electrical devices generate heat as electric current passes through them. For some devices like toasters or hair dryers, that's a good thing, but for other devices like laptop computers or smart phones, it's not. In fact some devices like incandescent light bulbs convert most of the power they consume into heat, rather than light as you might expect.
Generating heat isn't a bad thing if it can be dissipated as quickly as it is generated, but high powered solenoids generate heat much more quickly than they can dissipate the heat. When a high powered solenoid is left on too long it can overheat, start to smoke, melt its plastic parts or even burn nearby combustibles. Pinball machines have fuses which should blow before overheating can happen but sometimes a blown fuse is mistakenly replaced with a larger fuse which can lead to problems.
These photos show a solenoid that was left on too long due to a fault somewhere else in the pinball machine. It had scorched its wrapper and melted its inner plastic sleeve before the fuse blew or someone smelled smoke and turned the power off.
To avoid overheating flipper solenoids while players hold on to a trapped ball, most flippers use solenoids with two coils of wire wound together around the same core.
The first coil is a high electric current/high power coil used by itself to kick the pinball up the playfield. The second coil is a much weaker, low electric current/low power coil that is really only strong enough to hold the ball behind the raised flipper. Because of the two coils wound together, flipper solenoids have three terminals instead of two as other solenoids do.
The flipper assembly has a switch built into it called the End Of Stroke switch which switches the flipper from high power to low power once the flipper reaches the end of its swing. You can see how the switch works in the video above. The flipper only uses high power for a fraction of a second before switching to low power - just enough time to kick the ball and no more. The flipper switches from high to low power seamlessly; most players never realize that it's happening.
The following schematic diagrams may give you a better idea of how the two coils work together with the flipper button and End Of Stroke switches. If schematics and switches are unfamiliar, reading through the Switches and Electric Current lesson first may be helpful.
The first schematic shows the flipper button switch open which means that the flipper button on the side of the cabinet is not pressed in. In this situation the flipper is idle and no electric current is flowing through either coil. Note that the End Of Stroke switch here is a Make/Break switch which was used in some EM pinball machines.
When the player presses the flipper button on the side of the cabinet, the flipper button switch closes which completes the circuit and allows current to rush through the high power coil as shown by the green path through the schematic. The End Of Stroke switch allows the current to bypass the low power coil entirely. When the high power coil activates by itself in this situation the flipper can move with enough strength to kick to ball up the playfield.
Once the flipper has reached the end of its swing, a small lever attached to it changes the End Of Stroke switch which prevents electric current from flowing through the switch directly to the high power coil. This forces the electric current to flow instead through both the high power and low power coils. The amount of electric current flowing through the circuit decreases because the combined resistance of the two coils is more than the resistance of the high power coil alone. As Ohm's law says, increasing the resistance in a path will decrease the electric current through it. With the reduced electric current flowing through the coils the flipper is only strong enough to hold the ball, but neither coil is in danger of overheating so they can remain in this situation indefinitely.
While some games used a Make/Break End Of Stroke switch as shown above, most games used instead a simple, normally closed End Of Stroke switch as shown in the schematic drawings below.
When the flipper button switch closes, most of the electric current flows through the normally closed End Of Stroke switch. A trivial amount of current flows through the low power coil too, but because the resistance of the low power coil is so much higher than the negligible resistance of the closed End Of Stroke switch, nearly all of the current flows through the path of least resistance. The Make/Break switch shown earlier prevents any current from flowing through the low power coil when the flipper button is first pressed, but the small amount of current allowed to flow through low power coil here makes no real difference.
Once the flipper reaches the end of its swing and the End Of Stroke switch is forced open, all of the current is forced through both flipper coils.
Here are examples of schematic drawings from Gottlieb, Williams and Chicago Coin:
The final example is from a Bally schematic:
Notice how in the Chicago Coin example they explicitly describe the low power coil as a "Resistance" to clarify that it's for limiting power and not kicking the ball.
You might think that two coils should be better than one, and that the examples showing both coils working should represent the strongest configuration of the flipper solenoid. After all, there are more wire loops being used which should create a stronger magnetic field (as described in the Solenoids, Relays and Electromagnetism lesson) which should make the the solenoid stronger.
The reason that this isn't the case is that the resistance of the two coils, which is really dominated by the resistance of the low power coil, reduces the electric current that flows through the coils. The penalty of the reduced electric current far outweighs the benefit of additional wire loops so the net effect is a weaker magnetic field, and weaker solenoid that uses less power.
The schematic drawings above use the same symbol for both the high power and low power coils which makes them look the same. But really they are constructed in different ways to give them their different characteristics. The photo below shows a low power coil from a relay and a (very likely overheated) high power coil from a pop bumper for comparison.
Notice that the smaller relay coil is wound with very fine wire and the pop bumper coil is wound with much thicker wire. The thick wire has a much lower resistance to electric current than the fine wire for a given length of wire. Think of it as the difference between drinking a milk shake through a straw and drinking it through a piece of garden hose of the same length.
Lower resistance lets the thick wire carry more electric current (as described by Ohm's law) which lets it consume more power. More electric current also makes a stronger magnetic field inside the coil which makes the solenoid stronger for a given number of loops. (Refer to the Solenoids, Relays and Electromagnetism lesson for reasons why.)
It's hard to tell from the photos, but lower power coils are often wound with more turns or loops of wire than high power coils. The extra loops require more wire which adds to the resistance of the coil and further limits the electric current that can flow through it.
Flipper solenoids require both high and low power coils wound together in a single package. If you look closely at the flipper solenoid terminals in the photo below you'll notice that the red wire used for the high power coil is much thicker than the green wire used for the low power coil.
When flipper coils are replaced the question often comes up whether they are wired correctly or not. The replacement coil may not look the same as the original or the flipper may not be as strong as expected. To verify that a flipper coil is wired correctly into the game have a close look at the three solder lugs to see if you can identify how the two coil windings are wired to the solder lugs.
Note that the solder lugs in this example may be different than the solder lugs on your coil. Visually check for the wire thicknesses to determine how your flipper coil is wired.
On this flipper coil the solder lug on the left (#1) has a single thick copper wire connected to it. That indicates that it is connected to the end of the high power coil that is not also connected to the low power coil.
The middle solder lug (#2) has both a thick wire and a thin wire. That indicates that it is connected to both high and low power coils.
The right solder lug (#3) has just a thin wire connected to it which indicates that it is connected to the end of the low power coil not also connected to the high power coil.
The normally closed End Of Stroke switch in this example should be connected across the low power coil, or between solder lugs 2 and 3.
Some manufacturers use descriptive part numbers which tell you a lot about the construction of their solenoids. Consider for example the FL-20-300/28-400 or AF-25-600/31-1000 flipper solenoids. These tell you exactly what size wire and how many turns or loops are used in each of the coils.
The AF-25-600/31-1000 flipper solenoid uses 600 turns of 25 gauge wire for the high power coil and 1000 turns of 31 gauge wire for the low power coil. The wire gauge in this case is defined by the American Wire Gauge (AWG) standard. According to the AWG standard 25 guage wire is 0.0179 inches in diameter and has a resistance of 32.37 milliOhms per foot. 31 gauge wire is 0.00893 inches in diameter and has a resistance of 130.1 milliOhms per foot.
Stated another way, the high power coil in a AF-25-600/31-1000 flipper solenoid uses wire that is:
Also, the high power coil has a little more than half the number of turns as the low power coil which means that it uses a little more than half the length of wire used in the low power coil. All of these factors combine to give the high power coil:
Find more background information about other pinball machine devices is on the Things to Learn page.