Fun With Pinball

Flippers, Coils and Power

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.

Trapped BallTrapped BallLining up the next shot

So the question is, how can a flipper stay on indefinitely without overheating and blowing a fuse?  The video in the flipper section gives a quick explanation.  Read on for a more thorough answer.

Electric current generates heat

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.

Burned solenoid 1An overheated solenoidwith a burned wrapper Burned solenoid 2An overheated solenoidwith a melted plastic sleeve

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.

Flipper solenoids use two coils

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.

Two coil typesA normal coil and a flipper coilThe flipper coil has three terminals Flipper coil closeupFlipper coil closeup

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 in the flippers section.  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.

Flipper circuitry

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 switch.  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.

Flipper solenoid off schematicThe flipper is offThe flipper button switch is open and no coils are active.

When the player presses the flipper button on the side of the cabinet, the flipper button switch closes and electric current rushes through the high power coil and through the end of stroke and flipper button switches.  Most of the electric current passes around the lower power flipper coil because the coil has a much higher electrical resistance than the switch. Ohm's law says that the electric current flowing through a path is inversely proportional to the resistance of the path.  Since the end of stroke switch has almost no resistance most of the electric current chooses that path.  In this situation the flipper is moving with enough strength to kick the ball up the playfied.

Flipper solenoid on schematicThe flipper is kickingElectric current passes through the high power coil but mostly around the low power coil.

Once the flipper has reached the end of its swing, a small lever attached to it opens the end of stroke switch which prevents any more electric current from flowing through the switch.  This forces all of the electric current flowing through the circuit to flow 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.

Flipper solenoid holding schematicThe flipper is holdingLess electric current passes through both coils.

But wait, is that right?

You might think that two coils should be better than one, and that the last schematic of the flipper holding the ball 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.

High power coils vs. low power coils

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.

Wire sizesLow and high power coilsfrom a relay and a pop bumper

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.

Flipper coil closeupFlipper solenoid terminalsThe red wire is thicker than the green wire.

Flipper solenoid examples

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:

  • twice the diameter of the wire used in the low power coil, or
  • four times the cross sectional area, and
  • 1/4 the resistance per foot

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:

  • less resistance than the low power coil,
  • more electric current,
  • more power,
  • more heat,
  • a stronger magnetic field, and ultimately,
  • more strength


Find more background information about other pinball machine devices is on the Things to Learn page.