How Flippers Work

Many people are confused on just how flippers work with the double coils and switches and such. Here's a good explaination on just how these things actually work.

First off, a brief bit of info about Ohm's Law: The flipper coils work off 43 Volts DC (VDC). According to Ohm's Law, a small resistance draws a large current, and a large resistance draws a small current. This also implies that current flow will always try to follow the path of least resistance (like everything else in nature). Finally, for this example, just assume that current flows from positive to negative (ground).

If you look at a Bally flipper coil, you'll notice 3 lugs instead of two, like in the picture below:

There are two coils in this assembly, a high power coil, and a low power, joined together by the middle lug, like in this drawing:

The high power coil has a small amount of wire (500 wraps), which results in a small resistance, around 10 ohms. Using Ohm's law, at 43 volts, 10 ohms will draw about 4.3 Amps, which is quite a bit of current. The low power coil has over 5000 wraps of wire, resulting in about 350 ohms. At 43 volts, this coil will draw about .12 Amps, or 120 milliamps, which is a whole lot less than the high power coil. The high power coil is used to move the flipper when you first push the button, resulting in a nice strong flip. If you want to hold the flipper in the up position to trap the ball, the high power could would eventually burn up, or you'd blow a fuse. A continuous draw of over 4 Amps is just too much. So what happens next is when the flipper reaches the top of the flip, a little tab on the flipper pawl opens up a switch. This switch is called the End of Stroke (EOS) switch. In the two pictures below, you can see a before and after photo of this phenomenon (look at the black tab thing that pushes the switch):

Before Flip

After Flip

When the switch is normally closed, it bypasses the low power coil and most of the current flows through the switch, then just the high power coil (very little current will flow through the low power coil when the switch is closed). This gives the flipper it's punch. Then at the top of the stroke, the tab opens up the switch which makes the current flow through the low power coil IN ADDITION to the high power coil. This adds lots more resistance, which drops the current flow, which allows the player to keep the flipper engaged in the up position without burning anything up. When the flipper button is released, the switch again closes and everything's back to normal.

The picture above is what's happening just as the flipper button is pushed. Current flows through the EOS switch and then through the high power coil. This coil has a very low resistance, so it draws a lot of current, making for a nice strong flip.

In this second picture, now the flipper has hit the EOS switch and it opens, allowing current to flow through both coils as long as the flipper switch is closed. By switching in the low power coil, it's resistance is added to the resistance of the high power coil and now the total resistance draws a lot less current. Enough to hold the flipper up, but not enough to burn anything up, or blow a fuse.

Some Bally machines, like Power Play, have "upper" flippers. For machines like this, things are a little different. The lower flippers pretty much behave as above, but things change just a little bit for the upper flippers. The upper flippers have their own EOS switches, which act just like the lower flippers. The difference is in how they're powered. On machines with upper flippers, the EOS switch on the lower flipper is a double pole switch - one pole normally closed (like above) and the other pole is normally opened. It's this 2nd pole that powers the upper flipper.

When you push the flipper button, the lower flipper goes first. When it hits the EOS switch, and does the normal low-power coil cut-in operation described above, the other pole of the 2-pole EOS switch closes at the same time the first pole opens. This second pole, when closed, supplies power to the upper flipper coil. Once powered, the upper flipper's coil activates and pulls in it's plunger using it's high powered coil. Then, just like the lower flipper, it activates it's EOS switch (single pole) which switches in the lower power coil to allow the player to hold the flipper up.

Below, I've redrawn the second and third pictures above to illustrate a dual-flipper setup, plus added a fourth:

The picture above shows what's happening just as the flipper button is pushed. Notice the 2-pole EOS switch on the lower flipper, and see how it is not yet supplying power to the upper flipper. As before, all the current is flowing through the lower flipper's high powered coil only.

A fraction of a second later, the picture above shows the lower flipper's EOS opened which will switch in the lower flipper's low powered coil. Also, you can see how the normally opened pole is now closed, supplying the +43VDC to the upper flipper. The upper flipper now starts the whole sequence all over, by first supplying power to the high power coil only, since it's single pole EOS switch is closed at first.

Then, after another fraction of a second, the upper flipper EOS switch opens up too, which switches in it's lower power coil, and now both flippers are in the up position, and safely too as the lower power coil on both flippers have been switched in.

Now that you know how the flipper coils work, look at this drawing below:

If you replace the boxes labeled "Flipper Coils" on the left with the pictures of the flipper coils and EOS switches from up above, you can see exactly how the flippers work. The flipper enable relay is located on the Solenoid/Regulator board and is driven by the Q15 driver circuit. This is a "continuous solenoid" driver which is controlled by the MPU. When the relay is energized, the two switches inside the relay pull down and complete the circuit from the flipper buttons to the flipper coils. As long as the relay is energized, the flippers will respond to the buttons.

If all of your flippers do not work, you should check the Q15 driver circuit and make sure it's OK. You can ground the tab on Q15 and that should make the relay pull in. While that DOES NOT test Q15, it does verify the circuit traces from Q15 to the relay are good, and that the relay itself is working properly. If the relay is working properly when you ground the tab of Q15, make sure the flipper relay pulls in during the solenoid diagnostic test. If it does pull in, then Q15 is OK. If not, replace Q15. If the relay is working properly but your flippers are not, then you probably have a problem with the wiring, the flipper switches, or the coils and/or EOS switches.

On a rare occasion, the relay will fail and neither flipper will work. On an even rarer occasion, the switches inside the relay will fail and one or the other (or both) flippers will not work. You can usually hear the relay pull in at the end of the MPU boot process, just around the time you see the 7th flash on the MPU's LED.