Dual-Wound versus Single-Wound coils

It’s common for pinball machines to include coils that are “held on” for periods of time longer than the maximum pulse time of 255ms. The obvious example of this is for flipper coils, though other types of devices use these too. (Diverters, the trolls in Medieval Madness, certain ball release coils, etc.)

In many cases, these types of coils need to have strong initial pulses to quickly move the mechanism from its resting to active position, but they also need to be able to be held “on” for a long period of time.

These two requirements are conceptually incompatible.

The way you make a coil strong is you give it lots of power and make it really big. Unfortunately the byproduct of that is heat, which means if you make a nice, big, powerful coil that’s strong enough to move the mechanism with the quick power it needs, then when the coil is left in the “on” state, it generates so much heat that it will burn up the coil. :(

Fortunately the pinball companies solved this 60+ years ago with the concept of “dual wound” coils. A dual-wound coil is essentially two separate coils in one. (There are literally two separate wires wrapped around the coil sleeve instead of one.)

Dual-wound coils have a strong (often called the “main” or “power”) winding which is used for the initial “kick” of the coil, and they also have a lower-powered (“hold”) winding which is used to hold the active position.

Note

You can tell if a coil is dual-wound because the coil will have three wire connection points instead of two. There’s a power winding connector, a hold winding connector, and a common connector that’s shared by both.

The way these are used is that the strong winding is pulsed initially (usually for a fraction of a second) to provide the initial strength to move the mechanism, then it cuts off, leaving just the weaker hold winding active to keep the mechanism active. The hold winding can safely be enabled for a long time, even multiple minutes. When the machine wants to disable the device (or when the player releases the flipper button in the case of a flipper), the power to the hold winding is cut, and a spring causes the mechanism to return to the initial position.

Transitioning from the power to the hold winding: The old way

In old pinball machines (from the 1940s through the early 2000s), the “transition” from the power winding to the hold winding was purely mechanical and done using something called an “end of stroke” (EOS) switch.

The EOS switch is a physical leaf switch in the mechanism under the playfield with a switch that is mechanically opened by the movement of the device. When the coil is first activated, the current flows to both the power and the hold windings, and the mech starts to move. A few fractions of a second later, the mech reaches its full “up” position, and a little arm under it hits the EOS switch which opens it and breaks the connection to the power winding, leaving only the hold winding energized.

When the hold winding is de-energized, the spring causes the mechanism to move back to the original position, and the EOS switch is closed (by the movement of the mech) meaning that the next time the mech is activated, the current will again flow to both the power and hold windings.

Advantages of using this “old style” EOS switch

  • It’s simple. No computers or fancy timing has to be involved, and the transition from the power to the hold windings is automatic.
  • If a coil gets dirty, gummed up, or weak, the transition from the power to the hold winding always occurs only after the mech is all the way in the “active” position.
  • Only a single “driver” connection from the control system is needed since that single control line is used for both the power and hold windings.

Downsides to using this “old style” EOS switch

  • No fine tuning. Since the transition from the power to the hold winding is purely mechanical, you can’t change the power of the mechanism unless you physically switch out the coil and/or change the voltage used.
  • For flippers, you don’t get any “novelty” flipper modes. You can’t do things like “weak flippers” or “no hold flippers” since the flipper behavior is mechanically controlled.

Transitioning from the power to the hold winding: The modern way

Modern machines do not use EOS switches in the same way they have been used in older machines.

The main reason for this is that modern pinball control systems (including all the control systems that MPF supports) have the ability to activate coils with millisecond-level precision (something that was not possible even in 1990s WPC machines).

Using flippers as an example, in modern machines, when the player presses the flipper button, the control system will send current to both the power and hold windings at the same time, and then at a very precise moment (e.g. 27ms later or 14ms later or whatever), the control system will cut off the power winding, leaving just the hold winding active.

This has the same effect of the mechanical EOS switch in that the power winding is only used for the initial power motion, and the lower-current hold winding is then used to keep the flipper in the up position.

Advantages of using the modern transition from power to hold

  • You can fine-tune coil strength by changing settings in software.
  • You can use novelty modes like weak flippers, no hold flippers, etc.

Downsides of using the modern transition from power to hold

  • You have to play with your settings to get them right.
  • A dirty, gummed up, or worn-out coil or mechanism might mean that the initial power timing setting you originally configured might not be strong enough to move the mechanism all the way into the “up” position.

Single-wound coils

So far both options (EOS and non-EOS) we discussed use dual-wound coils with power and hold windings.

However there’s a third option that some modern machines use as well. The third option is to use more traditional (e.g. “single wound”) coils for your machine that do not have the dual “power” and “hold” windings.

Of course you might be thinking, “How does that work? Wouldn’t the coil burn up if the mechanism was active for too long?”

This is another case where modern technology can be used to address that.

In electronics, there’s a concept called “Pulse Width Modulation” (or “PWM”), which (in this case) basically means the control hardware turns the power on and off really fast. (Like, hundreds of times per second.)

So the way this works is that you have a high-powered, strong coil which is activated a full strength in order to provide the strong initial motion. However once the mechanism is in the up position (based on either an EOS switch, or based on the millisecond-level precise timing), the control system stops powering that coil at 100% and instead cuts the power back (using that PWM thing) to a smaller percent (like maybe 12.5% or 25% or so). That reduced power is enough to keep the mech in the up position, but not enough to cause the coil to overheat and burn out.

Advantages to using single-wound coils

  • You only need a single driver output per coil (instead of two).
  • You can still do the modern things, like use software to tune the strength of the coil and novelty flipper modes.

Downsides to using single-wound coils

  • You have to figure out the PWM (low power) settings which need to be strong enough to hold the mechanism up but not too strong so they don’t burn it up.
  • Sometimes the PWM “hold” makes an annoying buzzing sound (since the power is being turned on and off hundreds of times per second).

We should note that the decision to use a single-wound versus dual-wound flipper coil is technically a separate decision from whether or not to use an EOS switch. See the Flipper end-of-stroke (EOS) switches for more on that decision.

Which option should you choose?

Ok, so basically there are three options for coils that need to be held on for more than 255ms:

  • Dual-wound, with a mechanical EOS switch to transition from power to hold.
  • Dual-wound, with the control system timing to transition from power to hold.
  • Single-wound

The good news is that MPF supports all three options.

If you’re retheming an existing machine, and you’re using the original driver boards and power supplies, then you should probably just use whatever method was used in that machine and keep it simple.

If you’re building a new machine, most people choose the second option, where you use a dual-wound coil but with the transition of the power to hold windings done via software and the modern control systems. The reasons for this include:

  • It’s simple. You don’t have to mess with trying to figure out the PWM timings for the hold winding.
  • It works. You know the hold winding was designed to be held on at full power, so you don’t have to worry about breaking things.
  • It’s less wear-and-tear and emissions. Rapidly cycling power (in the PWM way) for the hold phase in a single-wound coil has the potential to add wear to the components in your system and potential to cause EMI emissions.

People have also pointed out that Stern’s S.A.M. system (which they used in from about 2006-2015) used the single-wound PWM-style flippers, but then with the SPIKE system (from 2015 onwards) went back to the dual-wound computer controlled option. So even in this modern era, there’s precedent for using dual-wound coils.

Really the only reasons to use the single-wound coils are:

  • You already have mechanisms that use single-wound coils
  • You’re running out of driver outputs in your control system and you don’t want to “waste” two drivers per mech.