Diodes across coils on solid state pinball machines

I read so much false information on pinball related sites when it comes to technical electronic information.  Even on sites such as Pinwiki and Pinside there is usually about 50% to 70% misinformation.  These people state these false facts with such authority.  I worry about the people who believe it. (But hey, this is the internet.  You have to take everything you read with a grain of salt.)

I should start a regular feature here called False Facts.

Today’s false fact: Diodes are connected to flipper coils to help them release faster.

Actually, it’s just the opposite.  The flippers would release faster if the diodes were removed.  I’ll explain why in a second.  But first, don’t even think about removing your diodes to improve flipper response.

The diodes are there to suppress the voltage that is generated when the magnetic field of the coil collapses, after the power is removed from the coil.  This applies to coils that are powered by a DC voltage, which would be all solid state machines and a few of the later EM machines.

OK. So here is the slow motion version of what’s happening.  You press the flipper button and power is applied to the coil. Actually there are two windings in the coil, one strong one for “pull-in”, and a weaker one for “hold”, but just pretend for this discussion there is only one.  In most machines, the flipper button is in the ground circuit of the coil. The power supply voltage passes through the coil, through the switch, to ground. The magnetic field builds up and pulls in the plunger.  This takes a little bit of time, maybe 20-50 milliseconds (I’ve never measured it on a flipper coil).

(This is simplified.  Usually there is also a relay involved to keep the flippers off when the machine isn’t playing a game.  And the Williams Fliptronic system adds quite a bit more complexity to the scenario.  I’m going to omit the phase relationships between voltage and current. I’m also going to refer to current flow from positive to negative.)

A regular diode allows the current to flow in only one direction, like a one-way check valve. When the coil is energized, there is no current flow through the diode because the current is going in the opposite direction for which the diode is installed.  If you were to turn the diode around, all of the current would flow through the diode and not the coil, causing a short and likely burning up the diode. So at this point the diode is invisible to the coil.

As long as current is flowing though the coil, a magnetic field is maintained.  When you release the flipper button, power is removed from the coil.  But due to a variety of factors, including the plunger being inside the coil, it takes a little bit of time for that magnetic field to collapse. As the magnetic field collapses, it generates a current in the coil in the opposite direction that was used to create the magnetic field.  So for a brief time, the current starts running backwards.  Since it’s now going in the opposite direction, it goes though the diode to the other side of the coil causing a momentary short across the coil.  This short dissipates the power until the magnetic field is gone and the plunger is released.  But the point is, for a brief time, there is still current flowing in the circuit, through the diode, after the button is released and a magnetic field is still holding the plunger.

If the diode was removed, the magnetic field collapses, but there is no current flow because there is open circuit and no place for the electrons to go. Instead the voltage keeps rising and rising across the coil until it arcs across the switch contacts or sends a big voltage spike into the rest of the pinball machine. (This is basically how sparks are created across spark plugs in your automobile engine.)

But since there is no current flow (very little anyway) the magnetic field goes away faster and the plunger releases faster.  But as mentioned, the downside is a voltage spike, which can cause havoc in a solid state machine.

So that is why the diodes are installed across every coil in the pinball machine, to reduce the voltage spikes.

There are no diodes in EM machines because they are running on AC instead of DC.  The voltage is lower to begin with and the coils not as strong, there isn’t much of a spike.  But it could be suppressed with a resistor and capacitor across every coil (called a snubber).

Now… there is a way to decrease the release time of flippers, but I’ve not tried it.  And I’m not sure how noticeable it would be and don’t recommend it. Replace each regular diode with two zener diodes in series, back to back (opposite polarity).  The diodes would need to be rated at the maximum voltage seen by the flipper coils (about 70V for a 50V system) and a current rating of 2-3 amps. A voltage spike will still be generated, but it will be a bit more controlled and may still cause havoc with a solid state machine.  This is how fuel injectors in an engine are handled (essentially a coil and a plunger, just like a flipper) and those are switching on and off at a very rapid rate.

10 thoughts on “Diodes across coils on solid state pinball machines

  1. Thanks for this explanation. I have a Captain Fantastic EM machine from 1976. There are five coils on an 80 volt DC bus – the three pop bumpers and the two slingshots. The schematic shows 1N4004 diodes across the coils for the pop bumpers, but not for the slingshots. I’m not really sure why that would be the case. I assume they are in place to protect the bridge rectifier? They were cut out at some point, and the pop bumpers worked fine. I just soldered in some new ones where they are shown on the schematic, and now I’m wondering if I should do the same for the slingshot coils.

    • It wouldn’t hurt to put the diodes on the slingshot coils. But I can’t say why they are on the pop bumpers to begin with. Perhaps to protect the rectifier, but maybe to give a little longer life to the switch contacts.

  2. I have a question maybe you can help me with. I am repairing a Close Encounters of the Third Kind (solid state) machine. The issue is one of the switch matrix chips (a 74156) on the NiWumpf board keeps blowing after playing a couple games. It seems to be related to the drop target bank, or the Roto Target assembly. I cannot find any shorts anywhere on the playfield, but I am wondering is a bad coil diode would cause this?

    • Usually if a coil diode is bad, it will cause the processor to lock-up. It usually won’t blow a chip. I don’t have a NiWumpf schematic handy, but my guess is there is a coil circuit shorting into part of the switch matrix, maybe at the coin door.

  3. I have a 1979 Williams flash that I had to replace 2 flipper coils on (SFL-20-300/30-800). I noticed the lower right flipper coil has 2 diodes on it – one between to the tow outer lugs and one between the far left and middle (when looking at it from the non-lug end).

    Do I need to move that diode from the old coil to the new one?

    • You will want to wire everything exactly the same as it was with the old coils: same number of diodes, same wire colors going to the same ends of the diodes. A diode is polarity sensitive so make sure the banded end of the diode is wired the same as the old coils.

    • Unfortunately, yes.

      Since you have to solder it to reconnected it and since diodes are so inexpensive, you may as well just replace it.

  4. What a great article post – Excellent – Thank You!
    Very helpful.
    Would be great to clarify – failure-modes, symptoms, test procedures…

    Can you clarify – symptoms/difference between diode failure in short VS open?

    Here’s my attempt – would you agree? Am I missing anything ?

    I’m rebuilding flippers – I’ve examined – confirmed I need new EOS switches and capacitors… It it recommended trying to proactively replace the diodes on old coils as well – just to prevent issues reducing back spike voltage? I know lifespan of the CAPs is around 20 years? Would you refresh flipper diodes as well on coils…

    DIODE FAILURE MODE: SHORT
    ===========================
    Causes: No voltage drop over diode
    Current flows direct to capacitor?

    Result: Flipper Won’t stay up
    Eventual Stress – failure of downstream yellow flipper capacitor
    Once Capacitor sink is gone – voltage can spike- carbon-build-up
    on blades of downstream flipper relay on boards.
    Eventually – weak flippers – dead capacitor
    Pitting of EOS Switches

    Diagnose: Diode shows short/<1ohm on DMD Ohm Test

    DIODE FAILURE MODE: OPEN
    ===========================
    Causes: Voltage Spike To Flippers button/switch-blades
    OR Back into "System?"?
    Because – no current flow after coil de-energizes – coils can spark discharge

    Result: WPC- Resets
    OR
    Nothing? No Symptoms earlier games <1990

    Diagnose: Needs to be removed from circuit to confirm ?
    Shows open circuit – Over-limit on Ohm test.

    • For flippers:
      Diode failure mode: Short
      Most likely would blow a fuse or burn up a coil. It’s different for different pinball manufacturers and if the coil has two windings or not. For a Williams 1980’s machine, if the shorted diode was on the “hold” winding, it would cause the “power” winding to overheat and eventually fail. If the shorted diode was on the “power” winding it would blow a fuse immediately. On a WPC Fliptronics system, it would probably blow a fuse regardless of which winding due to each flipper having a separate fuse.

      Diode failure mode: Open
      On Williams/Gottlieb/Bally 1980’s machine it would probably cause the MPU board to immediately lock up or reset. It would be obvious. Just one or two flips would be enough to do it. On a WPC Fliptronics system it would blow one of the drive transistors for either the “hold” or “power” windings. There is a transistor for each. On a Stern/Sega/Data East it would blow a transistor/MOSFET.

      It’s difficult to measure if a diode is open with a standard multimeter while it’s connected to the coil. You’d have to remove one leg of the diode from the coil to check. But the symptoms are so obviously harmful to the system that if you have blown transistors in the drive circuit, suspect that the diode is open. This goes for any other coil in the pinball machine. There is no harm in connecting an additional diode across a coil if one is suspected of being open.

      For a shorted diode, the coil will appear to be shorted. I’ve known people to replace a shorted coil when all along it was the diode and the coil was fine.

      As for capacitors across the flipper button switches or the end of stroke switches, they are primarily for EMI suppression (https://en.wikipedia.org/wiki/Electromagnetic_interference). They might help reduce arcing, which occurs as the switch opens, and the air gap is still sufficiently tiny as to allow the air to ionize. As the air gap increases, the arc will eventually stop. The lifetime of ceramic and film capacitors is many decades. The most common failure I see in capacitors is broken leads due to the severe vibrations from the flipper mechs. Pinball machines with capacitors across these switches are in the minority. And even on Black Knight, I’ve seen machines with and without capacitors. It’s possible the machines with capacitors were exported to countries in Europe that had stricter limits on EMI.

      Getting back to diodes, a diode can’t be tested by measuring resistance on most multimeters (unless you’re only testing for a short). But most meters will have a diode mode and the value displayed is the voltage drop across the diode. It should be open in one direction, and for a silicon diode, should measure about 0.6 volts in the other direction (usually black lead on the cathode (banded end)).

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