Bally logo Bally Solenoid Driver
for Dummies
Bally logo

First off, take a look at the picture below

This is a typical Bally coil from a Mata Hari machine. Notice 3 things: The two big fat yellow wires going to one lug, the small skinny wire going to the other lug, and the diode connected to the two lugs.

One lug on every coil is visited by these fat wires, in what's called a daisy-chain. This is the wire that supplies each coil with positive 43 volts DC (+43VDC). So each coil is connected to the +43VDC bus. Most have two fat wires, but some may have one. Flipper coils have these wires too, but they are connected a little differently, and are discussed elsewhere. For now, just assume we're talking about regular solenoids here.

Next, you'll notice each coil has a small skinny wire on the other lug. This wire goes to the control circuits on the solenoid/regulator board. In order to energize the coil, there must be a path to ground for the +43VDC. Normally, there is not so the coil is relaxed. When the small skinny wire gets connected to ground, the path is complete and current will flow. This current flow turns the coil into an electro-magnet and then pulls the plunger into the coil. When the wire is disconneted from ground, current flow stops, the electro-magnet is turned off, and the plunger returns to it's normal position, with help from either a spring, or gravity.

Finally, the diode. When the current is quickly turned off on an energized coil, the magnetic field around the coil collapses quickly and causes the coil to generate a huge voltage spike. The job of this diode is to prevent the majority of this spike from reaching the solenoid driver circuity. If the diode is bad, or installed backwards, you'll pop the driver transistor the first time the coil is energized, then released. It's like the ignition in older cars - when the points open, the 12 volts is removed from the car's coil quickly, which causes another coil to generate a huge voltage spike, to the spark plug. The computer program that runs the machine also tries to limit this spike by turning off the coil near the zero crossing of the line AC. This helps because the DC that drives the coils is rectified, but not filtered, so it's not smooth DC, but "humpy", like in this picture. By energizing the coils just after the zero crossing, the in-rush of current caused by a coil is limited, and by turning them off just after the zero crossing, the voltage spike caused by the collapsing field is also kept to a minimum.

So, in the simplest form, the solenoid driver circuits in your Bally look like this:

Look at it as a bunch of coils all connected to the +43VDC bus, and the other lugs going to switches which are also connected to ground. Then, if you were to close a switch, that would connect the circuit from +43VDC to ground, and the coil would energize as long as the switch is closed.

Now see how the circuit is complete due to the switch being closed, and the coil is energized. Then you open the switch and the coil turns off and you're back to the first picture. If the diode were not there, when you opened the switch, there's be a big arc across the switch contacts at the moment they opened up.

Finally, take this one step further and replace the manual switches with transistors. Transistors are normally used as amplifiers, but you can also use them as switches too. There are 3 leads on a transistor, the base, the emitter, and the collector. For NPN transistors like the ones on your Bally solenoid driver, you can used the emitter and collector like a switch. With no current supplied to the base, there is no current flow between the collector and emitter, so the transistor switch is open, or OFF. If you supply a current to the base, current will then flow between the collector and emitter, so now the switch is closed, or ON.

Without getting into too much detail - what happens is a current is applied to the base which is high enough to 'saturate' the transistor. This means the collector-to-emitter current will be amplified as high as it can, and the transistor will then conduct a large amount of current from COLLECTOR to EMITTER, in relation to the current flow from the BASE to the EMITTER. This is how it acts like a switch. The base goes high to turn it on, and low to turn it off. Since the collector is connected to the wire that goes to the coil (the small single wire), and the emitter is connected to ground, turning the transisor as the effect of connecting the collector to ground. This completes the circuit to the coil and it fires.

You may have heard that you can test a coil by grounding the tab on the coil's driver transistor. For the TIP-102 transistors used in the Bally solenoid driver, the metal tab is connected to the collector. Knowing this, and what you've just learned, you can now see that grounding the tab is the same as grounding the collector, which will complete the circuit to ground and fire the coil. Note that this test only tests the wiring from the solenoid driver to the coil. It DOES NOT test the transistor, or any circuitry before the transistor.

So, you can now replace the transistor and "control signal" in the simplified drawing above, with the actual circuit found on the page you just came from, and get an idea how the entire driver circuit works.

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Created 1/7/06 - Last Modified 1/20/07 - Steve Kulpa Mail Icon Nolensville, TN
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