Fun With Pinball

Solenoid Strength Test
(or, Why Does Removing Windings Make A Solenoid Stronger?)

Solenoids used in pinball machines are made of long lengths of copper wire wrapped in loops around a bobbin with a cylindrical hole through the middle.  These solenoids commonly have hundreds or thousands of loops of wire, or windings around the bobbin.  When current passes through the windings a magnetic field is created in the hole which draws in a steel plunger that provides motion and power to whatever device it is attached to.

Some pinball machine owners remove windings from solenoids in their games to increase the power and liveliness of playfield devices like pop bumpers and flippers.  The fact that removing windings from the solenoid makes it stronger is somewhat counterintuitive however because it's hard to imagine that a solenoid with a single winding would be more powerful than a solenoid with a thousand windings.

In general terms the strength of the magnetic field created in a solenoid is proportional to the number of windings around the bobbin and the amount of current passing through the windings.  If removing windings can make a solenoid stronger why don't solenoids have just a single winding?

The Experiment

To better understand what's going on I devised an experiment to try to measure the effect of removing solenoid windings.  Initially I thought I could approach the problem mathematically and simply plot the calculated solenoid strength against number of windings but the math gets complicated in a hurry and can have a fair number of assumptions baked in.

I realized however that it's not really the theoretical solenoid strength that's interesting as much as how well the solenoid performs in a pinball machine. So I built a simple test fixture that uses a sling shot assembly to kick a pinball up an incline.  The idea is that the distance the ball travels up an incline could represent the power delivered by the solenoid.

Note that this isn't intended to be the last word on solenoid alteration. It's just a data point to ponder based on the observations of a tinkerer.

The Test Fixture

The test fixture is a simple rectangular box mounted at an incline of about 5.25 degrees with markings to measure the distance the ball travels from the lower end.  The plywood base is a piece of new cabinet drawer side with a hard finish similar to that of a pinball machine playfield.  A standard used Williams EM sling shot assembly is mounted to the lower end along with a rubber ring and ball guides to return the ball to the same spot in front of the sling shot.

Solenoid Strength Test Fixure Video

The base of the fixture was clamped firmly to a heavy workbench to minimize the vibration and motion that would otherwise draw some of the solenoid power away from kicking the ball.  The guides return the ball so that it just barely touches the rubber ring directly in front of the sling shot kicker arm.  The kicker arm pushes the rubber ring into the ball but does not reach the ball guides so that all of the power from the sling shot should be delivered to the rubber ring and to the ball.

The sling slot uses a standard Williams G-23 600 solenoid (600 turns of 23 gauge copper wire) that is wired up with a momentary push button switch to a garden variety EM pinball machine transformer that delivers 25 volts AC (alternating current).  When the button is pushed power is delivered to the solenoid and the ball is kicked up the incline.  How long the push button is held shouldn't have much effect on how far the ball travels because after the initial kick the ball loses contact with the kicker.  A longer button press just makes more noise...

Gathering the data

The experiment was a data collection exercise.  The sling slot was fired 25 times and the distance the ball travelled was recorded each time.  Then 4% of the remaining solenoid windings were removed and the sling slot was fired another 25 times.  The solenoid resistance and AC current were also measured for each solenoid configuration.  The experiment was repeated through 18 iterations reducing the winding count from the initial 600 windings to 300 windings, or half the original number of windings.

The Test SubjectThe Test SubjectThe original G-23 600 solenoid Altered Solenoid Mounted in the Test FixtureAltered Solenoid Mounted in the Test Fixture The Test SubjectThe Test SubjectThe solenoid after some windings were removed

Each time windings were removed the newly exposed end of the wire had to be sanded to remove the lacquer insulating layer.

The Results

In the end the results confirm what others have observed, that removing windings does make the solenoid more powerful.

Experiment ResultsExperiment ResultsDistances travelled compared to the number Solenoid Windings

This graph shows how the number of solenoid windings (along the bottom) correlate to the distance travelled (on the left).  The green line is the average distance travelled over the 25 attempts.  The violet and blue lines are the maximum and minimum distance travelled.  Note that from 416 windings to 300 windings most iterations had a few of the 25 attempts reach the end of the ramp so an actual distance could not be measured.  The orange boxes in the lower right show how many of the 25 attempts for each iteration "Hit the Wall".  In the future the slope of the ramp could be increased to avoid this situation.

The electrical measurements too are what you might expect as you reduce the number of windings.

Experiment ResultsExperiment ResultsSolenoid resistance and current compared to the number of windings

The resistance drops steadily as windings are removed and the AC current rises.  I stopped measuring AC current after 339 windings to avoid reaching the 10 amp limit of my meter.

Observations - Variability

The actual distance measurements plotted in the first graph above reveal something that a strictly mathematical exercise would not have.  There is remarkable variability in the distance the ball travelled across the 25 attempts.  An equation would have given a single result for a given number of windings but the measured behavior demonstrates that there are more factors that affect the power delivered to the ball than just the number of solenoid windings.

Take for example the unaltered solenoid with 600 windings (the first points in the three distance graphs).  In 25 attempts the ball travelled as little as 19.5" and as much as 29.75", or about 50% more than the minimum.  The widely variable results are likely due to when the button was pressed relative to the voltage delivered by the transformer at that moment.  Here are three consecutive frames of video shot at 60 frames per second:

It's hard to know exactly how long it takes the sling shot arm to kick the ball from just these three frames but it seems clear that the entire process likely takes about 1/60th of a second or less.  This is what a 60 Hertz AC voltage does in the time between these video frames:

60 Hertz AC voltage60 Hertz AC voltageVoltage sweeps from 0 through its maximum and minimum values and back to 0 in 1/60th of a second.

If the entire sling shot kick takes less time than is shown in this graph then it should be apparent that when the button is pressed will have some effect on how far the ball will be kicked.  Pressing the button when the voltage is near zero will give the solenoid a relatively weaker start than when pressing the button closer to the maximum or minimum voltage.  This probably accounts for most of the variability seen in the distance results. Mechanical variation in the sling shot mechanism (wear, dirt, vibrations, loose joints, etc.) may also contribute to the variability in distance travelled.

The variability in the measured distances was severe enough that even the average distance graph doesn't move consistently up and to the right.  In several cases fewer windings led to a lower average distance most likely due to the variability.  Perhaps if many more than the 25 attempts were measured at each iteration the graphs would be more consistently increasing.

The variability also points out that an altered solenoid might not always deliver more power to the ball than the original solenoid. Note for example that the maximum distance measured with the original solenoid (29.75") is more than the minimum distance of all of the solenoid variations meaning that the original solenoid configuration beat all the other configurations at least once. It's entirely likely that this kind of variability is what you'd see in a real game too since you have no control over when a sling shot, pop bumper or flipper is activated relative to the AC voltage.

What about DC variability?

After the initial tests a question arose about how a DC (direct current) supply might change the variability of the results.  A DC supply delivers a constant voltage to the solenoid while the switch is closed compared to a constantly changing AC voltage.  DC supplies were used for some solenoids in late electromechanical pinball machines and are widely used in solid state games.

As a followup to the AC solenoid test I did a brief test of 25 more kicks with a different, DC supply to get an idea how variability might change between AC and DC voltages.  The metric I chose to measure variability is the percentage that the maximum distance travelled is greater the minimum distance.  In the initial 600 winding test for example the ball travelled a minimum of 19.5" and a maximum of 29.75" or a little more than 50% more ((29.75-19.5)/19.5).

Across all 18 iterations of the AC test the maximum distance the ball travelled varied from a little more than 40% to almost 70% more than the minimum. In the DC test the maximum was only 18% more than the minimum so in that small sample of 25 kicks at least the variability in the distance travelled was significantly less.  So a DC supply should give a much more consistent kick.

This observation also supports the idea that most of the variability of the distances travelled is due to when the kick starts relative to the AC waveform.

Observations - Resistance and Current

The data show that the resistance of the solenoid decreases with the number of windings.  The resistance should decrease at the same rate as the number of windings or more specifically with the length of the wire that makes up the windings so there are no surprises there.

The AC current measurements on the other hand increase more dramatically as the number of windings decreases. As the number of windings decrease from 600 to 300 the resistance decreases by about half (from 2.5 ohms to 1.2 ohms), but the AC current more than triples (from 3.3 amps to more than the last measurement of 9.3 amps at 339 windings). That is somewhat surprising because current and resistance are often inversely proportional. Understanding the current is key to understanding what's happening to the solenoid.

The strength of a solenoid is primarily determined by the strength of the magnetic field it creates.  The equation used to describe the magnetic field,

B = (μ * I * N)/l
Magnetic Flux Density = (Magnetic constant * Current * Number of Windings )/Length of the Solenoid

means that the strength of the solenoid (B) is proportional to the current through the solenoid (I) and to the number of solenoid windings (N).

Over the course of the experiment the number of windings was cut in half (½) but the current through those windings more than tripled (3).  That means that the magnetic field or solenoid strength increased roughly 1.5 times from its starting value (½ * 3).  If you look at the distance graph again you'll notice that the average distance travelled also increased roughly 1.5 times or 150%:

Average Distance TraveledAverage Distance TravelledDistance increased roughly 150% while as windings were cut in half.

So in this experiment the increase in the AC current more than makes up for the decrease in the number of windings because the current through the windings is increasing faster than the number of windings is decreasing.  This is why fewer windings results in a stronger solenoid.

Practical limits

There are of course practical limits to reducing the number of windings in a solenoid used in a pinball machine.  Pop bumpers for example are often wired so that they go off in pairs as shown in this schematic example. 

Pop Bumpers that fire togetherPop Bumpers that fire togetherThe top pair of pop bumpers will fire together, as will the bottom pair.

The top two pop bumpers will both fire when the No. 1 Bumper Relay fires; the bottom two behave the same way.

A pair of pop bumpers fired together with unaltered G-23 600 solenoids would draw close to 7 amps of AC current.  The same pop bumper solenoids with just 339 windings would draw close to 19 amps which would blow the 10 and 15 amp fuses commonly found in EM pinball machines and would likely stress the transformer and momentarily lower its output voltage.

More current also implies less efficiency.  While solenoids with fewer windings draw more power from the transformer, proportionally more of that power is wasted as heat.  The graphs above show that the altered solenoid with just 339 windings drew 280% more current than the unaltered solenoid with 600 windings (9.3 amps vs. 3.3 amps) while its average distance travelled was only 40% more (33.1" vs. 23.5").  Towards the end of this experiment the solenoid was noticeably warm after the 25 attempts while the original solenoid was not.

Taken to an extreme a solenoid with a single winding might work in a pinball setting but would require more current than could reasonably be delivered to it and would probably melt the first time it fired.

A Closer Look at the Data

The data also show that the measured current increased at a faster rate than the resistance decreased as windings were removed from the solenoid. But given Ohm's law:

Voltage = Resistance * Current
Volts = Ohms * Amps

that might not seem possible assuming that the voltage doesn't change - the resistance and current should change at the same rate.

Current and Resistance resultsCurrent and Resistance resultsCurrent and resistance change at different rates

Where the resistance cut in half over the course of the experiment you might expect the current to double, not triple.  Something else must be going on.

Another inconsistency in the data is that the voltage (25 volts AC) is not equal to the solenoid resistance multiplied by the current (e.g. 2.5 ohms x 3.3 AC amps for the original solenoid) as Ohm's Law would suggest.  These anomalies cannot be explained by Ohm's law without the inclusion of the electrical property inductive reactance.

Resistance vs. Inductive Reactance

The simplified version of Ohm's Law describing the relationship between voltage, resistance and current as shown above really only applies to circuits with voltage sources, switches and/or resistors.  Electromechanical pinball machines are made up of circuits that primarily use switches and solenoids or relay coils driven by AC voltages.

Solenoids don't behave like resistors because they are essentially inductors.  The difference is that while resistors have a resistance that limits current, inductors also have inductive reactance that limits changes in current.  The inductive reactance of a solenoid varies with the number of windings and with the location of the plunger that slides through the center of the solenoid among other factors.

Even though a resistor limits the current that can flow through it, the current can change its magnitude or even its direction instantly to match any change in voltage across the resistor.  The inductive reactance of a solenoid on the other hand makes the current seem sluggish in comparison, behaving more like it has momentum or inertia.  The magnetic field that forms inside the solenoid and draws the plunger in has a damping effect on the current through the inductor that resists any change to the current.

If the voltage across the solenoid changes (as AC voltage does constantly) it takes some time to change the current and the magnetic field in the solenoid in the same way that it takes time to change the speed or direction of a freight train.  Inductive reactance is essentially a measure of resistance to change in the current - the higher the inductive reactance (and the companion magnetic field) the harder it is for the current to change.

If you are familiar with the behavior of capacitors, the resistance to change of voltage across a capacitor (called the capacitive reactance) is a corollary to the resistance to change of current through an inductor.

As the number of solenoid windings is reduced the resistance of the remaining solenoid wire is reduced but so is the inductive reactance of the solenoid. In other words pushing AC current through a solenoid gets harder as more windings are added and easier as windings are removed.  So as solenoid windings are removed in this experiment the current rises faster than the change in wire resistance might indicate because the inductive reactance too is decreasing which allows even more current to flow.

So What's the Answer?

Deciding whether or not to remove windings from your pinball machine solenoids is a personal, subjective matter.  Personally I'd want to be sure that the related mechanism is in top shape before altering its solenoid. You could for example:

  • chase out all of the mechanical inefficiencies like friction, worn linkages and parts, loose screws, dirty playfield, etc..
  • check for over stretched springs or other factors that compete with the solenoid.
  • double check that the device has the correct solenoid.
  • consider the possible side effects of boosting solenoid power like broken plastics, targets, etc..
  • experiment with a steeper playfield pitch if faster game play is your goal.

Putting more power into the solenoid before addressing other issues could lead to faster wear.  But many have found that boosting some solenoids gives them the edge they're looking for in their pinball machines.


I'd like to thank Steve Young for reviewing this material and providing valuable feedback.