(Topic ID: 272038)

Comparing two flipper coils FL20-300/28-400 and FL21-375/28-400

By Peruman

3 years ago

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#1 3 years ago

The left flipper on the Little Chief I recently got to work has always been weaker than the right flipper. Not only does it have less power when sending the ball up the playfield, but also "gives" when trying to catch the ball. The right flipper has neither of these issues.

What I have done so far is replace the bushings, springs, and plunger/links. I have also checked and corrected the gap on the EOS switches. What I had not done until tonight was check the coils themselves. Each flipper has a different coil.

The right flipper (which works as expected) has the correct coil for this game, FL20-300/28-400. The left flipper has a different coil, FL21-375/28-400. From what I have read online, the secondary windings are the same (28-400) but the primary windings are not. Which coil is stronger? And could the difference in coils explain why the left flipper does not work as well?


#2 3 years ago

Without remembering the math, a smaller gauge wire like 21awg would need more windings to be equivalent in magnetic strength, so I would expect the coils to be sorta similar. But there is a lot more to the formulas than that. Best thing is to do is replace with the correct coil. As for the "gives" you mentioned, remember the 2 windings are in series during the hold, so yes the coil could have weaker strength overall in both of its duties. Power stroke = heavy winding only (high current), hold = both windings, (Low current) for max power the eos switch should be gapped maybe 1/32" when flipper is in hold position.

#3 3 years ago

The right flipper has the stronger coil. It has fewer turns of slightly larger diameter wire so it will draw more current leading to stronger magnetism.

#4 3 years ago

I agree with Student Prince. The FL20-300 is definitely the stronger coil.
The 20-300 coil should have a resistance of about 1.1 ohms where the 21-375 should be approximately 1.5 ohms. The 20-300 will draw about 1/3 more current than the 21-375 and yield more power on the flipper stroke.

#5 3 years ago

Had the same issue on my OXO recently. replaced the incorrect left 21-375 coil and it was a dramatic difference. 20-300 is much more powerful

#6 3 years ago

Adding the correct coil and some new sleeves to my PBR order! Thank you for all the help!

I do need to find the formula for calculating the resistance of the coil/magnetic field strength for times like this.

#7 3 years ago
Quoted from Peruman:

I do need to find the formula for calculating the resistance of the coil/magnetic field strength for times like this.

It's complicated. You'll find formulas and calculators online that will usually show that the magnetic field strength
increases with:
- the number of wire turns or windings
- the amount of current passing through the wire
but decreases with:
- the length of the coil (which decreases the winding density)
- the distance between the windings and the plunger or core

Many online calculators simplify things and assume a single layer of wire windings but coils usually have many layers of windings, each a bit further from the core. Calculating the current can be problematic too as the current through a coil will be determined by the wire gauge (thickness) and the configuration of the coil winding.

These factors all combine in an elaborate and non linear way to determine the magnetic field or coil strength.

Consider the common practice of removing some windings to make a pop bumper coil stronger. Fewer windings should decrease the strength but in reality it increases the strength because you're removing the outer windings which are relatively weaker and because you're lowering the overall coil resistance which lets more current through. The effect of the extra current through the coil outweighs the effect of fewer coil windings to a point. If you were to continue removing windings however eventually your coil would become weaker. Imagine removing all but one winding. That single winding would quickly overheat and would have little if any magnetic strength.

Electricity and Magnetism is typically a 2nd semester college level physics course so it's no wonder that there isn't a simple and accurate formula for coil strength.


#8 3 years ago

We are dealing with AC here. More turns means higher inductance hence higher reactance, further reducing the current taken by the coil. The coil with fewer turns will be stronger.

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