I'm using some old EM pinball relays in a project and find myself wondering how the coils for EM relays were selected by the game designers. I knew that there are higher resistance, lower power coils intended for things like hold and coin lockout relays that are on most of the time. But what about the variety of coils that are available for most other relays?

Most of the relay coils for a given manufacturer in a given era are about the same size, presumably so that they'll all fit into the same relay frames. So the designers could specify any of a number of coils for each relay. My initial thought was that the number of switches in the relay frame might dictate the strength of the coil in the relay. So a coil switching 6 switches in a relay frame would require more strength than another switching just 2 switches. But scanning through a few Williams game manuals from 1970 I couldn't find that pattern. On the contrary, as an example a stronger Z27-1000 relay coil was usually used to switch 3 or 6 switches while a weaker Z28-1150 relay coil was usually used to switch 4 or 5 switches. And the M29-1100 relay coil was used to switch from 2 to 8 switches in the three games I looked at.

So does anyone know how the coils were selected for a given relay?

As a little background for the curious, the strength of a coil is roughly comparable to the magnetic flux of the coil which is:

flux = (magnetic constant) * (number of turns) * (current) * (cross sectional area of the coil)/(length of the coil)

Since relay coils that fit in a given frame have the about the same (magnetic constant), (cross sectional area) and (length), the equation can be simplified to show the relative strength of similarly sized coils:

strength ~ (number of turns) * (current) or
strength ~ (number of turns) * (relay voltage) / (relay resistance)

So if you know the number of turns in a coil (e.g. 1000 turns in a Z27-1000) and can measure the resistance, you can estimate the strength of the coil relative to another coil of the same diameter and length.

More detail at: https://en.wikipedia.org/wiki/Solenoid#Inductance

/Mark

Yeah, I'll have to go through more of the manuals looking for the patterns.

Your point about the faster reaction time is interesting. I wonder what might require a faster reaction time. Playfield targets? Impulse switches on the score motor? I wonder too if the timing tolerances were so tight that various relay response times were needed. I'd be surprised but I guess it's possible.

I wondered about the equations and wire gauge too. Higher gauge wire has a smaller cross section which means more resistance and less current. So since a change in wire gauge implies a change in current, I think the equations have that covered. After all, the magnetic field created doesn't really care about what kind of wire is used in the coil, just how much current is flowing through how many loops of wire.

Another interesting nugget is that more wire turns implies more resistance which implies less current. Apparently the extra magnetic field created by another wire loop outweighs the reduction in current due to adding the extra wire.

One thing I'm not clear about is why removing a few loops on a flipper coil apparently makes it 'hot'. That seems to contradict the equations above, but that may be a discussion for another thread.