Antique Arcade Game AC Coil problem

[morke]

An arcade game from the 1940s uses a small 120 VAC coil to pull in a latch that releases a power switch. The coil present in the game was fried and seems impossibly small --a medium gage wire (I forget the # turns, but maybe .75" diamater). I rewound the coil with the same gage wire and approximate overall size, but it only has a few ohms resistance. In operation, the coil is only on ong enough to pull in the plate and release all power, but the switch contacts arc badly.

On top of this, a 250VAC 0.1MFD cap was wired IN PARALLEL with the coil. I've seen this setup before but the cap is always across the coil switch contacts to minimize arcing, not across the coil itself.

When I apply power to the coil the switch arcs terribly as I've mentioned. I don't know if the cap is any good as it's ancient as well.

So...

Could this be correct that such a low-ohm coil would be used with a high voltage, and was the original coil something other than the one I found in the machine?

What would be the purpose of a cap across the coil (and was the original configuration having the cap across the contacts)?

Keith







, but only a few ohms resistance.

 

[OperaHouse]

The current is limited more by the inductance of the coil than the resistance. It can be difficult to rewind the coil with the same number of turns because of packing efficiency or use of a slightly larger wire. The capacitor across the coil performs much the same function as on the contacts. If it is actually from the 40's the capacitor is probably leaky and should be replaced. Your rewound coil may be saturating at a higher voltage. Rather than rewind the coil, you can use a 120V to 24V transformer to make an auto transformer and operate the coil at a lower voltage of maybe 100V. I would typically expect your coil should have a resistance of 70-100 ohms. Are you sure your new coil isn't now shorted?

 

[morke]

Thanks. The wire is 0.025"dia which looks to be 22-gauge. It can't have but a couple hundred turns as the coil diameter is about 0.75" and the length can't be more than 1". I just don't see how a coil like this could've worked on 120V! Coils rated for 120VAC in similar machines seem to be wound of 36-gauge wire and 5000 turns! Unless the cap in parallel makes a big difference. I'm beginning to think this relay mechanism came from a lower-voltage machine.

Keith

 

[geekEE]

I've heard that the technicians that repair arcade games in the field will do whatever they need to get the machine working, and that they don't generally have engineering backgrounds. It may be that someone rewound the coil in the field with whatever wire he had in his toolbox and it may have worked...for a while!

 

[bogeyman]

The cap across the contacts should have a 100ohm 1/2 watt resistor in series to limit the cap discharge current when the contact closes.
else wise the cap discharge current is massive and will help destroy the contacts it is intended to protect.
You can get a ready potted CR suppressor which has a suitably rated capacitor for this duty from most electronic catalogue suppliers look for contact arc suppressors.

 

[coconutalley]

morke,

The voltage generated across the inductor is V = L x dI/dt
where L = inductance of coil, dI/dt is the rate of
rise for the current. In other words, the steeper the change in current the bigger the voltage which would cause
arcing. The other thing: coils and latches usually have iron housing that completes the magnetic circuit. The coils generate magnetic flux which goes through the latch and comes around the housing and back to the latch again. This gives it more inductance. The more inductance, the slower the time constant which also is related to impedance. More impedance would limit the current spike. So, if there is a housing, it should be used with the coil and latch.
If not, more coils is needed to bring up that inductance.
The resistor idea suggested by the gentleman, might work in limiting the current spike from the cap but not from the 120VAC source unless you add resistance there too.

 

[geekEE]

I would probably say not to add resistance in series with the coil as that could keep it from pulling in at all (unless something like a lightbulb was used as a series resistance). I would say that adding more turns of wire is the way to go.

 

[morke]

Thanks for your help.

I didn't mention that there is a 3 amp 120VAC motor running when the contacts that energize the coil that pulls in the latch that breaks the contacts to the whole machine.

But in any case, the original circuit did not have a resistor and had simply the coil and cap.

Also, I tested the cap and it has rated capacitance and 35 microamp leakage at 200V -- does that sound good?

 

[OperaHouse]

"35 microamp leakage at 200V"

Is that DC? I've rebuilt a few old magic eye capacitor testers and even back then they thought 9uA was really bad. It's not your problem, but this is not a good cap to have across the line. I for one love light bulbs. A 15W 120V lamp may be just what you need. A low resistance for a quick pulse and the resistance will increase about 8 times that if the situation persists. That will protect the solenoid from burning up.

 

[coconutalley]

morke,

Rereading your post again, I think the cap is just for filtering and does not really contribute much to the arcing.
It may be that the arcing was always an issue and the reason for the fried coil. You could try using an
MOV [metal oxide varistors] ones rated for 250VAC - 500VAC [experiment]. It will clamp the high voltage spike generated by the coil. Put it across the coil.

The real solution would be to use an SCR or TRIAC to replace the large switch contacts. You'd still need to use a smaller switch to trigger the TRIAC. Solid state switches do not arc. Of course, this might be more design
engineering than you want to do.

 

[morke]

The TRIAC is a good idea, but I'm trying to keep this antique as close to original as I can. The original schematic shows only a relay coil and cap across the contacts. I've checked some more and I am convinced the coil is not original to the machine and is WAY overpowered by 120VAC. I'm going to replace it with a coil designed for 120VAC and that should fix the problem.

Thanks for all the help.

 

[MacGyverS2000]

Can you tear apart a 120V relay for the coil inside? Not handmade, but at least it's easy to get...


Dan - Owner

http://www.Hi-TecDesigns.com