(Topic ID: 165968)

Why would a coil weaken over time? Dumb question of the day


By Oldgoat

3 years ago



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  • 38 posts
  • 24 Pinsiders participating
  • Latest reply 3 years ago by ramegoom
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    #1 3 years ago

    I see posts about people replacing weak coils and it started me thinking. And given that it's been awhile since I have asked a stupid question, here goes...
    What makes a coil weak? I understand friction related causes such as gummed up sleeves as well as causes related to power being fed to the coil. I also get the 'lost force' impact if there is play in the mechanism, but what else would cause a coil to become weak over time? It seems to my simple mind that since the diameter of the wire and the number of windings stays consistent, so too should the magnetic force of the coil.

    #2 3 years ago

    That is correct. Theoretically it should never weaken unless acted upon by outside forces, like vibration. It is simply a winding of enameled (insulated from itself) wire. However, if the enameling on the wire should crack and fail due to vibration that would change its properties in the circuit and could change its magnetic field.

    Coils rarely "go bad". Rather they are ruined by outside forces, usually being powered for too long.

    #3 3 years ago

    Wire heats up, changes characteristics slightly. the varnish melts, exposing bare wire. can cause shorts in windings.
    the magnetism causes the coiled wire to swell.
    all these can cause a coil to weaken. but, usually, causes a short.
    mostly, bad connections cause weak action.

    #4 3 years ago

    So then, if the enamel comes off in the 'right place', I would guess that it could short the windings at a point where some of the total turns are not included, thereby reducing the magnetic force?

    #5 3 years ago
    Quoted from Oldgoat:

    So then, if the enamel comes off in the 'right place', I would guess that it could short the windings at a point where some of the total turns are not included, thereby reducing the magnetic force?

    Correct!

    http://www.allaboutcircuits.com/textbook/direct-current/chpt-15/factors-affecting-inductance/

    #6 3 years ago

    I've had lots of success by just changing out the sleeve. Can't recall having a bad coil. Just bad voltage, adjustments and the sleeve.

    #7 3 years ago

    Corrosion on the end of stroke switches.

    #8 3 years ago

    Also.....take a look at the end of stroke switches. For example, take a look at your flipper coils and you'll see that at the end of the stroke the armature separates a switch that is normally closed. You want the switch to open at the very end of the stroke because both switches are actually feeding current to the coil. The switch opens at the end of the stroke to keep from burning up the coil. Because of the amount they are use these switches get out of adjustment and therefore weaken the coil creating a loss of power in the flippers. Its a quick easy fix that has a big impact on play. Also make sure those end of stroke switches are clean.......I use a small metal file on them because of the metal used in those high impact switches. Should be the same on your pop bumpers as well.

    #9 3 years ago

    I like this "dumb" question - I'm learning from it too.

    #10 3 years ago

    During my first two restorations, when I was replacing the coil sleeves, I had a few coils with coil sleeves that seemed to be melted or fused inside the coils. Not knowing better at that time, I struggled and almost destroyed the coils while trying to remove the sleeves. After I finally got the sleeves out, I struggled again to insert the new coil sleeves that just did not want to go back in.

    I learned later on that this is a sign of a bad coil and that it most likely needed replacement. Only wish I could remember which coils I had these problems with so I could go back and replace them.

    Someone else with more knowledge on this example could probably explain this better than me.

    #11 3 years ago

    Since bad coil sleeves or poorly calibrated EOS switches result in poor performance but don't impact the magnetic field, it appears that the only cause of a coil losing magnetic strength is the result of a short somewhere in the bowels of the coil.

    #12 3 years ago
    Quoted from Mikala:

    After I finally got the sleeves out, I struggled again to insert the new coil sleeves that just did not want to go back in.
    I learned later on that this is a sign of a bad coil and that it most likely needed replacement.

    This is kind of interesting, if it were an indicator of a short in the winding. Perhaps if the coil got real 'toasty' at some point, the varnish melted and the coil distorted and then cooled out of round. Although the charred wrapper is a pretty good sign of that as well. Maybe it gets hot enough to cause distortion and a short without going to the full charcoal briquette stage?

    #13 3 years ago

    [from a CARGPB]

    Even if the coil is swollen a little bit, it's "damaged", not "bad", IME. At home, it's no big deal, but if you're out on a call and don't have the right coil, it can still be used.
    You have to carefully remove the stuck sleeve (I use a very thin screwdriver to slide in between the sleeve and...bobbin, is that the right term?...and crack it down it's length so I can get it out with brute force). Then I "shave" the new sleeve so it will fit into the swollen coil. Then if the plunger will slide freely reassemble and it's good to go. I rarely replace flipper coils on service calls.

    Repair HACK!

    #14 3 years ago

    I've also run into the occasional case where a flipper coil sleeve gets "cemented" in place by black dust and gunk; breaking out the sleeve as Cody mentions above and cleaning the bore of the coil bobbin sometimes allows a new coil sleeve to slide in easily.

    #15 3 years ago

    You will also occasionally run into a coil where someone has rammed a screwdriver into it, or otherwise punctured the coil through the label. Nicks in the wire will cause increased resistance, which will add to the heat... which will eventually lead to a short or full open.

    I never replace a coil unless it is obviously bad, currently on fire, etc.

    #16 3 years ago
    Quoted from bingopodcast:

    I never replace a coil unless it is obviously bad, currently on fire, etc.

    I've never seen one burning, but when I bought my Eight Ball I saw the results of a coil fire in the chimes. It was a melted sooty mess in there. But my favorite part was what the tech did. Looks like he just cut the wires to the chime box and kept on playing. The clipped pop bumper wires with the melted coil and spoon confirmed this was their standard operating procedure.

    #17 3 years ago

    While that seems like it would be bad - at least he protected the rest of the game from the burned out chime/pop. When the load on a circuit suddenly disappears or increases dramatically, bad things happen elsewhere. And it's not too hard to repair, thankfully.

    Much better than the alternative where multiple relays or a stepper or two need serious repair.

    #18 3 years ago

    Need to be careful in interpreting some of this text.

    Practically speaking, the magnetic force produced by a coil on a plunger will get stronger as one removes coil windings. This might also apply to shorting out some percentage of the windings by burning off the insulation, although with damage to the coil like this, it is hard to predict.

    Removing windings has the effect of reducing the impedance of the wire, and since the source voltage is fixed, the current through the windings will increase (Ohms law). The electromotive force by the coil produced is proportional to the current and thus, force produced is greater as the impedance of the coil drops, until you get less than about 1.5 to 2 ohms, when the entire coil begins to resemble a short.

    #19 3 years ago
    Quoted from bingopodcast:

    You will also occasionally run into a coil where someone has rammed a screwdriver into it, or otherwise punctured the coil through the label. Nicks in the wire will cause increased resistance, which will add to the heat... which will eventually lead to a short or full open.
    I never replace a coil unless it is obviously bad, currently on fire, etc.

    I guess they didn't notice the chime unit jones plug tucked underneath there...

    #20 3 years ago

    I've never run into a coil that just 'weakens'. I have run into coils that just stop working (broken winding...I had one just happen again last night) or blows a fuse (shorted winding). If it seems like the coil is sluggish or doesn't have enough ooomph, it's always been a mechanical issue with the linkages or coil sleeve, tarnished connector or dirty switch contact to the coil.

    Yes there have been situations where the coil has overheated, at some point in it's life from being shorted on, and the inside diameter has decreased. This is still a result of something happening to the coil indirectly that caused the coil to become damaged.

    #21 3 years ago
    Quoted from schudel5:

    I've never run into a coil that just 'weakens'. I have run into coils that just stop working (broken winding...I had one just happen again last night) or blows a fuse (shorted winding). If it seems like the coil is sluggish or doesn't have enough ooomph, it's always been a mechanical issue with the linkages or coil sleeve, tarnished connector or dirty switch contact to the coil.
    Yes there have been situations where the coil has overheated, at some point in it's life from being shorted on, and the inside diameter has decreased. This is still a result of something happening to the coil indirectly that caused the coil to become damaged.

    Yea, stop holding the flipper up people! We know who you are!

    #22 3 years ago

    So it seems we are have we come full circle, i.e., that a coil never weakens by virtue of what is happening in it's innards, it is always the result of a mechanical issue (e.g., bad sleeve) or an electrical issue (e.g., reduced voltage)?

    If so, then people who replace a working coil are probably also (and maybe inadvertently) fixing the real issue when they install the new coil.

    By the way, is there some chart or easy formula to determine if the impedance on a coil is correct? I've always just measured two of the same type, but if you only had one of a particular type in a game, how would you know what the correct reading should be? Of course, based on my first statement, this may be moot since it seems like the only test needed is 1) Does the coil fire

    #23 3 years ago
    Quoted from Oldgoat:

    By the way, is there some chart or easy formula to determine if the impedance on a coil is correct?

    These may or may not include all coils, but probably a majority of the more common ones.

    https://www.flippers.com/coil-resistance.html
    http://www.pinballmedic.net/coil_chart.html

    #24 3 years ago

    What would cause a coil to over heat and warp the coil sleeve inside? I had this happen on the ball launch coil on my Frankenstein recently and ordered a new coil and sleeve but want to make sure it does not happen again when I put in the new coil.

    #25 3 years ago
    Quoted from Oldgoat:

    By the way, is there some chart or easy formula to determine if the impedance on a coil is correct? I've always just measured two of the same type, but if you only had one of a particular type in a game, how would you know what the correct reading should be? Of course, based on my first statement, this may be moot since it seems like the only test needed is 1) Does the coil fire

    As long as it isn't 0 ohms, you can just compare with an identically wound coil to get an idea of what it should be. Two identical coils will probably vary slightly as well in ohms. Nothing wrong with that.

    As for a table or chart, I'm not sure that is really even necessary. You can always A/B reference coils in a pin. More windings = higher ohms = more resistance = lower power.

    In a case where the wrapper fell off, and you're not sure what you're looking at, you can clip the diode on one end, read ohms, and compare it to another similar coil to make sure it isn't much lower or higher power, but you'd probably know it is weird just by the gameplay. You would want to clean up slop first (tight coil stop, clean plunger and new sleeve) before assuming anything though. There is really no rule for the strength of coil you use in a particular location either, except for overly powered stuff that could cause things to break on the game.

    For instance, the upper flipper in my Sorcerer I think is way overpowered for what it needs to do. I dropped it down in power to a higher winding coil to prevent damage.

    #26 3 years ago
    Quoted from Don44:

    What would cause a coil to over heat and warp the coil sleeve inside? I had this happen on the ball launch coil on my Frankenstein recently and ordered a new coil and sleeve but want to make sure it does not happen again when I put in the new coil.

    If the coil locked on, that would melt the sleeve. This is likely a transistor issue on the driver board. When you put the new coil in, turn on the game and see if it immediately locks on. If not, start a game, see if it locks on when it is activated. If so, turn the game off immediately and inspect and possibly replace the TIP102 driver transistor for it (if you're experienced with electronics) and possibly the pre-driver transistor as well. You won't burn up the new coil as long as it is turned off right away after locking on.

    If not a transistor issue, something gave the coil a path to ground to energize it. Look for loose wires or metal touching nearby the coil solder tabs. Coils always have power on them at all times. They turn on when they are allowed a path to ground.

    #27 3 years ago
    Quoted from schudel5:

    These may or may not include all coils, but probably a majority of the more common ones.
    https://www.flippers.com/coil-resistance.html
    http://www.pinballmedic.net/coil_chart.html

    Thanks. Lots of good info, particularly in the second document

    #28 3 years ago
    Quoted from DaveH:

    he just cut the wires to the chime box and kept on playing. The clipped pop bumper wires with the melted coil and spoon confirmed this was their standard operating procedure.

    This ticks me off more than anything. I look at working on pinball like a doctor, DO NO HARM! Cutting Wires is not fixing a problem.

    I'm not sure if the removing a stuck coil sleeve counts as a hack, or something "bad," I do it all the time! I agree with Bingo- if it's not broke, or on fire, don't replace it!

    I find a little sandpaper on the inside of a coil will also help slide a new sleeve in a little bit. To take down a coil sleeve a little bit, I put a drill inside the sleeve to maintain it's integrity as I spin it, and then sand it ever so slightly.

    Does anyone else just use one size sleeve and trim them down for the shorter coils? Is THAT someone how considered bad form?

    -dp

    #29 3 years ago

    I keep a stock of standard sleeves on hand, and cut down the length of a sleeve when I need a short one. I use a plumber's tight-space copper tubing cutter, which is small and light and fits easily into your hand. I slip a salvage plunger inside the sleeve while cutting it, so it won't collapse. Makes a perfect cut, and only takes about 10 seconds.

    - TimMe

    #30 3 years ago
    Quoted from bdPinball:

    ....
    I'm not sure if the removing a stuck coil sleeve counts as a hack, or something "bad," I do it all the time! I agree with Bingo- if it's not broke, or on fire, don't replace it!
    ...

    I'm currently working through a row of '60s Williams pins. Nearly every coil sleeve in the playfield is worn out and stuck. Cut 'em out with a saw if necessary. Replacing 10 or 12 coils per game simply will not happen.

    #32 3 years ago

    If you've got a worn aluminum sleeve stuck in a good coil, try energizing the coil for about 20 seconds or so until it gets fairly hot in your hand. The heat will often loosen up the sleeve enough so that you can get it out without cutting. It doesn't always work, but when it does it saves time and (for me) the aggravation of cutting the sleeve out.

    - TimMe

    #33 3 years ago

    Had a few coils burn up in a bad transistor locked on situation a few years ago. Other than that, no issues.

    #34 3 years ago

    Flipper coils can heat up with extended play, which I assume increases resistance of the wound up coil wire. With solenoids, more resistance = less power.

    #35 3 years ago

    Over time you can get micro fractures and corrosion in the solder joints at the terminal. This can reduce the current carrying capacity of the coil and hence its "power". Check to make sure the lead wires from the coil haven't broken loose and are just laying on the solder instead of being embedded in it. Also, in older coils, make sure the EOS surfaces are flat and clean. Cupping or burrs will reduce the surface area available for carrying current and will reduce the "power".

    I know this was an electrical discussion, but don't discount the effects of friction inside the coil sleeve. Both from the sleeve melting/warping and the armature mushrooming. I suspect that there is an inductive heating component as well on the armature, especially in coils that get a lot or rapid repetitive power cycles, like flippers and pop bumpers.

    #36 3 years ago
    Quoted from thedefog:

    If the coil locked on, that would melt the sleeve. This is likely a transistor issue on the driver board. When you put the new coil in, turn on the game and see if it immediately locks on. If not, start a game, see if it locks on when it is activated. If so, turn the game off immediately and inspect and possibly replace the TIP102 driver transistor for it (if you're experienced with electronics) and possibly the pre-driver transistor as well. You won't burn up the new coil as long as it is turned off right away after locking on.
    If not a transistor issue, something gave the coil a path to ground to energize it. Look for loose wires or metal touching nearby the coil solder tabs. Coils always have power on them at all times. They turn on when they are allowed a path to ground.

    Thanks for the help, sorry to hijack post op. I replaced the coil and same problem and the wires connecting the coil are not touch any anything so maybe it is the transistor. What board is the tip 102 driver transistor on or where is it? I have replaced some transistors in the past.

    #37 3 years ago
    Quoted from Don44:

    What board is the tip 102 driver transistor on or where is it? I have replaced some transistors in the past.

    Transistors are typically always on the driver board or power board in the backbox or in the case of flippers on the flipper driver board. There will usually be about 20 to 30 of them lined up along the bottom.

    #38 3 years ago

    I have used TIP142 transistors in borderline cases where the current drain to the coil is higher than usual. The TIP102 is a Darlington (high gain) transistor with a maximum 8 amp load. The 142's can carry up to 15 at the expense of a slightly lower gain. the package is larger, but the pinouts are the same.

    Usually a lower gain is not a problem since they're fully saturated when in operation, and there's enough drive behind the transistor.

    The suspect coil should be inspected for excessive friction within the armature when in operation - hot. Most high power coils like flipper coils and reset coils are rated for intermittent duty instead of continuous duty, which is why they tend to overheat on occasion.

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