Help understanding a sewing machine pedal...

[corrado33]

So I collect and use old sewing machines. Generally, they're pretty easy to repair, however recently the foot pedal on an old singer of mine stopped working. I took it apart to find some... weird... things. On similar vintage pedals I'd generally see about 8-9 different contacts with resistance wire between them and an arm that sweeps across them, so as the arm moves, the resistance decreases and the machine speed increases. Almost like an autotransformer... but with resistance wire and not a massive coil.

Anyway, this pedal is... different. I can't figure out how it's supposed to regulate the resistance. Here are some pictures.

Fully assembled (minus the top cover) from the top.


Fully assembled (minus the top cover) from the back.


Back of the "unit" with a few things removed and one side filled up with most of it's carbon pieces.


Plunger that moves.


Mostly taken apart w/ parts showing. See all the carbon disks? See the "nipple" piece at the bottom right of all the organized carbon disks? That's the part that the springy part of the plunger (picture above this one) contacts. Not the spring itself, but the thin piece of bent metal to the right of the spring in the picture.


Unfortunately, I did not take a picture of the back with the "ends" on, so I've made a drawing.



So basically the path for the electricity is this.

• From the wire
• Through the electrode/contact
• Through the thick carbon piece on one side
• Through all of the thin carbon pieces on one side.
• Through the nipple piece on one side
• Through the springy contact attached to the plunger seen in picture 4 above
• The reverse on the other side

Now, I know this is AC, but still that's the path for the electricity, regardless of which way it flows.

When you press the pedal ALL of the way down, the two electrodes/contacts for the wires are shorted by the bent pieces of metal on the left of the first picture. Even THAT fails to work sometimes.

The thing I can't understand is this. How the heck is the resistance supposed to change? The only thing I can think of is that the "springy" piece of metal that contacts the "nipple" pieces changes its contact area, therefore changing resistance? That's virtually the only thing that changes throughout the actuation of the pedal. Am I missing something? Or maybe the springy piece is like resistance wire and as you press the pedal the carbon "nipple" pieces are touching more closely together and therefore less resistance?

When I took it apart I did my usual "clean the contacts, make sure the wires have conductivity, etc. but none of that workes. The disks are made from graphite.

When I test the resistance as I press the pedal, it SOMETIMES jumps from kOhms to 29 ohms, nothing in between. Sometimes it just sits at MOhms regardless of how hard I press the pedal. If it does work, it just sorta sits at 29 ohms until the contacts are shorted, then it jumps to ~1 Ohm. Before I took it apart, it was MOhms and kOhms throughout its entire range, hence why it didn't work.

Pedals from similar vintage machines go from ~100 Ohms to ~200 Ohms throughout the stroke of the pedal, but that counts wire and contact resistance, so it's probably lower than that. When I measured the resistance I did it directly from the wire contact (screw) points.

After the first time I reassembled it and it didn't work, I figured I must have assembled it incorrectly, and the only thing I could think of was that I put the carbon pieces in incorrectly. So I organized them by size (some seem to be .1mm thicker than others.) and assembled them in size order, both directions. No luck.

I'm not sure how to fix this honestly. Any ideas?

[keethrax]

Does this help:

http://archaicarcane.com/what-a-pile-of/

[corrado33]

Does this help:

http://archaicarcane.com/what-a-pile-of/

While that does give better pictures than mine, they didn't seem to run into the same problems I did and it just "worked" for them after they reassembled it. I did clean all of the carbon disks off (by rubbing them on a piece of paper.)

[Vtile]

Oh, carbon pile reactor the atomic sewing machine. 
https://en.wikipedia.org/wiki/Chicago_Pile-1

Jokes aside, this is interesting, I remember pulling one of those apart a years ago and after while taking it back together only to be totally

[corrado33]

After looking up carbon pile resistors, I found that I may have tightened the "piles" too tight. The bent piece of metal may do more than just change its contact area, it may compress the piles, although I don't remember them being loose when I took it apart. I'll try to loosen up the screws that adjust how tight the piles are pre-compressed.

I kinda just want to rig up a modern adjustable power resistor (or two in parallel.) Everything I've read about carbon pile resistors is that they're awful and unreliable. The only question is what kind of resistance I'd need. I have a resistance box, but it's only populated with 0.5 W resistors... I think.

[CatalinaWOW]

You seem to be on the right track, but I would say you have two problems.  Reread the links and drill into your mind the fundamental operating principal of this pedal.  There is a pile of carbon disks.  The resistance of the pile (when all the moons align) varies with the pressure applied to the pile.  I would guess the design intent is for that resistance to vary from a couple of dozen ohms up through a few hundred ohms.  The pressure is applied by the spring bar across the top of the two nipples.

1.  You have a bad contact at one or both of the attachments to the piles.  This is the explanation for the very high resistances (many kOhms).  Look for a ledge on the housing or other physical barrier between the metal and the carbon and verify that the nipples can freely move to make contact with the piles.

2.  You either have too much pressure or haven't cleaned the piles enough.  Hence when you do have contact the resistance is too low.

[Vtile]

Also is this one of those rare cases where clean is no good, since carbon / graphite dust is might be easier to pressure adjust than solid. I am shooting in the dark here, it may work in both ways.

Edit. Correction of bad wording.

[corrado33]

Also is this one of those rare cases where clean is no good, since carbon / graphite dust is easier to pressure adjust than solid. I am shooting in the dark here, it may work in both ways.

Yeah, I hate when this is the case. Luckily, graphite is readily available. (And I have a mortar and pestle that I can use to crush it up.)

[corrado33]

You seem to be on the right track, but I would say you have two problems.  Reread the links and drill into your mind the fundamental operating principal of this pedal.  There is a pile of carbon disks.  The resistance of the pile (when all the moons align) varies with the pressure applied to the pile.  I would guess the design intent is for that resistance to vary from a couple of dozen ohms up through a few hundred ohms.  The pressure is applied by the spring bar across the top of the two nipples.

1.  You have a bad contact at one or both of the attachments to the piles.  This is the explanation for the very high resistances (many kOhms).  Look for a ledge on the housing or other physical barrier between the metal and the carbon and verify that the nipples can freely move to make contact with the piles.

2.  You either have too much pressure or haven't cleaned the piles enough.  Hence when you do have contact the resistance is too low.

Thanks. I only recently found out what that type of resistor is called, so I haven't been able to look much into it. I think you are right though, I tightened the screws down that pre-tensioned the carbon piles. I didn't crank on them, but they were certainly snug. I also placed the "retaining clips" in the position that would provide more pressure on the piles (they're slightly bent.) So perhaps those are backwards. I think loosening up the pressure on the piles would be a good idea. I'll let you know when I try it.

If I find it's not very controllable, I may try sprinkling some graphite powder between the piles, but that seems like it'd be a pain.

[joseph nicholas]

I have one of these things and it looks like what I really want is a thermistor regulator, but hooking it up seems almost not worth it without using a pic, an optoisolator and a diac.  I tried to find the disks, but no one knows what I'm talking about at the local electronics store.  A new pedal is expensive and then you are back to square one if the new pedal stops working.

[Gregg]

Carbon pile resistors have been used for decades in automotive battery load testers and are quite reliable if not abused.  They are generally carbon disks about 90mm diameter and about 10mm thick with a hole in the center.  They are stacked over an insulation sleeve with a threaded rod through the center to tighten them together producing lower resistance as they are compressed and therefore a higher load on a battery.  A stack about 20 cm long can easily load a 12V battery to 500 or more amps for a short test, limited by how much heat the  resistor can dissipate before burning up.  The disks are generally not compressed at all until a test is being performed.  The pile is generally oriented horizontally to allow the broken bits of carbon to fall away when there is clearance between the disks.  The resistance is fairly constant across the disks.  The resistance changes occur at the surfaces; the greater the area and the greater the pressure, the lower the resistance.  The longer the stack and the diameter determine the minimum resistance and overall heat dissipation capability. 
McMaster sells graphite rods that may work for you.  You can cut off sections with a hacksaw and sand them close to parallel with sandpaper on a flat surface.  These are not tight tolerance devices and don’t need to have the surfaces finished to a fine degree nor do all the disks need to be exactly the same length.
Since most of the older sewing machines use universal motors and will run on DC, you could hide a nice little PWM supply in the foot pedal and it will run smoother with more torque than the old resistor setup.