finding a potentiometer switch

[Clear as mud]

I am having trouble finding a replacement potentiometer for a lamp.  It's one of those tall lamps, but instead of a 300W halogen bulb like they normally have, this one has a 55W fluorescent, with a ballast right under the lamp, and the dimmer switch on the pole supporting the light has a circuit board, but there is only one component on the board, a rotary potentiometer with a switch built in.

I searched the part number on the potentiometer itself, FPS270S1554, and only found one result, a forum thread where someone else was trying to repair the same model of lamp: http://www.justanswer.com/electrical/314zc-ge-41334-torchn-lamp-uses-f55w2d-830-lamp-feh552d-dv-120.html.  They talk about the problem possibly being with the ballast, but in my lamp, the ballast works but the lamp just doesn't respond properly when trying to dim it, so the potentiometer is bad.

The thread mentioned that the pot is 10k resistance, and that is approximately what I get when I measure it, but it also has a switch built in which is in series with the incoming 120VAC.  It has a strange footprint, and I can't find anything like it.  It is similar to the CTS 270X series, with a round body, bent leads, but instead of mounting tabs on the bottom, it has the switch terminals on the bottom.

So, I thought since I can't find any device with the same footprint, I'll take the circuit board out and just use a potentiometer with solder lugs.  The hole it fits in is about 9 mm, and the shaft sticks out a total of 12 mm.  I found some similar ones, but am having trouble finding one the right size, with the switch rated for high enough voltage, and linear taper.  I am pretty sure I need linear taper, but I am also pretty sure I found one that would work that is audio taper.  I tried measuring the resistance as I rotated the knob, to verify the taper, but the old potentiometer is so sketchy that it is very hard to tell what the resistance value is supposed to be at various knob locations.

Anyone have insight on this?  Perhaps know of a part that would work?

-Daniel

[Belgarion]

Something like these? P170S-FC20BR10K http://www.mouser.com/ProductDetail/BI-Technologies-TT-Electronics/P170S-FC20BR10K/?qs=sGAEpiMZZMtC25l1F4XBU9IghlYGyeD%252b7gXESbwHTcg%3d

[Clear as mud]

Those are almost right.  The voltage rating is OK, but I need 12mm length instead of 20, and a knurled shaft instead of round with a flat spot, so that I can re-use the same knob and mounting hole.  At least the shaft diameter seems to be right.  According to the data sheet, I would need P170S-QC12BR10K or P170S-QA12BR10K, but nothing comes up when I search Mouser for those.  I'll check with other suppliers, but if they don't even come up on a search on Mouser, I don't know if it is likely that anyone stocks them.

I am still not quite sure whether this was originally linear taper or not.  I guess maybe it doesn't matter much.  Anyone have an idea whether it was likely originally manufactured with a linear taper pot?

[Clear as mud]

Those didn't come up on Digikey or Element 14 either.  I think it's probably an option that is not generally manufactured, so I'd probably have to order 10,000 of them, and that is not feasible.  Maybe there is still another brand that has the features I want.  Or maybe I can order a replacement knob to fit the other shaft type.

[Richard Crowley]

It seems quite likely that the pot is linear taper.  And even if it isn't, the adjustment will only be slightly more fiddly.

Just finding a 10K pot of the proper dimensions will be challenging enough.

You would have better luck finding the holy grail than find a proper-size 10K pot with a built-in switch, and with fluted shaft of the right length.   

Use whatever 10K pot will fit.

Buy a new knob if you have to.

Add a toggle switch to the lamp in lieu of power switch on the pot.

[Clear as mud]

For reference, in case someone else is trying to repair the same model of lamp:

I successfully repaired the lamp, using a potentiometer made by Piher, the PC16SH07CP06103A2020CTA.  This 10k potentiometer with a built-in switch is available from Mouser.  The shaft is way too long for the lamp I was repairing, so I cut it off with a hacksaw and then smoothed the corners (which can be done with a grinder or with smooth concrete, or probably with sandpaper).  I also had to put an extra washer on the shaft.  After that is done and the wires are soldered on and everything is put together in the lamp, the original knob will fit back on.

If you try this, buy an extra potentiometer in case you accidentally destroy one.  The first time I did it, I accidentally broke off one of the tabs for the switch by bending it back and forth, so I had to start over with a new potentiometer again.