The mechanical on/off detent of the dimmer on a 120 volt, 100 watt halogen lamp failed (i.e.: lamp always on but would still dim/brighten). The failed pot was labeled SB250K and part of a diac & triac circuit I could figure out.

Using a digital meter, I confirmed my suspicion that the pot's resistance was ~250kΩ and was linear. When searching for a replacement, I found identical mechanical layout (e.g.: diameter, shaft length, contacts positions, etc.) Electrically, those replacements were also 250kΩ with linear taper and had five identical contacts: 3 traditional pot contacts (top of photo) and 2 on/off switch contacts (rear). But in all cases, were labeled B250K (and not SB250K). I took a chance, ordered one of these and it has worked fine.

Is it important that I could not find a pot labeled SB250K? What does the letter S signify?

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  • \$\begingroup\$ It looks like the SB250K has a switch on the back. Does the B250K have a switch. If not, S probably stands for switch. \$\endgroup\$ – Charles Cowie Mar 27 '17 at 16:34
  • \$\begingroup\$ @CharlesCowie As stated in description, both the original (SB250K) and replacements (B250K) have identical contacts. These include 3 traditional pot contacts (top of photo) + 2 contacts for on/off switch (on rear). \$\endgroup\$ – BCorbeil Mar 27 '17 at 16:54
  • \$\begingroup\$ Is there any physical difference it terms of colour or shape? Otherwise I would just assume it is probably either manufacturer specific, or has some indication of what the resistor/wiper material is. \$\endgroup\$ – Tom Carpenter Mar 27 '17 at 17:01
  • \$\begingroup\$ The S could designate a different taper. That would be easy to test with an ohmmeter if the manufacturer doesn't have an online catalog. It could also not be properly marked especially if you didn't buy it from a regular dealer. \$\endgroup\$ – Charles Cowie Mar 27 '17 at 17:08
  • \$\begingroup\$ Edited my original description to clarify it based upon the helpful comments already received. \$\endgroup\$ – BCorbeil Mar 27 '17 at 17:22

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