I am looking for help in my effort to identify a SMD on the circuit board of a key fob. The SMD is the one located on the lower right in the image. The large copper pad on the very bottom right connects to the +3v side of the lithium coin cell in the fob. To the left of the copper pad is the spring loaded connector that connects the circuit to the negative side of the lithium coin cell battery. From my limited knowledge derived from a couple semesters of electronics nearly 40 years ago, I am guessing that it is a diode to protect the circuit from a reverse polarity situation but that is just sheer speculation at this point. I was hoping that someone with a much greater depth of knowledge would impart some of their wisdom to me, if it is glaringly obvious as to what this SMD is.

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The tan-colored component with two terminals is a capacitor, connected in parallel with the cell. It's used to "decouple" noise from the circuit from the relatively high source impedance of the typical coin cell.

  • \$\begingroup\$ Thanks Dave, that is helpful but is there anyway to determine the size of the capacitor? I am guessing not and that I'm sol but I obviously don't know much about the subject. There were no markings on the SMD or on the circuit board. When I took the electronics courses years ago you didn't need a microscope to be able to look at the components. \$\endgroup\$ – Jim Wheeler Jan 13 '19 at 17:13
  • \$\begingroup\$ You'd have to measure it. SMD ceramic capacitors typically have no markings. \$\endgroup\$ – Dave Tweed Jan 13 '19 at 17:17
  • \$\begingroup\$ I think that I probably ruined it by attempting to solder it back on with my nearly 40 year old soldering iron. I made quite a mess and the fob does not work. \$\endgroup\$ – Jim Wheeler Jan 13 '19 at 17:26
  • \$\begingroup\$ Dave, is there a general range in the size of a capacitor that is used to "decouple" noise from the circuit from the relatively high source impedance of the typical coin cell or is the size dependent upon the circuit that the coin cell is connected to? \$\endgroup\$ – Jim Wheeler Jan 13 '19 at 18:19
  • \$\begingroup\$ I would expect to see something in the range of 0.1 uF to 10 uF in that position. Probably on the larger side of that range, given that there's a second set of pads for a smaller device (handling higher frequencies) in parallel with it. \$\endgroup\$ – Dave Tweed Jan 13 '19 at 19:19

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