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I have a selection of salvaged inductors, and I don't (yet) have an inductance meter.

I would like to know what the inductances are, but I have a problem.

They are numbered with 3 digit numbers - I assume this is in the same format as capacitors and resistors - 2 digits and a number of 0's.

However, what is the "base" value of the inductance?

e.g., I have a "221" inductor:

enter image description here

Is this going to be 220pH, 220µH or 220mH?

Where do they start counting from?

With resistors it's easy enough - it's from 0. With capacitors it starts with pF. What do inductors with this kind of numbering system start from?

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That looks like a 220 µH inductor. These size inductors (like a lot of electronic parts) are labeled with something a lot like a floating point format in a computer. The first N-1 digits are the mantissa and the last digit is the exponent of 10 to apply to the integer mantissa value. In this case, the mantissa is 22 and the exponent 1. The value is therefore 22 x 10^1 = 220. The fact that the units are µH is obvious from the general size and windings. Some very small inductors are labeled like this in nH, but this one is obviously a lot bigger than 220 nH.

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  • \$\begingroup\$ Excellent, thanks - so there is no "standard" like with other components? Typical ;) \$\endgroup\$ – Majenko Dec 17 '11 at 15:51
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    \$\begingroup\$ @Majenko: What I described is a standard, mostly, except when it's not of course ;-) \$\endgroup\$ – Olin Lathrop Dec 17 '11 at 15:52
  • \$\begingroup\$ As in most "standards" ;) So µ it is then unless it's obviously too small. \$\endgroup\$ – Majenko Dec 17 '11 at 15:54
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    \$\begingroup\$ @Majenko, verify with an LR circuit! You have an o scope or one you could build with an arduino? \$\endgroup\$ – Kortuk Dec 17 '11 at 17:02
  • \$\begingroup\$ @Kortuk Do you fancy expanding on that in an answer? Formulæ would be nice. \$\endgroup\$ – Majenko Dec 17 '11 at 17:06
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A quick google search yielded Electronic color code. If you go to the section on printed numbers it will explain it. What you have shown in the image is 220uH

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  • \$\begingroup\$ Hard to find the actual bit in there that tells you - I have to look several times. Nice link though, thanks. \$\endgroup\$ – Majenko Dec 17 '11 at 15:55
  • \$\begingroup\$ You don't answer OP's question why it couldn't be 220nH or 220mH. \$\endgroup\$ – stevenvh Dec 17 '11 at 17:24

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