SMD Inductor shorted (cooked) need to find replacement

Thread Starter

bxdobs

Joined May 11, 2014
23
Can someone possibly identify the value and or manufacturer of this SMD part?

Thanks in advance for any insight you can provide.

The coil is being used in some sort of embedded computer-controlled voltage regulation circuit which
is attached to a National Semiconductor MLAC 2951 (8 pins) ... No schematic or documentation is available ...
thinking this IC may be an adjustable LP2951 low power regulator ... there are also 2 x 22uF 35V Tantalum Caps
hanging off one of the legs of this Coil. Haven't found any app notes for the LP2951 that use a coil

Coil physical markings (see photo attached):
220
MH

my best guess is that this coil is 22uH ( M to signifying 6 digits from E-12 and 0 being a multiplier)

SMD Plastic Spool physical dimensions:
LxWxH = .2"x.2"x.1"

Coil dimensions:
copper wire 10 mil diam total length of wire (unwound) 11"
approximately 30 turns around the plastic spool
80 mil coil (inner spool) length
160 mil outside Coil diam

- the markings to me suggest this might be 220 mH (but expecting the part is physically too small for that large of an inductance)
- if the M is similar to Cap designations, then this might be .000 000 220 000 H (220nH) or .000 220 000 000 H (220uH)
- if the 0 is a multiplier, then maybe this is either 22 uH or 22 nH (again, expecting mH would be physically much larger)

there is a possibility that the M and or H may actually provide more information about specifications rather than just the inductance value
the only way to know for sure is to know the manufacturer and find their part spec sheet.

there are no noticeable manufacturing markings (see attached photos; the cooked part and it's pristine mate)

have looked at various manufacturers for coding examples with no results

have tried some online inductance calculators none of which provide an expected value prefixed with 22 ... about the only thing these calculations prove to me is that this part can't be in the mH range. (using air-coil formulas even though the windings are around a plastic spool so this may account for the wide range of values)

N = 30
l = 80 mil (length of spool | coil)
r = 60 mil (mean radius of coil ... max radius = 160 mil / 2, min radius = 80 mil / 2, mean of 80 and 40 is 60)
d = 40 mil (depth of coil = 160 / 2 - 80 / 2 = 40)
Copper Wire Diam 10 mil
Wire length = 11000 mil (11")

From: https://en.wikipedia.org/wiki/Inductor
(multi layer air coil)
L =.8* r^2N^2/(6r + 9l + 11d) uH
r = .06, l = .08, d = .04
L = .498 uH = 498nH

(flat spiral air coil)
L = r^2N^2/(8r+11d) uH
L = 3.52 uH

length calculation validates turns count
10 mil wire should provide 8 turns per row on an 80 mil spool ... this provides a max 4 rows (40mils) which matches the coil depth (d above)
Circumference = 2 * 3.14 * r
using the center of the wire as the circuference radius:
length of 1st row would be 8 rows * 2 * pi * (80/2 + 10/2) = 2.262"
length of 2nd row would be 8 rows * 2 * pi * ((80 + 20)/2 + 10/2) = 2.765"
length of 3rd row would be 8 rows * 2 * pi * ((80 + 40)/2) + 10/2) = 3.267"
length of 4th row would be 6 rows * 2 * pi * ((80 + 60/2) + 10/2) = 2.827"
----------
Total length closely matches measured length 11.121"
 

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AlbertHall

Joined Jun 4, 2014
9,281
I suspect the spool is ferrite not plastic which would increase the inductance considerably. Check if it is attracted to a magnet.
 

Thread Starter

bxdobs

Joined May 11, 2014
23
Thank you for your suggestion ... confirmed, this core is actually MAGNETIC ... the size and shape of this core along with it being non-conductive led me to the conclusion that it was plastic.

Wheeler's inductance formula for air includes a Ur factor of 1 ... per wiki, Ferrite Ur factors can be anywhere between 10 and 2300 ... applying a Ferrite Ur factor to Wheeler's formula, prefixed as 22 or 220, eliminates all but 2 calculations by simply ruling out Ur < 10 and Ur > 2300 ... Wheeler's formula with the given metrics and an Ur = 1 works out to 498nH
Ur = 220n / 498n = .44 (Ur <10)
Ur = 22u / 498n = 44
Ur = 220u / 498n = 440
Ur = 22m / 498n = 44000 (Ur > 2300)

Thinking I will just order both a 22u and a 220u inductor and then select whichever physically best resembles the existing part
or remove the pristine part and measure it against the new parts
 
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