# marking schemes for SMD resistors

Discussion in 'Electronic Design' started by Winfield Hill, Apr 15, 2006.

1. ### Winfield HillGuest

The conventional marking scheme for SMD 5% resistors is two
digits plus an exponent of 10, e.g., 120 would be 12 ohms,
and 122 would be 1.2k, etc. But, do _any_ manufacturers use
a direct method, so 120 means 120 ohms and say 1k2 = 1.2k?

2. ### James WaldbyGuest

Among 0805's I've got on hand in the 1 to 10 ohm range,
x.x seems to be a little more common than xRx, but both occur
repeatedly. Eg, 1.1, 2R7, 3.3, etc. Besides those, I
haven't seen any 5% SMD resistors or data sheets with direct
values.

-jiw

3. ### Rene TschaggelarGuest

Na.
It always is the number of significant digits
plus the exponent. For some reason, character
were avoided.

Rene

4. ### Guest

That reason would probably be that e.g." 120K" would require one more
character than "124". And the printed text is already small enough.
More characters mean an even smaller font.

Joop

5. ### Winfield HillGuest

James Waldby wrote...
For resistors under 10 ohms, yes. Nothing above that?

I ask because I've been analyzing my technician's washing-
machine VFD motor-control circuit board. This has an 0805
resistor labeled 120, which must be near 120 ohms, because
if it was 12 ohms the circuit not only wouldn't work, it'd
go up in smoke. Actually, it did go up in smoke, burning
out the "120" resistor (and many other components), so I
can't measure it. The circuit board has mostly US-made or
familiar parts, but also a few foreign-looking parts. The
other 0805 resistors have conventional labeling, a 33-ohm
part says 330. But this 120 fellow simply can't be 12 ohms!

Further complicating the issue is that this 120 resistor was
thrown away when my technician replaced it, but both he and I
clearly remember it said 120, I had even written it down. To
my memory it looked just like the others. But hey, it really
couldn't have been 12 ohms! :-<>

6. ### Fred BloggsGuest

Which brand? A universal motor, transmissionless drive?

7. ### colinGuest

Ive come accros some marked 1R0, 1210 case size, ive never seen 1k0 etc
though, maybe its just the low values. the coding system I came acros on
some 1% 0603 is rather anoying, you have to look up a 2 digit code to find
the 3 digit value and look up the letter to find the multiplier value.

Colin =^.^=

8. ### Michael A. TerrellGuest

The "R" is used in place of a decimal point to make sure the value
can be read. I wrote some javascripts to decode the three digit 5%
values and another for the the four digit 1% and .1% values for
production and stockroom workers. I'll put them on my website if anyone
is interested. I also have one for SMD capacitors with the number/letter
ID codes if anyone needs it.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

9. ### Winfield HillGuest

Michael A. Terrell wrote...
That sounds both useful and interesting, Michael. I like reading
javascript code. Put it up for us, with a link from here.

10. ### colinGuest

Ive only just got into using javascript for some elementry calculations, I
got fed up calculating reactances etc ever since i lost my walchart from an
early issue of everyday electronics, and a single web page is far easier
than creating a whole c++ application wich I would otherwise have done, it
was just a smallish table of hand entered values, but I will try and make it
so moving the cursor over the table gives all the inbetween values.

I also just did a page to list all the frequencies available from a PLL
within a range of N and R divisor values. only drwaback is it seems to get
suddenly get very slow once a certain number of array entries is reached
although this can be minimised.

Incidently the problem I find with the 2 digit + 1 letter 0603 1% codes is
they are extremly hard to distinguish (I need to use a microscope to
distuinguish the last letter) its so easy to read 10C (12.4kohm) as 100ohms

I dont know if this is common but I always used to surprise people when I
could read colour resistor codes instantly, (especialy when I was working as
a software engineer)

Colin =^.^=

11. ### joseph2kGuest

Gosh, the "typographic" resistor value markings date back to Apollo times
about 40 years ago. they haven't changed much either. i was surprising
people by reading the much more varied capacitor color codes.

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