0603 Resistor Codes

I was looking at a Printed Circuit Board populated with 0603 size surface mount resistors and puzzling over the markings. In particular R45 and R56 in the photo below. They are marked 18C, but the actual value is 15k.

 Now in the old days resistors were colour coded, and we all knew the code off by heart, Black = zero, brown = 1, red = 2 etc. There were usually three colour bands - 1st digit, 2nd digit and a multiplier which represented the number of noughts. So 4.7k was yellow, violet, red, for example. There might then be a gold band for 5% tolerance, or brown for 1%. Even when "E96" values came in, and many more "preferred" values were needed, it was just a case of adding another colour band to make three digits and a multiplier. This system was carried over onto surface-mount parts, but in numerical form. You can see some examples below - there is a 473 (47k) and a 223 (22k). But there is also 01C  and 30C which don't make much sense.

The answer to this is, of course printed in the data sheet for the resistors. It is is a way to address the problem of marking E96 values - presumably 0603 size parts are deemed too small to put four digits on. 

Perhaps an aside about "preferred values" might be appropriate here (although there is a very good Wikipedia Article about it). When a resistor is manufactured there is a tolerance in the value, perhaps +/- 10%, or 5% or 1%. So a 1000 Ohm resistor might be, say, 5% high (1050 Ohms). So there is no point in making a 1050 Ohm resistor because the value would overlap with 1000 Ohm. The next "preferred value" is 1100, because a 1100 resistor which was 5% low would measure 1050. For 5% tolerance resistors there are 24 non-overlapping values in each decade, and the sequence of values is called the E24 series. 10k, 11k, 12k, 13k, 15k, 16k, 18k, 20k, 22k, 24k, 27k, 30k, 33k, 36k, 39k, 43k, 47k, 51k, 56k, 62k, 68k, 75k, 82k, 91k, For 1% values there is an E96 sequence which requires three digits to represent each value. You can see this sequence in the table below which gives the manufacturer's 0603 resistor code on the left and the E96 value on the right. The Letter is the multiplier code.

So "18C" is indeed 150 x 100 = 15k Ohms. 30C is 20k Ohms, and 01C is 10k. All has become clear.

Now to print out this table and pin it up at my work-bench!
73
Hugh