One product I ran into used MELF diodes with a single blue color band. The circuit they were used in was very strange, appeared that it would draw excessive current. Fortunately when studying an earlier revision of the product that used thru-hole the number 1N5305 was marked on the diodes, turned out to be not a true diode at all but a 2 mA current regulator made from a FET with gate and source tied together through an internal resistor.
SOT23 packages with their 2 or 3 letter codes are not quite as bad, there are several code-lookup sites online. Where it does get tough is when the code is not listed on these sites.
I think SMD MOVs have never had any marking on them. I have a product that uses a 5V5 and a 33V MOV and you cannot tell the difference. Of course you know what happened- they got interchanged. We got some very strange results reported from the test department of our sub.
The only way to test an MOV is to force it into clamping using a current limiting resistor- It is not a simple exercise, especially when you are using 1206 components since you first have to remove the part from the design- connect a resistor and then apply a voltage.
Even more concerning is the fact that if they used a 33V in place of a 5V, test would not normally detect it, but the product would be unprotected.
The production house could even place a capacitor there and we would be none the wiser.
It would be so much easier if they were marked!!!!
It's a conspiracy, probably started by Analog Devices and Analogic when they both made ADC, ADC, and PGA modules. They copied each other all the time. I know because I was once given the job of tracing out a schematic of one onf "Bernie's" modules when I worked at ADI right out of college. We copied them.
@Dom Lodge: I wonder what future impact this may have on counterfeiting if it was to be applied to all components. One fo the first things to check is whether the markings are correct / have been changed
Good point! Without markings we certainly lose a lot of traceability.
@Zeeglen: You CAN get SMT capacitors marked with value codes - http://www.vishay.com/doc?45028
You had me all excited for a minute there....until I looked on Digi-key. The only ones I can find listed are more than 10X what I had been paying for the same values! It's hard to find them, since there's no parametric search option that I can find for markings.
Do you know of any available marked caps that won't break the bank?