Resistor Color Code Question

[Balaur]

Hello everybody,

I was dismantling some boards recovered from a large computer monitor and I have found a few color-coded resistors.

Since I'm somewhat proud in being able to manually decode 4/5-ring values, including tolerance, I was a bit perplexed by a few resistors following a non-standard code.

Example:

• Yellow-Violet-Red-Gold-Black
  
  If you read the value as a standard 4-ring code, it's a 4K7, 5% resistor. Measurement seems to indicate that this is indeed the case.
  Fair enough, but what's the black ring for? It's not a valid color for neither Temperature Coefficient or Fail Rate
  There are several other resistors in this case, with a last black ring.
  
  
• Brown-Black-Gold-Gold-White
  
  It's a 1R, 5% resistor, but the white ring should not be used there. Again, there are several resistors with a last white ring.
  
  
• Brown-Gray-Black-Silver-Brown
  
  At least the 5-ring code it's valid: a 1R80, 1% resistor, but the 1R80 is a non-standard 1% (E96) value (measured OK).
  I know that the manufacturers can produce any value they want, but come on.
  

So, do you have any insight to shed on these facts? One explanation could be that the resistors are manufactured according to the end user specifications, including value, tolerance and whatever the last ring means.

Thank you very much,
Dan

[orbiter]

Being a noob myself and having newly learned resistor ident skills I'm interested too. The way I see it is... The fourth band (which can be silver or gold) is the standard multiplier (Gold = x0.1 Silver = x0.01)
The fifth band would still be the tolerance value (although the black and white ones are throwing me also) Are you sure the fifth band colours are correct and not discoloured or something?

John

[DJPhil]

• Yellow-Violet-Red-Gold-Black
  
  If you read the value as a standard 4-ring code, it's a 4K7, 5% resistor. Measurement seems to indicate that this is indeed the case.
  Fair enough, but what's the black ring for? It's not a valid color for neither Temperature Coefficient or Fail Rate
  There are several other resistors in this case, with a last black ring.
  
  

There is a vaild tempco for a black band, 250ppm/K. It doesn't seem to be listed often, as it's pretty crummy.
It's also a valid tolerance, +/-20% (last table on the page), when it shows up as a fourth band.

• Brown-Black-Gold-Gold-White
  
  It's a 1R, 5% resistor, but the white ring should not be used there. Again, there are several resistors with a last white ring.
  
  

This one gets a bit arcane. I found a few references in google books to 'military reliability' bands in the fifth slot.
The orange 0.01/1khrs failure rate is said to be certified for missile electronics.
The yellow 0.001/1khrs failure rate is said to be certified for space flight systems.
The white band seems to indicate 'solderable leads'. Huh? Beats me.

• Brown-Gray-Black-Silver-Brown
  
  At least the 5-ring code it's valid: a 1R80, 1% resistor, but the 1R80 is a non-standard 1% (E96) value (measured OK).
  I know that the manufacturers can produce any value they want, but come on.
  

Oddly, this one would be valid as an 18R 5% with a 1/1khrs tested failure rate. Of course if you measured it at 1R8 then you've interpreted it correctly, but it just goes to show how messy these codes get.

Hope that helps.

[orbiter]

Nowhere in my books did it say anything like that DJ

Thanks though, very interesting.

John

[Balaur]

There is a vaild tempco for a black band, 250ppm/K. It doesn't seem to be listed often, as it's pretty crummy.
It's also a valid tolerance, +/-20% (last table on the page), when it shows up as a fourth band.

Very good finding about the black tempco. I fail basic internet search. I've looked everywhere *except* Wikipedia and didn't found the value for the black ring.
Thank you very much.

• Brown-Gray-Black-Silver-Brown
  
  At least the 5-ring code it's valid: a 1R80, 1% resistor, but the 1R80 is a non-standard 1% (E96) value (measured OK).
  I know that the manufacturers can produce any value they want, but come on.
  

Oddly, this one would be valid as an 18R 5% with a 1/1khrs tested failure rate. Of course if you measured it at 1R8 then you've interpreted it correctly, but it just goes to show how messy these codes get.

Hope that helps.

Yes, the resistor is a 1R80. I've also measured that on a 4W lab multimeter and it showed a very nice 1.799? value.

Thank you all very much,
Dan

[Zero999]

Because the E24 series doesn't match with the E92 series, the E24 series is also available in narrower tolerances than 5%.

In my opinion, they should've designed it so the E96 is just an extension of the E24 series.

[Neilm]

There are occasions when a designer will require some really weird value resistors. I have had to use values like 1.8 ohms on occasion. Usually I buld them up out of standard parts, but occasionally I need special resistance value at a tight tolerance, good voltage rating or maybe I don't have room to put multiple resistors on the PCB.

Neil

[Zero999]

Series/parallel is normally fine.

I can't say I've have ever had that problem, as for most circuits it's the ratio that's important rather than the resistance so getting the right ratio of E24 and E96 values is normally easy.

[Neilm]

Series/parallel is normally fine.

I can't say I've have ever had that problem, as for most circuits it's the ratio that's important rather than the resistance so getting the right ratio of E24 and E96 values is normally easy.

True, but I have had some reasonably specialist applications when the exact value was important. Generally these happen in precision measurement circuits.

Neil