I would wire a pot in series with the existing resistor. That way, you can't burn out the LED regardless of where or how far you turn the knob on the pot.

Resistor color codes get real confusing because of changes that have occurred over the last 90 years or so. For that reason, when in doubt, use an ohmmeter to put you in the ball park. That will also help you determine about what value of pot to use to dial in the brightness.

Note that the 1st band from the left in the photo is WIDER than the others. That signifies the FIRST band. On my monitor it looks like brown, brown, black, gold(?), gold(?). That translates to 11 ohm, 5% which is a standard EIA value. The last color band could represent either the temperature coefficient or the reliability. (REF http://www.resistorguide.com/resistor-color-code/). You would have to know the manufacturer of the resistor to know for sure.

So a standard EIA 5% resistor is going to have three value bands?

While it would be nice if the first band were always wider than the others, or if the last band were spaced further apart than the others, there is no requirement or even widely accepted practice of doing either.

The first band is on the right side. It is a 3.3 kΩ, 1% resistor. This matches what the TS measured after removing it from the board.

 

So what's the fifth band?

And how do you explain the fact that the measured resistance of that resistor is no where near whatever value you seem to be zeroing in on, while if you read it as a 5-band resistor starting from the other end the value is 3.3 kΩ, 1%, which matches what the TS read with an ohmmeter?

That's probably why the "brown" bands at one end are lighter than the other - I always check with a DMM to make sure.

With the advice I gave; it doesn't matter if the 5 band system is open to misinterpretation.

 

That's probably why the "brown" bands at one end are lighter than the other - I always check with a DMM to make sure.

With the advice I gave; it doesn't matter if the 5 band system is open to misinterpretation.

The problem is that removing the resistor so that it can be measured with a meter is frequently not a viable option. Measuring a resistor in place can place a lower limit the value, but some circuits can be damaged even by the voltage and currents that a typical ohmmeter can produce. So we do need to be able to interpret the bands as best we can as reasonably as we can.

 

I would wire a pot in series with the existing resistor. That way, you can't burn out the LED regardless of where or how far you turn the knob on the pot.

Resistor color codes get real confusing because of changes that have occurred over the last 90 years or so. For that reason, when in doubt, use an ohmmeter to put you in the ball park. That will also help you determine about what value of pot to use to dial in the brightness.

Note that the 1st band from the left in the photo is WIDER than the others. That signifies the FIRST band. On my monitor it looks like brown, brown, black, gold(?), gold(?). That translates to 11 ohm, 5% which is a standard EIA value. The last color band could represent either the temperature coefficient or the reliability. (REF http://www.resistorguide.com/resistor-color-code/). You would have to know the manufacturer of the resistor to know for sure.

In the "old" days (my time), a wider first band indicated a wirewound resistor.

 

In the "old" days (my time), a wider first band indicated a wirewound resistor.

That's a new one to me. Never heard that before. Interesting. I wonder how widespread that was. Was it common practice amongst most manufacturers, or was it a convention adopted by a particular manufacturer for their product line? (That's a rhetorical question in that I would be surprised if you had an answer, but if it happens you do, please do share.)

 

In the "old" days (my time), a wider first band indicated a wirewound resistor.

Must've been before my time - and I'm no spring chicken.

 

That's a new one to me. Never heard that before. Interesting. I wonder how widespread that was. Was it common practice amongst most manufacturers, or was it a convention adopted by a particular manufacturer for their product line? (That's a rhetorical question in that I would be surprised if you had an answer, but if it happens you do, please do share.)

Wow, I still have one. Now, having documentation is another thing. As a young lad, I thought this was really good information and have retained that in my mind.

 

Okay, I found a reference. See the "Notes".

http://www.lift-net.com/?p=293

 

Okay, I found a reference. See the "Notes".

http://www.lift-net.com/?p=293

I'm pretty skeptical of how widely applicable that table is. I've never heard of any tolerance level in the 1% to 10% range other than 1%, 2%, 5%, and 10%. I can't imagine any reason for making a 9% tolerance resistor, for instance. Then the table indicates that even 1% resistors only have two value bands, while every 1% (or even 2%) resistor I've ever encountered had three color bands. So even if two-value-band 1% resistors exist (which I certainly can't rule out) the fact that the table doesn't allow for the far-more-common case of three value bands on such resistors indicates that it is not general in nature at all. That they say that a fifth band means it's flame proof further indicates, to me, that this chart is not general but rather is specific to a particular line of resistors. An additional band generally either indicates a temperature coefficient or a reliability rating (mostly for military suppliers).

 

I'm pretty skeptical of how widely applicable that table is. I've never heard of any tolerance level in the 1% to 10% range other than 1%, 2%, 5%, and 10%. I can't imagine any reason for making a 9% tolerance resistor, for instance. Then the table indicates that even 1% resistors only have two value bands, while every 1% (or even 2%) resistor I've ever encountered had three color bands. So even if two-value-band 1% resistors exist (which I certainly can't rule out) the fact that the table doesn't allow for the far-more-common case of three value bands on such resistors indicates that it is not general in nature at all. That they say that a fifth band means it's flame proof further indicates, to me, that this chart is not general but rather is specific to a particular line of resistors. An additional band generally either indicates a temperature coefficient or a reliability rating (mostly for military suppliers).

Another:
https://wiktel.com/standards/resist.htm

 

Another:
https://wiktel.com/standards/resist.htm

What is WikTel? As best I can tell it's a telephone and/or cable TV provider?

I'm still having a hard time taking that as a general purpose standard and not one that is highly specific to a subset of resistors (perhaps power resistors?). Notice that that table doesn't acknowledge the existence of 5% or 10% resistors nor any resistors with three value bands.

I did some searching (not a lot), including looking for sites that would tell me how to tell if a resistor was wirewound, and I was unable to find any site that mentioned anything about a wide first band. I would imagine if this was at all a widely used convention, that I would have found several mentions of it. I similarly searched for how to tell if a resistor was flameproof. I found several sites that mentioned different markings, but none that mentioned the presence of a fifth band.

Clearly the tables you've referenced indicate that some set of resistors used this convention, the question is how widely used was it?

Regardless, it's interesting to hear about. If nothing else, it's one more thing to store in the back of my mind if I run across resistors that appear to be marked this way.

 

Thanks for the continued discussion and input from everyone! I got some parts and hope to try some things out this weekend.

Steve

 

No, it's a 1% 3.3 kΩ resistor. The colors are Orange,Orange,Black,Brown,Brown

I agree with this assessment of the band colours. . . In the original post, it is mentioned that the positive terminal of the LED is connected to the left-most solder (which I impulsively assumed to mean the left leg of the resistor according to the picture). This would suggest that for a forward bias, current should flow from right to left (in the picture) and hence my quick conclusion that the bands must be orange, orange, black, brown, brown.

Of course I realised later that my assumption that the left solder corresponds to the left leg of the resistor could be wrong. However, the MM reading confirms that reading the colours from left to right in the picture is clearly incorrect.

 

Wow, I still have one. Now, having documentation is another thing. As a young lad, I thought this was really good information and have retained that in my mind.

First moulded WW resistors I've ever seen - they were at best dip coated as far back as I can remember.

Moulded construction would defeat the whole object of WW.

 

I agree with this assessment of the band colours. . . In the original post, it is mentioned that the positive terminal of the LED is connected to the left-most solder (which I impulsively assumed to mean the left leg of the resistor according to the picture). This would suggest that for a forward bias, current should flow from right to left (in the picture) and hence my quick conclusion that the bands must be orange, orange, black, brown, brown.

Of course I realised later that my assumption that the left solder corresponds to the left leg of the resistor could be wrong. However, the MM reading confirms that reading the colours from left to right in the picture is clearly incorrect.

Since resistors aren't polarized, which end connects to the LED has absolutely no meaning on which way to read the color bands. Resistors are often placed without concern for orientation. When care if given, they are usually oriented so that, holding the board so that the silkscreen can be read (or most of it) that the first color band is on the left of the top, without any regard to the resulting orientation in the schematic.

 

Since resistors aren't polarized

Resistors aren't polarized. . . But diodes are. We also know that the resistor will likely come before the diode (am I right?!). Using this logic then, we can infer the direction of the current will be through-resistor then through-diode.

*just to clarify what I meant earlier

 

Resistors aren't polarized. . . But diodes are. We also know that the resistor will likely come before the diode (am I right?!). Using this logic then, we can infer the direction of the current will be through-resistor then through-diode.

*just to clarify what I meant earlier

The resistor can come before or after the diode (even what "before" and "after" mean is ambiguous) and whether or not we can infer the direction of the current through the resistor is completely irrelevant with regards to reading the color bands on the resistor. The two simply are not related in any way, shape, or form.

 

The resistor can come before or after the diode (even what "before" and "after" mean is ambiguous) and whether or not we can infer the direction of the current through the resistor is completely irrelevant with regards to reading the color bands on the resistor. The two simply are not related in any way, shape, or form.

*noted

 

So here's the final result! I kept it simple!

I started off trying a few different values of resistors -- 22K, 47K, 100K -- with the blue LED. Since it was clear though, it was annoying when looking directly at it no matter how dim. So, I decided to just use a diffuse red LED that came with a pack of LEDs I purchased. I ended up using a 22K resistor and got exactly the desired result. I couldn't find my solder sucker -- been a while since I worked on a PCB -- so I had to improvise with solder braid and a sharp pencil to clear out the holes. The result wasn't the absolute neatest but plenty good enough for this quick fix.

Thanks for all the input and ideas! I'm definitely going to keep them in mind for any future tweaks/fixes!