A little eletronic question .

clyst

clyst

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I have inline resistors on the live side of led indicators on my switch panels . Not being really conversant in eletronics I'v been told that they reduce the voltage down from 12V to 3V . I need some more but how do I order them ?? There are no markings other than the multi coloured bands. Is this where the answer lies ????. See I told you I was thick when it comes to eletronics . /forums/images/graemlins/blush.gif

Cheers

terry
 
The colour bands show the value of resistance and the tolerance of that resistance.
Describe the markings for the value in words, and then you'll know what to order...
 
The coloured bands tell you the resistance value and the tolerance. As already said a photo would be the best way to get them "decoded"; if not could you describe them in words and I (we) will tell you the value to order. There may be a description of how to decode the value on the Radio Spares web site ........ there is, and you'll find it at this address:
http://uk.rs-online.com/web/generalDisplay.html?id=infozone/calculators&file=4band

Should be easy to work out.

Alan.
 
The resistors reduce voltage but more accurately limit current. The LED has a voltage drop of about 1.5-3.5V (depending on the LED colour output)and an absolute maximum current rating of about 50-100mA (thousandths of an Amp) before it will blow. Typically you might want about 5 to 25mA so the resistor would be calculated to suit. For a 12V system and a 2Volt forward drop LED to run at 10mA you would have 10Volts across the resistor which gives 1000 ohms (Ohms Law)
 
Depends on LED .. Most are 25ma 3volt .. With a 12v supply you need to drop the voltage by 9v (12-3) 9/.025ma = 360ohms .. So get the nearest you can .. ( OHMS Law ) If you want them brighter reduce the value .. dimmer increase /forums/images/graemlins/cool.gif
 
Thanks for your promp replies chaps . Now I know what to look for I'll have to buy a magnifying glass to accurately read the bands . We are taking minuscule portions here . Got a job to see the colours even with my "new" varyfocals . /forums/images/graemlins/frown.gif
Thanks for all your advice.

Terry
 
Be careful how you read the bands. You can read the both ways round. If the colours seem to be spread evenly then look for a larger gap bewteen the third & fourth. The fourth is the tolerance. This won't really matter in your application.

Alternatively, there may be a gap between the fourth & the end of the resistor. Again the fourth is the tolerance.

The important bands are the first three.

Once you've read the value. Check you're right by calculating the current. Assume 2v on a diode & 12v on the battery. So you have 10v on the resistors. If you get 1000 ohms, you have 10 / 1000 = 0.01A or 10mA. That's reasonable. If you work out something like 1mA or 1A, you have it wrong.
 
[ QUOTE ]
Got a job to see the colours even with my "new" varyfocals

[/ QUOTE ] In the Maplin catalogue they explain how to calculate the value of the resistor you'll need.
 
If you want to remember the code, in the 50s I was taught the following " Big Boys Rape Only Young Girls But Virgins Give Way" which represents Black Brown Red Orange Yellow Green Blue Violet Grey White for the numbers 0 to 9. The first 2 numbers are the value and the third is a multiplier which represent the number of zeros. if there is no 4th band it has a 20% tollerance and I think pink silver and gold are 10% and better. An examble is as follows Brown Black Green = 1, 0, 00000 = 1Megohm @ 20% and Yellow Violet Red = 4, 7, 00 = 4.7Kilohms @20%.
Interesting that I can remember that when I often don't know what day it is.
Stan
 
The last delivery of resistors we had some had gained a another colour band ( 6 colour bands from memory ) , assumed they were a RoHS compliant identifier. But I have to investigate when I have 5 minutes.

So don't assume they will be 4 or 5 ring in future.

Brian
 
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