News How To Use Resistors in a Project

punkncat

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I live near Atlanta, and there is almost nowhere left to find 'off the shelf' resistors of a wide variety any more. We work in fire safety, and even the supply houses for the alarm parts have stopped carrying them. We had a small locally owned place that closed last year, and with the past death of Radio Shack and more recently Fry's there is simply nowhere left to pick these up outside Amazon and such.
 

Johnpombrio

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I was repairing a $150,000 Hewlett Packard cable tester system for UL Labs. Sometimes it worked great after a calibration, sometimes it worked terribly and showed numerous erroneous results. After spending 2 days and umteen hours troubleshooting, I found that one of the two $200 "precision" 10-ohm reference resistors mounted on a board was, wait for it, 100 ohms. I noticed the markings and, remembering the resistance chart from school, figured out that the label on the board it was mounted on was wrong. I measured the reference resistors with a simple voltmeter and replaced the errant reference resistor. 6 engineers and I missed this simple error for weeks!
 

Johnpombrio

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I live near Atlanta, and there is almost nowhere left to find 'off the shelf' resistors of a wide variety anymore. We work in fire safety, and even the supply houses for the alarm parts have stopped carrying them. We had a small locally owned place that closed last year, and with the past death of Radio Shack and more recently Fry's there is simply nowhere left to pick these up outside Amazon and such.
Working as a test and measurement repairman for HP for 25 years, I watched most of the boards in the instruments go from having discrete components and potentiometers to surface mount and analog to digital chips. Towards the end, swapping boards was really the only way to fix anything. Woe betides the guy trying to fix the very old stuff, like your building alarm/fire/intrusion systems, heh. It wasn't all roses back when there were electronic component stores around tho. The parts like resistors were mostly the most used values, usually tiny, and could not handle any sort of current. Fuses were really tough (remember fuses?) and for a long time I had a bin full of dozens of fuses of all sizes. Chips were pretty much non-existant except for hobby timers, some inexpensive A to D chips, and low-end CPUs. At least I was spared having to fix wire-wrapped boards as breadboards and circuit board kits were available when I first started. Nowadays, we are BLESSED with overnight delivery of pretty much anything you can think of for parts. No, the "good old days" weren't that great.
 
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Johnpombrio

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there was a simple sentence for recalling the color bands, but I can't recall it any more. I just use a meter or look it up at this point.
Bad Boys Rap e Our Young Girls, But Violet Gives Willingly. Gawd, that just popped into my head when you mentioned it. I am sure there are some better PG versions out there!
 
They cost less than 1 cent each, but humble resistors are an essential tool no matter what electronics you are building. In this reference 6hKT5juHjBn3qymNbofaEje we look at using them in projects, and how to identify the correct resistor for your project.

How To Use Resistors in a Project : Read more
One important thing is to "Know thy wattage" Resistors are rated in 1/8th, 1/4, 1/2, 1W+ varieties. If you pick the wrong one, your resistor can burn out. While 1/4W is more than sufficient for most applications, it might not be for all. For example, your pi/arduino might be driving a transistor/MOSFET which can carry a MUCH higher current load.

The equation to know here is VV / R. So if you have a voltage drop across the resistor of 10 V and a resistance of 20 Ohms, you get 1010 / 20 = 100 / 2 = 5 Watts of Heat. You would need a 5 Watt rated resistor.

Excellent Article.

For those looking for cheap parts: Amazon & AliExpress. For more reliable parts: Adafruit.com Mouser.com, and Digikey.com
 

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