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Basic Electronics » Help with choosing the right resistor

April 16, 2010
by CyberGod
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I am about to buy some 1K, 22K and 3.3K resistors. I see that there are 1/2-watt, 1/4-watt, 1/8-watt and 1-watt variations. Which one do I need for the Piezoelectric Sound Meter Project and what exactly is the difference ?

April 16, 2010
by mongo
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You can probably get by just fine with the 1/8 watt variety.

Most everything I use are the 1/4 watt but most available are 1/2 watt.

Rarely does a resistor ever have to actually handle the wattage designed into them. Power resistors on the other hand, generally do but that's a whole different game.

April 16, 2010
by bretm
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In a circuit limited to 5V, a 1K resistor will conduct up to 5mA of current. Watts = amps x volts, so it would be dissipating 0.005A x 5V = 0.025W. That's 1/40 of a watt, so any of those choices would leave you well within the power tolerance of the resistors. The larger resistors would conduct even less current and dissipate even less power.

If the resistance were only 100 ohms, then it could dissipate 1/4 of a watt of power in a 5V circuit and you'd want to avoid the 1/8-watt variety.

If you were to exceed the tolerance, the resistor could generate more heat than it could dissipate and it could burn out.

April 16, 2010
by CyberGod
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Thank you for the great explanation guys, this helps a lot!

April 16, 2010
by Farmerjoecoledge
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Hi, yeah, but "I" still don't understand why each of the watt values, 1/8, 1/4, 1/2, 3/4 all have their own complete set? .001 ohms to 9.99m ohms. Millions of resistors, don't you hink they could have eliminated half of them?

April 17, 2010
by bretm
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For hobbyists, yeah, but if you need to buy a million of them, the price difference matters.

April 17, 2010
by wayward
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It's actually not millions, but around 168 distinct values for 5% tolerance resistors. Each magnitude has 24 possible multipliers arranged exponentially so that no two values are more than 10% apart from each other; that way, you can always find one standard resistor within 5% of your desired value. That is why there is a 4.7k resistor, but not a 4.8k one (again, this only holds for 5% tolerance).

April 17, 2010
by mongo
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Back in my early days, we had resistors that were down to the ohm in value at a .01% tolerance. Those were specifically made so not much in variety.

Even in the 1% tolerance range, the selection is pared down to a minimum but most anything you would need could be obtained. They keep basically to the same values of the 5% variety but there are a few more as the values get larger.

One type is a carbon coating on a glass or ceramic core and painted outside. These can actually be trimmed with a file to get things tuned. Just remember to seal them up with something like nail polish to keep the air from corroding the internals down the line. You can only increase the value by trimming.

For experiments, I like to use trim pots and when all is said and done, I can always match with regular resistors if I decide to make a permanent circuit. Trim pots take up space but for learning and such, no problem, plus they can be used over and over again at different values. The come in a wide assortment of values from 10 ohms to 10 Meg ohms in both linear and log-rhythmic (like a volume control) forms. I prefer the linear ones.

April 18, 2010
by Farmerjoecoledge
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Just picking up on stuff that never quite got understood. On my old lcd I got pin3 on the lcd going to a pot. It doesn't work without it. But on the new lcd that pot is replaced by a resistor. That's quite fine, I suppose, I plugged it without the resistor, is that why it burnt?

April 18, 2010
by Farmerjoecoledge
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Duh!

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