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resistor Q Answered

I'm building an LED circuit and I have the right ohm but the W is 1/2 when 1/4 was calculated out?

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VIRON

11 years ago

A higher Wattage resistor is always just as good or better unless it's too big to fit on the board. Size and getting less hot is the difference... it doesn't mean it uses more power. A resistor smokes if it's wattage is too low. "Q" is something capacitors and inductors have, resistors do not have Q... but I know you meant "question", so forget about Q now.

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mrshow555VIRON

Reply 11 years ago

cool thanks. also what's a safe number in ohms to go over. say if I only have a 150ohm resistor when it calls for 120ohm. i know going over is better than under, but how much over is too much?

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NachoMahmamrshow555

Reply 11 years ago

. Since most resistors are +/-20%, there's a chance that 150 Ohm resistor you have IS 120 Ohms. ;) Ie, go for it. The LED will be slightly dimmer, but that shouldn't be a problem.

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westfwNachoMahma

Reply 11 years ago

"most" resistors haven't been 20% in years. I don't think I've seen anything less than 5% tolerance for sale from anywhere current. That said, how far off you can be (and in which direction(s)) depends on the actual circuit. The pullup resistors in a digital circuit can probably be anywhere between 1k and 100k ohms, the current limiting resistors for you LEDs can be 10 or 20% under or LOTS over (as long as they're "bright enough"), but the resistors in your op-amp digital equalizer probably need to be real close to what's specified.

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NachoMahmawestfw

Reply 11 years ago

> "most" resistors haven't been 20% in years . OK. Guess I need to catch up. Sorry for the misinformation.

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westfwNachoMahma

Reply 11 years ago

There was an interesting phenomena someone noticed (somewhat) recently. When you get a batch of resistors with, say, a marked 20% tolerance, you expect a sort of normal bell-shaped curve with MOST of the resistors closer to the actual value, and only a few at the edges, right? Well, apparently not. In some cases, the resistors that fall within 5% of the value get pulled out as 5% tolerance resistors, and those within 10% get pulled off as 10% resistors, and so the resistors you get marked 20% are the ones that are outside the 10% range. Bleh. (My understanding, though, is that 20% is from back in the "carbon composition" days, and it's hard to get the modernly more common "carbon film" process to behave so badly...)

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Goodhartmrshow555

Reply 11 years ago

how much over is too much?

if the LED doesn't light at all, you are too far over :-)

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its a lionGoodhart

Reply 11 years ago

i could (or well, maybe couldnt) see where that may be a problem.

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Goodhartits a lion

Reply 11 years ago

The battle cry of those of us that walk among (and are members of) the uncertain: Breadboard first ! LOL