# how to choose resistor

by onceinalifetim
Tags: choose, resistor
 P: 28 Hello I need to buy 100k and 1k resistor.. but how do i know what watts do i need???? 1 watt or 2 watts hope help
 P: 28 and i need to buy 800nf capacitor..
 Sci Advisor PF Gold P: 2,242 what is the resistor being used for ? you have told us nothing about the circuit its going into 800nF (0.8uF) isnt a standard value. The closest you could get would be two 0.47uF (470nF) capacitors in parallel EDIT Actually a better choice ould be a 470nF and a 330nF = 800nF I'm too tired LOL Dave
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P: 6,341

## how to choose resistor

From Ohm's law, you need 30 volts across a 1k resistor to generate 1W, so unless your project uses high voltages you won't need a higher wattage rating. Most likely a 1/4 watt or 1/2 watt would be enough. To get 1 watt through a 100k resistor, you would need a 300 volts. If you are asking this level of question, you probably shouldn't be building a project that uses high voltages, without proper supervision from somebody who knows what they are doing!

For the cap, 820nF is a standard value, and unless you need the "exact" value for some reason, that's likely to be near enough. (But if you got the 800nF from a circuit diagram, it does raise the question as why they specified that value)
PF Gold
P: 2,242
 Quote by AlephZero ............ For the cap, 820nF is a standard value, and unless you need the "exact" value for some reason, that's likely to be near enough............

Yeah I would have thought so but since it wasnt in the listed values of either of my 2 component suppliers, I didnt suggest it and hence went for the paralleled solution

Dave
 P: 28 that is my circuit for my project!!! similar design i mean Attached Thumbnails
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P: 6,341
 Quote by davenn Yeah I would have thought so but since it wasnt in the listed values of either of my 2 component suppliers, I didnt suggest it and hence went for the paralleled solution
I checked there were some suppliers on google before I posted it.
But I agree caps are usually only specified using the 10 - 15 - 22 - 33 - 47 - 68 series of values (or even just 10 - 22 - 47).

 Quote by onceinalifetim that is my circuit for my project!!! similar design i mean
So with a 5v supply, 1/4 watt resistors would be just fine everywhere.

L1 and C5 look like a tuned circuit, so the question is, how accurate does the frequency need to be? Since you don't have any way to fine-tune it, I would guess "not very", but that's just a guess.

What is the "sensor"? it is some gizmo and the data sheet says it has inductance and capacitance equivalent to L1 and C5, or are they "real" components?

You have R1 and R3 in series to make 1.56k, instead of just a single 1.5k resistor - does that mean you really do need accurate (and close tolerance) component values, or is it just that the person who drew the circuit had those two R values in their spares box but didn't have a 1.5K ...... ????

It's hard to guess what the circuit does, just from the diagram.
 P: 28 thanks for the reply folks.. may i know is there 800nf capacitor in the market???
 P: 389 No, but you can buy 820nF cap.
 P: 28 i need help!!! i need the capacitor value of 17nf and 151pf... is there in the market or is there any way to series or parallel it???

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