how to choose resistor


by onceinalifetim
Tags: choose, resistor
onceinalifetim
onceinalifetim is offline
#1
Feb14-14, 10:13 PM
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
Phys.Org News Partner Engineering news on Phys.org
A smart prosthetic knee with in-vivo diagnoses
Old tires become material for new and improved roads
Students take clot-buster for a spin
onceinalifetim
onceinalifetim is offline
#2
Feb14-14, 10:16 PM
P: 28
and i need to buy 800nf capacitor..
davenn
davenn is online now
#3
Feb14-14, 10:26 PM
Sci Advisor
PF Gold
davenn's Avatar
P: 2,243
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

AlephZero
AlephZero is online now
#4
Feb14-14, 10:41 PM
Engineering
Sci Advisor
HW Helper
Thanks
P: 6,379

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)
davenn
davenn is online now
#5
Feb14-14, 10:53 PM
Sci Advisor
PF Gold
davenn's Avatar
P: 2,243
Quote Quote by AlephZero View Post
............
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
onceinalifetim
onceinalifetim is offline
#6
Feb15-14, 12:13 AM
P: 28
that is my circuit for my project!!! similar design i mean
Attached Thumbnails
httpi30.tinypic.comwbdicx.jpg  
AlephZero
AlephZero is online now
#7
Feb15-14, 12:40 AM
Engineering
Sci Advisor
HW Helper
Thanks
P: 6,379
Quote Quote by davenn View Post
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 Quote by onceinalifetim View Post
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.
onceinalifetim
onceinalifetim is offline
#8
Feb16-14, 07:51 AM
P: 28
thanks for the reply folks.. may i know is there 800nf capacitor in the market???
Jony130
Jony130 is offline
#9
Feb16-14, 08:09 AM
P: 389
No, but you can buy 820nF cap.
onceinalifetim
onceinalifetim is offline
#10
Feb16-14, 08:10 AM
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???


Register to reply

Related Discussions
Help with Simple Resistor problem! from physics resistor chapter! Introductory Physics Homework 2
Use resistor simplification to find current across the 10 ohm resistor Engineering, Comp Sci, & Technology Homework 8
On non linear resistor and how to calculate a fixed resistor. Introductory Physics Homework 4
equivalent resistor of 2011 resistor web Introductory Physics Homework 7
induced current through a resistor - with variable resistor and two coils Introductory Physics Homework 15