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### Topic: Resistor (Read 903 times)previous topic - next topic

#### abbyo

##### Jun 27, 2012, 08:25 pm
hello,

Please how do you determine a resistor value to be used for a particular project

Right now I'm doing the Interactive Traffic Light project

Thanks!

#### jackrae

#1
##### Jun 27, 2012, 09:25 pmLast Edit: Jun 27, 2012, 09:32 pm by jackrae Reason: 1
You determine the voltage drop that you want across the resistor and the current flowing through it.  From that you calculate its value from the equation R = V / I

For example, say you have a 12 volt supply to power an LED that requires 25mA and has a forward voltage of 1.8volts.  By using a resitopr in series with the LED you can set the correct operating parameters.

Since the resistor is in series with the LED, the same 25mA flows through it.  The resistor is required to develop 10.2 volts across itself
(12 - 1.

So R =  10.2 / 0.025  = 408 ohms.    For general applications one chooses a resistor value from a standard selection range.  In this case our choice is 390 ohms or 470 ohms.    390 would entail more current flowing through the LED, which may damage it.  One cannot determine exactly how much more since the LED is a non-linear device.  So one would select the more conservative 470 ohm.  This would mean less current flowing so the LED would be somewhat (slightly) dimmer.

Resistor power rating, based on the original calculated value is W = V x I  which is 10.2 x 0.025 = 0.255 watts.   Since we are using a larger value value which will mean a reduced current it is reasonable to use this vale for component selection.   However, one should run components at lower than their rated value (for enhanced life) so either a 1/3rd or 1/5 watt unit should be chosen.

#### jackrae

#2
##### Jun 27, 2012, 09:33 pm
Bl**dy predictive text makes (that's an 8 followed by a closed bracket) into a smiley !!!

B**ger me, there it goes again !!

#### Docedison

#3
##### Jun 27, 2012, 11:42 pm
According to the math in the previous response 390 ohms would be a 4.4% increase... 470/408 = 1.15 or a 15% decrease in current. It is my not so humble opinion that the 390 ohm resistor would be a better choice as it would increase the current by 1.1 mA, the 470 ohm resistor would decrease the current by 3.75 mA...

Doc
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

#### JoeN

#4
##### Jun 27, 2012, 11:45 pm

Bl**dy predictive text makes (that's an 8 followed by a closed bracket) into a smiley !!!

B**ger me, there it goes again !!

So why not click Modify on your message and put a space between the 8 and the parens?

8 )
I will never ask you to do anything that I wouldn't do myself.

#### JoeN

#5
##### Jun 27, 2012, 11:46 pm

According to the math in the previous response 390 ohms would be a 4.4% increase... 470/408 = 1.15 or a 15% decrease in current. It is my not so humble opinion that the 390 ohm resistor would be a better choice as it would increase the current by 1.1 mA, the 470 ohm resistor would decrease the current by 3.75 mA...

Doc

You think like I do.  As long as LEDs are ten cents and not ten dollars I say WTF (and buy them from the discount Thai place and they are a penny).
I will never ask you to do anything that I wouldn't do myself.

#### jackrae

#6
##### Jun 28, 2012, 09:59 am
The original question came from someone who seemingly has no knowledge on the subject.  As a responsible tutor (well some of the time) I was detailing the cautious approach, rather than the "WTF" approach.  Sensible specification within reason for the newbies - do what you like once you've learned the game.

#7
R = V / I

#### dkl65

#8
##### Jun 28, 2012, 09:56 pmLast Edit: Jun 28, 2012, 10:00 pm by dkl65 Reason: 1
@abbyo: didn't you see the thread called "Resistors", talking about the same thing by SamKirkiles?

Ohm's Law

P.S. Before I knew the name "Ohm's law", I called it "Elements of Electricity".

#### jaredpi

#9
##### Jun 29, 2012, 01:28 am
I assume the resistors are for LEDs?Here is a formula:
R = (VS - VL) / I
VS = supply voltage
VL = LED voltage
I = LED current (make sure to convert mA to Amps(if needed))
or a calculator: http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator

#### Docedison

#10
##### Jun 29, 2012, 01:42 am
I learned it as ohms law and yes I Did Bypass the tradition a little bit by deliberately not taking the conservative approach but I thought the error calculations would show my thinking as the error is somewhat smaller than the error term in the Voltage regulator Specification and therefore not worth much consideration beyond estimating tha magnitude of the error. So as to have an idea of whether I am "In The Ball Park". Expressing the calculations as an absolute is good training but one must also talk of the Relative Errors VS Absolute Errors least one get thoroughly trapped in the esoterica. My single intent was simply to lighten up the details a little and make the concept a little more acceptable. In some area's of Electronics Absolute precision is required in many area's of electronics but unless one is a complete Dunce and totally unwilling to read the data sheets and ask questions where understanding is not forthright and clear, success will come with perseverance like the OP did in asking a question about something he didn't understand completely. I just re-read the post you made in response I am NOT criticizing your post it was Very correct as far as it went but it wasn't a real learning experience in that you imply that dangerous things might happen I Quote from your post...
Quote
One cannot determine exactly how much more since the LED is a non-linear device.  So one would select the more conservative 470 ohm.  This would mean less current flowing so the LED would be somewhat (slightly) dimmer.
That statement is totally incorrect and the sole reason why I responded. The Wise (IMNSHO) would have been to point out that a LED is a current operated device and a look at a Data sheet on a GENERIC LED would have shown the truth which IMO is that a 5 to 10% change in component values would MAKE NO DIFFERENCE whatsoever in the operation of the device and further had you mentioned the data sheet first your lesson would have taken on real dimension in TEACHING the subject. I was on my way out to the store and posted hastily without the description of my thoughts at the time and for that I apologize for I did essentially the same mistake as you did. There are Absolutes on ALL parts but they are better learned from the data sheets. In my not so humble opinion you should have shown him the simple math lesson lesson about power dissipation rather than the comment on conservationism... which leaves the OP in the dark as to the real reason for the resistor. in the first place.

IMO

Doc
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

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