470 Ohms.

Yoshti

Hi all,
A really stupid question:
My son was looking in my "What's a microcontroller" and at page 43, Lights on lights off, He sees a 470 Ohms resistor with a LED. Even when using a Stamp pin (Page 48) same thing 470.
In nowhere in the book it explains how did they get that number and why.
To my nooby look, I don't even know myself...

So how did 470 came up?

Cheers
Yoshti

Tubular

The resistor values are spaced 'logarithmically' within each decade.

The E12 series breaks say the 100ohm to 1000 ohm space into 12 logarithmically even gaps, and 470 happens to be close to 8/12 of the way through that log space.

I have a female cousin who looks at resistors for their jewellery potential, and cannot understand why they didn't decide on values based on the colours instead

Here's a link that explains it well
http://services.eng.uts.edu.au/pmcl/ae/Downloads/Lecture04.pdf

Kevin Wood

There is a section in the WaM book that describes Ohm's law. Basically, the resistor acts to prevent too much current from flowing through the circuit. By using Ohm's law, and knowing a couple of the variables, you can calculate how much resistance to use in a circuit to reach a certain maximum limit on the current.

That said, why they chose 470 Ohm instead of something smaller or larger is a question that only the manual authors can answer. It's probably a value that provides an optimum balance of circuit protection and led brightness. Or it could just be that 470 Ohm resistors were on sale when they bought them.

LoopyByteloose

Ohm's Law is how the calculation of proper size is arrived at. But the purpose is to limit the current. E= I x R so 5v/470 ohms = about 1/100th of an amp.

This leads to why limit the LED to 1/100th of an amp?

The reason is destructive heat from a combination too much current and a given voltage. You can run an LED at higher current and/or voltage, but too much higher and it burns up eventually. We all know about burnt out light bulbs, don't we? With the right limit to current an LED for a certain regulated voltage can last much longer, maybe forever.

Also consider that the amount of destructive heat is related to a number, Watts - which indicates how much power is being used. P = I x V And so 5 volts times .01 amps (10 milliamps) equals .05watts or 50 milliwatts.

So you see, an LED is a 50 milliwatt light bulb and needs to be protected from too much power - voltage and resistor size can vary, but one must limit the watts to protect the device.

And of course, you can ask how many 50 milliwatt light bulbs does it take to make one 50 watt light bulb? This will give your boy an appreciation for both math and the LED's ability.

Yoshti

Hi,
Well, to me, taking into account the forward voltage at 1.5v for the LED.
5-1.5 = 3.5 v
Lets put the Amps at 20mA
So, my R is 3.5 /.02 = 175
Looks like I'm all wrong...
Even at I=3.5/470 =.007
What the heck??? 7mA ?

Need to go back to square one... makes no sense...

Cheers
Yosh

PJAllen

3.5V / 20mA
3.5 / 0.02
175 Ω

Yoshti

YEah I saw it only after, was corrected ...

Yoshti

PJ Allen,
Do you have an idea idea, why they used 470 instead of 175?
All I can think of, is to OVER protect the STAMP PIN. Would that be a fair statement?

Cheers
Yosh

schill

Reasons for using a larger resistor:

1) The led won't be as bright. Some LEDs can be blinding/very distracting.
2) The led will probably live longer the further you are from driving it on the edge.
3) I often just use a 470 ohm (or 330 ohm) resistor when I'm doing something quick/temporary without doing any calculations (or even looking up the specs of the led) because I know that the led will probably be ok with this value.

Yoshti

schill,
Great, thanks...
Its all I wanted to know.
I'll use this , as my rule of thumb.

Cheers
Yosh

LoopyByteloose

My reasoning behind all that talk of watts is that this is a good opportunity to introduce a child to the core maths of electronics.

Regarding using 470ohm as a fall back value,
I find that if I have a blinking light, it is often hard to see with older LEDs that demand more power.

Blinking brings into play the whole concept of 'duty cycle' and 'averages of voltage, current, and power'',

For 5 volts, my usual starting point for LEDs is 330 ohm, and for 3.3 volts it is 220 ohms.

I still think it is very important to get started early with presenting Watts - some people never grasp their importance and become very frustrated with their inability to build anything stable.

schill

Loopy Byteloose wrote: »

My reasoning behind all that talk of watts is that this is a good opportunity to introduce a child to the core maths of electronics.

I agree wholeheartedly with this idea. I want to emphasize that when I say I use 470 or 330 ohm it's for convenience and based on experience but it's not the best approach to take.

Between things I've made and purchased, I have so many leds on different devices that I don't have to turn on my lights . I don't know if I need to keep them dim to prevent lighting up the room or make them bright so they stand out amongst all the other leds. I don't know who decided that TVs needed an led that lights up to let you know they are off (ok, they're letting you know they are in standby but still...) - maybe it's the people who sell black electrical tape.

yarisboy

I use ohms law to arrive at that first/best guess but at the breadboard after I have a stable power supply I also measure the voltage across the LED and the milliamps through it. To the eye some colors of the super-bright colors glare more than others so for a four color display assembly I'm adjusting the power in the different colors downward from maximum in order to get the appearence of uniform relative intensity. My resistances are aways a little higher than the minimum and I can come up with a reasonable set of current limiters using standard values. I don't know the exact value in Db of a just-noticible-difference for the human eye but the scale is fairly forgiving. Here in Arizona in direct sun light I have to take them up the power curve to make a really good visability. Conversly for night driving I have to tone them down. The diferences in intensity become more apparent as the driving voltages are dropped. I'd have to write a driver object with four different PWMs running to totally linearize my display. Each PWM would need it's own offset and gain parameters. In general red and yellow use the lower driving voltage and blue and green need more voltage to get there.