LED resistor Calculators...Am I doing it right?

[Hiwatt25]

Hey-

Just hoping for a head check to make sure I'm using the LED resistor calculators correctly.  Take for example an LED I bought at the Shack.  The package its in says "1.7V-40Ma-0.8mcd...

What is MCD?
Is the 1.7V the voltage drop across the resistor?
And is the 40Ma the current it is designed to run at?
Finally, if I'm using a battery or wall power the supply Voltage should be 9V correct?

Just want to make sure I'm putting the correct values into the calculator so that I get something meaningful out of it.

Thanks in advance.

[LyleCaldwell]

It's all here:  http://www.superbrightleds.com/led_info.htm

[SuperGeo]

 mcd is mili candelas = luminosity strengh of the LED

yes I think 40mA is the current it's designet to run at.

the voltage depends of your wall wart and battery you're using to power the led. If it's an 9v battery the voltage will be 9v.

[Hiwatt25]

Thanks for the link and the response.  Felt like I was on the right track but just wanted to be sure.

Cheers!

[Peter Snowberg]

40mA is a lot of current for an average LED. If that figure in the absolute maximum area? Do you really need 40mA?

To get 40mA through a 1.7V diode with 9.00V for a supply (check your actual voltage), the resistor needed would be 182.5 ohms. I would jump up to the next 5% value which is 220 ohms but that will set the current at about 33.5mA.

If this is for an indicator, you can use drastically less current. Try a 4.7K and vary from there.

[R.G.]

Just hoping for a head check to make sure I'm using the LED resistor calculators correctly.

Using packaged calculators damages your ability to understand what is happening. It's a bad idea until you can already do the math yourself and understand what is happening.

Take for example an LED I bought at the Shack.  The package its in says "1.7V-40Ma-0.8mcd...
What is MCD?

As noticed, measurement of the brightness of the LED at full current. I believer it's actually millicandelas. A candela is the brightness of an average (!?) candle on one square foot of area at some reference distance when measured on the night of a new moon in winter... or something like that. It's a measure of brightness. LEDs are available with thousands of millicandelas.

Is the 1.7V the voltage drop across the resistor?

No, it's the voltage drop across the LED - they have no idea what resistor you'll use it with, so the specs describe the LED.

And is the 40Ma the current it is designed to run at?

40ma is the MAXIMUM current you can run it with and have it live. You may want to have it run at less - except that 0.8mcd is an exceptionally un-bright LED so you'll maybe need it all. Go over 40ma average and it may burn out.

Finally, if I'm using a battery or wall power the supply Voltage should be 9V correct?

It depends entirely on what the battery voltage or wall power voltage is.


Here's the scoop. LED calculations are dead simple. You have a voltage source (battery or DC source),  resistor, and an LED. The voltage source has some voltage. You want the LED to have some designated current that you choose. LEDs have a forward voltage that doesn't change much with all kinds of currents.

That's where the resistor comes in. The resistor is the only thing in this setup that limits current. If you have a 9V source and a 1.7V LED, then the voltage across the resistor has to be what's left - 7.3V. So you can pick any current you want by making the resistor be R = 7.3V/(desired current). If you want 10 ma, then bango, R = 7.3/0.01 =7300 ohms. If you want 20 may, then R = 7.3 / (0.02) = 3650 ohms.

In all cases, R = (available supply voltage - LED voltage) divided by the current you want. And you don't even need a network connection to do that.

[LyleCaldwell]

One other downside to the calculators is they tell you what the maximum safe voltage to use is.  But that's usually too bright and will drain a battery fast.

[$uperpuma]

I learn something everytime I come here....

[Peter Snowberg]

Everyone should have these 12 memorized: 



Print it out now and paste it to your eyelids.

[Satch12879]

Everyone should have these 12 memorized: 



Print it out now and paste it to your eyelids.

+1

[Hiwatt25]

40ma is the MAXIMUM current you can run it with and have it live. You may want to have it run at less - except that 0.8mcd is an exceptionally un-bright LED so you'll maybe need it all. Go over 40ma average and it may burn out.

That explains why I destroyed two of them while trying to find the appropriate resistance value.  I hooked a pot up in series with a 9v supply and dialed it in till it was nice and bright...only thing is, it didn't stay bright for too long.  Sigh, at least I didn't hurt myself or burn anything down. 

As for Ohm's law, I am right there with you.  I was just a little fuzzy on the terminology that the calculator I was using employed.  Specifically, I didn't know if "voltage drop" and "forward voltage" were considered the same thing.  I stand in bewildered awe of Mr. Ohm.   

Thanks again all, I too learn something every time I come here.  Just last week I learned I actually like asparagus.

[$uperpuma]

Everyone should have these 12 memorized: 



Print it out now and paste it to your eyelids.

As a Mobile Electronics Installer...I had that nailed into my forehead with a 16 penny nail... invaluable