Help My Brains Switched Off Simple calculation

[rmn_tech]

Hi I'm Having a Bad day.

OK, Probably simple but my memory is not what it was. I'm Trying to calculate the value of the series resistor for some IR LED's.

Now I know the formula
Vs-Vf/If = R

My problem is I can't remember the value of If eg is it in Amps or mA

Vs = 12v
Vf = 1.4v
If = 100mA

I know this is stupid but I just can't get it today (senility setting in)

All help appreciated

 

[audioguru]

100mA is 0.1A. Then calculate the resistor's value:

1) 12V - 1.4V= 10.6V.
2) R= 10.6V/0.1A= 106 ohms.

 

[rmn_tech]

Thank you so much It's in mA then.

Thanks again Rick.

 

[rmn_tech]

Thank you so much It's in mA then


Thanks again Rick.

Sorry it's in Amps then

 

[3v0]

Should you ever forget again you can figure it out.

If you do the math using mA instead of amps you end up with a .1 ohm resistor which is so small the answer is obviously wrong.

Using amps you end up with 106 ohms which is in the range of what you would expect.

 

[Garibaldi]

Or, you can let one of these calculate for you



**broken link removed**

 

[philba]

Perhaps the best way to remember is that milliAmp isn't the basic unit - Amp is. It's the same way when calculating low pass frequency - don't use microFs, use Fs. and so on... For most electronics equations, you use the basic unit.

 

[Hero999]

Another thing is you'll soon learn to estimate what values you'd expect from a calculation, e.g. if the resistor value worked out to be 1mhm: or 100k then you'd be reasonably sure it's wrong.

 

[rmn_tech]

Thankyou All

Thankyou all for your replies.