Communities

Writing
Writing
Codidact Meta
Codidact Meta
The Great Outdoors
The Great Outdoors
Photography & Video
Photography & Video
Scientific Speculation
Scientific Speculation
Cooking
Cooking
Electrical Engineering
Electrical Engineering
Judaism
Judaism
Languages & Linguistics
Languages & Linguistics
Software Development
Software Development
Mathematics
Mathematics
Christianity
Christianity
Code Golf
Code Golf
Music
Music
Physics
Physics
Linux Systems
Linux Systems
Power Users
Power Users
Tabletop RPGs
Tabletop RPGs
Community Proposals
Community Proposals
tag:snake search within a tag
answers:0 unanswered questions
user:xxxx search by author id
score:0.5 posts with 0.5+ score
"snake oil" exact phrase
votes:4 posts with 4+ votes
created:<1w created < 1 week ago
post_type:xxxx type of post
Search help
Notifications
Mark all as read See all your notifications »
Q&A

Comments on Select resistor for a diode

Parent

Select resistor for a diode

+2
−0

Suppose we have this circuit

see me

How do I pick a appropriate value for the resistor R1?

History
Why does this post require attention from curators or moderators?
You might want to add some details to your flag.
Why should this post be closed?

0 comment threads

Post
+3
−0

You start by checking the forward voltage and rated current for the LED. 20mA is the usual industry standard for regular LEDs and the forward voltage will be specified for that rated current.

In this case the datasheet says VF = 2.0V typical @ 20mA.

So now we know the current and the voltage will be supply voltage minus forward voltage. Applying Ohm's Law, we get:

R = (Vsupply - Vforward) / I

In this case R1 = (9 - 2) / (20*10-3) = 350 ohm.

History
Why does this post require attention from curators or moderators?
You might want to add some details to your flag.

1 comment thread

Use max forward voltage, not typical. (4 comments)
Use max forward voltage, not typical.
Olin Lathrop‭ wrote over 3 years ago

The general form of this answer is correct, +1. However, you shouldn't be using the typical forward voltage drop, but the worst case (highest) for the current you want. The resulting calculation that you show than results in the minimum resistance to not exceed the maximum current rating of the LED.

Lundin‭ wrote over 3 years ago

Olin Lathrop‭ Wouldn't the worst case be the minimum voltage drop, not the maximum? The part didn't list a minimum so I used the typical value. It's better that a LED shines ever so slightly dimmer than supplying it with too high current. Also as I'm sure you know, In practice every modern LED out there can do with significantly lower current than 20mA which is the value for which they guarantee a particular luminous intensity. So if one uses bright modern LEDs with some >300mcd @ 20mA, they can be supplied with just a few mA and still shine like the sun.

Lundin‭ wrote over 3 years ago

Btw depending on what battery chemistry this is, then 9V fully charged might mean 10-11V. And down to somewhere around 7V when low. Obviously it isn't a good a idea to drive a LED directly from a raw battery voltage.

Olin Lathrop‭ wrote over 3 years ago

Oops. Yes, I meant to say "minimum".