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 »
Users Search
Help Dashboard Sign In Sign Up
Q&A Papers Meta
Q&A
Posts Tags Edits
Ask Question

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?

LED-driver
posted over 3 years ago
1y ago by Lorenzo Donati‭
user card
MissMulan‭
73 reputation 58 11 -52 61
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
+0
−2

A diode obeys the Shockley diode equation and for this circuit we have:hi

hi

The current through the diode must not pass a limit so for LTL-307EE we go to the datasheet to find the maximum current before the diode is destroyed:

For the LTL-307EE at 20C we have 10^4 A Isat and Vth=0.026V

hi

hi

(30mA).

Now in order to find the resistance we find the resistance of the resistor if max current was flowing through the diode

hi

If we solve this equation we get:

hi So in order for LTL-307EE to not be damaged the resistance of R1 > 295.05Ω

Hope this helps

posted over 3 years ago
3y ago
user card
MissMulan‭
73 reputation 58 11 -52 61
History
Why does this post require attention from curators or moderators?
You might want to add some details to your flag.

2 comment threads

Don't assume you know the voltage to current function (1 comment)
Don't pick a value based on absolute maximum ratings. (3 comments)
Don't assume you know the voltage to current function
Olin Lathrop‭ wrote over 3 years ago
copy link

The equation you use that implies the LED current as a function of the voltage across it is stated nowhere in the datasheet. You should only go by what it says in the datasheet. Also 5 digits is absurd in this case, especially due to the high temperature dependence of LEDs in general. "295.05 Ω" is actually wrong because it implies a precision that just doesn't exist here. Go back to high school math and look up "significant digits".