Previous in Forum: High Power LEDs , Junction Temperature   Next in Forum: All Schooled Up and Nowhere to Go
Close

Comments Format:






Change Mode
Close

Subscribe to Discussion:

CR4 allows you to "subscribe" to a discussion
so that you can be notified of new comments to
the discussion via email.

Close

Rating Vote:







Add Vote
12 comments
Rate Comments: Nested
fifahaywire
Active Contributor

Join Date: Nov 2011
Posts: 20

LED Driver

11/29/2011 8:12 AM

Iello. I'm currently carrying out a project where I am suppose to implement an LED driver.

So, basically my circuit starts off with a rectifier, and then to a RC filter and then through a buck converter. The output is a series of power LEDs with voltage drop of 3.5V and rated forward current of 350mA for each LED (I will be using 5 LEDs). Also it is a 1.4W LED.

I'm currently having problems with finding the internal resistance of the LEDs as I need to model it in Pspice.

Vo = IRin + 5xVf

Where, Vo is the output voltage of buck converter and Vf is the forward voltage of LED.

Or should i use P=V^2/R = I^2R to get the resistance I need?

I'm still new and this is my first project.

Thanks :)
Register to Reply
Pathfinder Tags: LED driver power LED
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Good Answers:

These comments received enough positive votes to make them "good answers".
PWSlack
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 32175
Good Answers: 839
#1

Re: LED Driver

11/29/2011 8:32 AM

If LEDs had a fixed forward resistance then they wouldn't be LEDs; they would be LERs.

They are diodes. They pass whatever current is presented to them subject to sustaining the forward voltage drop - provided that the forward current is not large enough to burn them out. That is why it is usual to introduce a series resistance, or failing that run them only from a high impedance supply.

__________________
 "Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Register to Reply
fifahaywire
Active Contributor

Join Date: Nov 2011
Posts: 20
#2
In reply to #1

Re: LED Driver

11/29/2011 8:50 AM

Thanks PWSlack,

i understand that now :)

but is the resistance of a random value?

i'll try to simulate it out with 50 ohms then.

thanks again.
Register to Reply
PWSlack
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 32175
Good Answers: 839
#4
In reply to #2

Re: LED Driver

11/29/2011 8:53 AM

The apparent resistance is a function of the forward current, which could be anything from zero to the maximum rating of the device. Without knowing what the supply is limited to providing, plucking resistance values out of cyberspace, or anywhere else for that matter, is both nonsensical and abstruse.

__________________
 "Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Register to Reply
fifahaywire
Active Contributor

Join Date: Nov 2011
Posts: 20
#5
In reply to #4

Re: LED Driver

11/29/2011 9:07 AM

PWSlack,

i was also researching while posting here. i found this regarding dynamic resistance. http://www.national.com/appinfo/webench/led/Rd.pdf

this is what you mean, right?

i'm sorry for taking your time.

thanks!

Register to Reply
fifahaywire
Active Contributor

Join Date: Nov 2011
Posts: 20
#3

Re: LED Driver

11/29/2011 8:53 AM

Hi SolarEagle,

thank you for the fast reply. i'm checking out the website now.

thanks a lot
Register to Reply
Usbport
Guru
Engineering Fields - Optical Engineering - Member Engineering Fields - Engineering Physics - Member Engineering Fields - Systems Engineering - Member

Join Date: Apr 2010
Location: Trantor
Posts: 5363
Good Answers: 647
#6

Re: LED Driver

11/29/2011 11:24 AM

In my experience, you can apply Ohm's law as a first approximation when the LEDs are operated per the manufacturer's recommendations.

V = I x R; Since V = 3.5 volts and I = 0.35 amps, R must equal 10 Ohms.

__________________
 Whiskey, women -- and astrophysics. Because sometimes a problem can't be solved with just whiskey and women.
Register to Reply
PWSlack
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 32175
Good Answers: 839
#7
In reply to #6

Re: LED Driver

11/29/2011 11:35 AM

...and if the forward current is 0.1 amp instead, then R must equal 35 ohms. The point is that they are LEDs, not LERs; the apparent resistance varies with the forward current QED.

__________________
 "Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Register to Reply
Randall
Guru

Join Date: Aug 2005
Location: Hemel Hempstead, UK
Posts: 5826
Good Answers: 322
#8

Re: LED Driver

11/29/2011 3:36 PM

For once I disagree slightly with PWSlack. Look at this VF against IF graph for the first diode I found a spec. for:-

Just look at the typical 25° C curve for now. The dynamic resistance of the diode is the slope of the curve around the area that you want to use it.

Not too accurately: from 250 mA to 450 mA the Voltage rises by about 0.2 V so the dynamic resistance is 0.2/0.2= 1 Ohm (that seems like too much of a coincidence). Extend the tangent to the curve around the area you want to use it to the x axis and you get 0.8V

So for your model V=0.8 + I*1

__________________
 If you spend all your time looking for people and things to complain about: trust me, you will find plenty to complain about.
Register to Reply
PWSlack
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 32175
Good Answers: 839
#9
In reply to #8

Re: LED Driver

11/30/2011 4:31 AM

Respect.

__________________
 "Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Register to Reply
3
Winfield Hill
Power-User

Join Date: May 2010
Location: Cambridge, MA
Posts: 390
Good Answers: 82
#10
In reply to #8

Re: LED Driver

11/30/2011 8:41 AM

Right idea, wrong part. When fifahaywire says 3.5 volts, that tells us it's a white LED he's using. These use far-blue LEDs to excite a white phosphor, and physics tells us they need nearly 3-volts to create the energetic blue photons and operate.

Philips Luxeon is a big supplier, and I grabbed an I-V plot from one of their 350mA LEDs (right). These LEDs show a dynamic resistance of 1.1 ohms, up to about 400mA.

rs = dV/dI = (3.0-2.8) / (390-210mA) = 1.1Ω

Just above 400mA the plot shows the dynamic resistance beginning to decrease, to perhaps 0.7 ohms. That matches the datasheet spec of 3.2 volts for 700mA pulsing. (But another plot shows that doubling the current only increases the light by about 1.7x.)

If we extend the 100-to-350mA portion of the line down to 0V, it crosses at about 2.55V. So the dynamic I-V formula, for ID from 100 to 400 mA, would be

VF = 2.55 + 1.1 ID

The datasheet tells us the LED also has a -2 to -4 mV/°C temperature coefficient.

__________________
 Thanks, Win
Register to Reply Good Answer (Score 3)
No more Good Answers.
fifahaywire
Active Contributor

Join Date: Nov 2011
Posts: 20
#11
In reply to #10

Re: LED Driver

11/30/2011 9:22 AM

thank you, guys for the explanation. it now seems clear to me already :)

Winfield_Hill, yes you're right. i'm using a white LED.

http://uk.farnell.com/hlll/t21d1/led-pwr350-to220-daylight-white/dp/1226709

but the one im going to use doesnt have IV characteristic, so i can't really calculate the dynamic resistance. but thanks for the detailed explanation

Register to Reply
Winfield Hill
Power-User

Join Date: May 2010
Location: Cambridge, MA
Posts: 390
Good Answers: 82
#12
In reply to #11

Re: LED Driver

11/30/2011 9:34 AM

Your T21 LEDs are probably similar, maybe a little lower resistance. I like the TO-220 package.

Philips make a series of LEDs in sort of TO-247 packages, that are convenient. They're rather serious 14-watt LEDs, p/n LAFL (link). They're meant for automotive "forward lighting". Ahem!

__________________
 Thanks, Win
Register to Reply
Register to Reply
Have a Different Question?
12 comments

Good Answers:

These comments received enough positive votes to make them "good answers".
Copy to Clipboard

Users who posted comments:

fifahaywire (4); PWSlack (4); Randall (1); Usbport (1); Winfield Hill (2)

Previous in Forum: High Power LEDs , Junction Temperature   Next in Forum: All Schooled Up and Nowhere to Go

Advertisement

Got Something to Say?
Ask a Question
Start a Discussion

Search this Forum
Electrical Engineering:
Go

CR4 Sections
Aerospace
BioMech & BioMed
Chemical & Material Science
Civil Engineering
Communications & Electronics
Education and Engineering Careers
Electrical Engineering
General Discussion
Instrumentation
Manufacturing
Mechanical Engineering
New Technologies & Research
Software & Programming
Sustainable Engineering
Transportation

Site Directories
Forum Directory
Blog Directory
User Group Directory
RSS Feeds

CR4 Rules of Conduct
CR4 FAQ
Site Glossary

Who's Online (151 right now)
Show Members
Advertisement