Topics Topics Edit Profile Profile Help/Instructions Help Member List Member List  
Search Last 1|3|7 Days Search Search Tree View Tree View  

Shed some light please!!!

:: EPE Chat Zone ≠:: ≠Radio Bygones Message Board :: » EPE Forum Archives 2007-2009 » Archive through 05 July, 2008 » Shed some light please!!! « Previous Next »

  Thread Last Poster Posts Pages Last Post
  ClosedClosed: New threads not accepted on this page        

Author Message
Top of pagePrevious messageNext messageBottom of page Link to this message

techno
Frequent Contributor
Username: techno

Post Number: 63
Registered: 07-2005

Rating: N/A
Votes: 0 (Vote!)

Posted on Thursday, 26 June, 2008 - 08:16 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

hi guys iam connecting 10 white leds in parallel i want to use a 24 v dc supply .what value resistor must i use so that i can get the leds as bright as possible and i wont damage them thanks techno
Top of pagePrevious messageNext messageBottom of page Link to this message

neroku
Regular Contributor
Username: neroku

Post Number: 32
Registered: 09-2007


Rating: N/A
Votes: 0 (Vote!)

Posted on Thursday, 26 June, 2008 - 08:30 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Don't connect 10 leds in parallel, it won't be efficient, this is a waste of power.

If you can't reach the voltage needed to connect them in series with a 24v power supply, just connect 5 leds in series and put it in parallel with another 5 leds in series.
Top of pagePrevious messageNext messageBottom of page Link to this message

grab
Frequent Contributor
Username: grab

Post Number: 762
Registered: 05-2005


Rating: N/A
Votes: 0 (Vote!)

Posted on Thursday, 26 June, 2008 - 10:11 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Techno, we can't answer that without knowing what the forward voltage of the LEDs is - check your LED's datasheet. After that, the LED datasheet may also give a max current. Resistor value is then

(24 - forward voltage) / max current

You'd need one resistor per LED for this. If you're putting all 10 in parallel, the resistor value would be

(24 - forward voltage) / (10 * max current)

And as Neroku says too, this is less efficient than putting several in series. Power supplies aren't always 100% on their rated voltages though, and nor are the LED forward voltages, so you usually want to leave at least 2V difference between the total forward voltage for the LEDs in series and the supply voltage. This means your LEDs would need to have a forward voltage drop of at most 4.4V if you want to put five in series.

If you do put five in series, and another five in parallel with them, the resistor you need will be

(24 - (forward voltage * 5)) / (max current * 2)

Graham.
Top of pagePrevious messageNext messageBottom of page Link to this message

stylers
Frequent Contributor
Username: stylers

Post Number: 115
Registered: 04-2005

Rating: N/A
Votes: 0 (Vote!)

Posted on Thursday, 26 June, 2008 - 10:54 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

you would be better to connect 5 of them in series, and use two strings. to parallel them all will mean you will need approx 3 V @ 500mA (assuming 50 mA per LED, which is the norm for bright white LED's). thats a lot of current at low voltage, so a dropper resistor for 24V would need to be low value and huge size to dissipate the wasted power. so instead, use 2 strings of 5 led's in series with a separate resistor for each string. now for the maths bit (simple ohms law stuff)..
3V * 5 leds = 15V, 24V supply - 15V = 9V drop, 9V / 0.05 A = 180 ohms. power = 9V * 0.05A = approx 0.5 Watts, so use a 180 ohm 0.5 W resistor for each string (they will get hot though, so maybe better to use 1 watt resistors).

if all the leds were being used in parallel, the only practical way to supply them from 24V would be to use a switching regulator, and there are many available nowadays specifically for driving big LED's
Top of pagePrevious messageNext messageBottom of page Link to this message

techno
Frequent Contributor
Username: techno

Post Number: 68
Registered: 07-2005

Rating: N/A
Votes: 0 (Vote!)

Posted on Thursday, 26 June, 2008 - 11:35 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

thanks alot for the info stylers much appreciated
Top of pagePrevious messageNext messageBottom of page Link to this message

grab
Frequent Contributor
Username: grab

Post Number: 764
Registered: 05-2005


Rating: N/A
Votes: 0 (Vote!)

Posted on Friday, 27 June, 2008 - 10:38 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Techno, do check that forward voltage though. Newer white and blue LEDs might be better than they used to be, but I do remember having seen blue LEDs with forward voltages not much below 5V. If the forward voltage is more than 4.4V, a string of 5 in series ain't gonna work.

Graham.
Top of pagePrevious messageNext messageBottom of page Link to this message

paul_goodson
Frequent Contributor
Username: paul_goodson

Post Number: 850
Registered: 05-2005


Rating: N/A
Votes: 0 (Vote!)

Posted on Monday, 30 June, 2008 - 08:52 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

You need to find out what forward voltage (FV) and current you need to run the leds at!

The forward voltage would also depend on how much current you are running them at and the brightness required!!!

The data sheet will often refer to the light output and FV at a given amount of current

For a white led I would expect the FV to around a little under 4 volts at say 20 ma
But remember this can differ a good deal.

So I would probably run say 10 leds in 2 serial strings (one resistor for each string) that are intern in parallel

The formula is simple say you wonted them to run at a FV of 3.8V at 20 ma (0.02amp)

24v divided by 3.8V =6.3 ledís maximum in a string
As you will need 2 strings anyway then I would simply only have 5 in each string
The total FV in each string is then 5 x 3.8V =19V
So 24V-19V =5V divided by the amount of current needed 0.02 amp =250R
You will obviously need to use the nearest preferred resistor

Try the link below in schematic form then scroll down for different configurations

http://led.linear1.org/led.wiz
The bluntest pencil is better than the sharpest memory!!!!!!!

Administration Administration Log Out Log Out   Previous Page Previous Page Next Page Next Page