GideonTech.com Forums

  • May 31, 2020, 02:22:28 AM
  • Welcome, Guest
Please login or register.

Login with username, password and session length
Pages: [1]   Go Down

Author Topic: Double Check my Led Calcs Please  (Read 1574 times)

fuware

  • Senior Modder
  • ***
  • Karma: +0/-0
  • Offline Offline
  • Posts: 913
Double Check my Led Calcs Please
« on: September 20, 2003, 08:38:01 AM »

I'm going to hook 2 red 3mm 1.7v 20ma 6000mcds to a 5v line.

According to X24's handy dandy calculator, 1 led on the line would require a 180ohm capacitor, so 2 would require a 82ohm capacitor on the line.  Right?  Just double checking that I haven't retarded this up somehow.
Logged

DurocShark

  • Senior Modder
  • ***
  • Karma: +0/-0
  • Offline Offline
  • Posts: 513
    • http://www.donimages.com
Double Check my Led Calcs Please
« Reply #1 on: September 20, 2003, 11:48:13 AM »

"180ohm capacitor, so 2 would require a 82ohm capacitor on the line"

Umm... I think you mean resistors... ;)

You could prolly do 3 LEDs in a series and not mess with the resistors at all.  3x1.7=5.1
Logged

fuware

  • Senior Modder
  • ***
  • Karma: +0/-0
  • Offline Offline
  • Posts: 913
Double Check my Led Calcs Please
« Reply #2 on: September 20, 2003, 12:56:46 PM »

woopsy daisy, my mistake.  Yes Resistor.
Logged

Skylined

  • Moderator
  • Elite Modder
  • *****
  • Karma: +4/-2
  • Offline Offline
  • Posts: 1,842
    • mtb.com.uy : Mountain Bike Uruguay
Double Check my Led Calcs Please
« Reply #3 on: September 20, 2003, 01:07:14 PM »

fuware: it's right, connect them in series and put an 82ohm resistor.
Logged

PetaHertz

  • GT Fanatic
  • **
  • Karma: +0/-0
  • Offline Offline
  • Posts: 73
Double Check my Led Calcs Please
« Reply #4 on: September 20, 2003, 01:16:14 PM »

Ummm.... for DurocShark, I think you need a resistor in there anyways.  LEDs have negligable resistance, which means if you put any current to it more than what it's rated for, you'll burn it out.  And just having an LED in front of it in series, isn't going to change that.

I think what you need is a 250 kohm resistor in series with those LEDs.  It's not the voltage that kills LEDs, it's current.

If I'm wrong, enlighten me.
Logged

viridius

  • Moderator
  • Elite Modder
  • *****
  • Karma: +15/-0
  • Offline Offline
  • Posts: 4,258
  • You know what I mean.
    • http://www.getyourlanon.com
Double Check my Led Calcs Please
« Reply #5 on: September 20, 2003, 05:58:48 PM »

No, you're right.  LEDs are diodes, which means that they will act as a short in one direction.  They also exhibit a thermal runaway effect.  This means that even if you hook up the LEDs to the 5 volt rail so that their current drop is more than 5 volts, any fluctuation on the rail (from a HD spinning down or a fan turning off) can be enough to overdrive the LEDs, causing them to overheat, burn out, and trip your PSU's overcurrent protection.  There may also be some damage to other components.  To recap, using LEDs without a resistor may seem like a good idea, but there is absolutely nothing protecting them from shorting.
Logged
"I'm totally going to send ninjas after you." - BalefireX
Pages: [1]   Go Up
 

Page created in 0.062 seconds with 34 queries.