Previous in Forum: What Happens to Light Energy?   Next in Forum: Looking for collaborators for study...
Close
Close
Close
10 comments
Guru

Join Date: Sep 2007
Location: Reno, NV (USA)
Posts: 608
Good Answers: 66

LED Reactive Lighting Circuit

12/13/2007 1:19 PM

Short Question: What are the consequences of driving a LED at lower voltages?

Long Story: I want to design a reactive lighting circuit for an E-stop in a low-light environment. Circuit is 12V DC. I have the candidate switch (IDEC) that has the 12V rated LED installed in the button, 1 NC & 1 NO sets of contacts. I want the E-stop to glow slightly at all times (so operator can easily find in low light) but come on at full power if E-stop is pushed. Panel space is limited, and I feel this is an elegant alternative to a separate E-stop indicator light. My thought is to drive the LED through the NC contacts with a series resistor do deliver approximately 4V DC to the LED. (4V from my bench power supply seems to give the desired light output.) When the E-stop is pushed, the NO contacts would deliver the full 12V to the LED. I can easily hook a variable resistor up to my power supply and determine the resistor value to deliver 4V to the LED, but what wattage range would I need and will I be damaging the LED by driving at 4V most of the time?

Electronics was never my strong suit, and any feedback is greatly appreciated.

__________________
Aequam memento rebus in arduis servare mentem.
Register to Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".
Power-User

Join Date: Nov 2007
Location: Alberta Canada
Posts: 182
Good Answers: 9
#1

Re: LED Reactive Lighting Circuit

12/13/2007 3:55 PM

I'm a little rusty, but from what I recall as long as you have sufficient forward voltage to make the thing work (i.e. the required 'activation voltage' drop across the diode) its the current flow through it that determines the brightness, the more current the brighter it'll glow, till you reach the point that it melts!

Connect it in series with a high ohm low wattage resistor for "standby" mode, and replace this with a low ohm high wattage resistor for the "on" mode.

As for running it on a lower voltage, I don't think it will hurt it.

IPG

__________________
Vote for something useful this time, vote to repeal the second law of thermodynamics!
Register to Reply
Guru

Join Date: Sep 2007
Location: Reno, NV (USA)
Posts: 608
Good Answers: 66
#2
In reply to #1

Re: LED Reactive Lighting Circuit

12/13/2007 4:15 PM

Thanks, Zaphod!

That is what I was thinking as well. I'm presuming the switch has the current-limiting resistor for the LED internally. My research indicated that a typical red LED has a forward voltage of about 2.1V @ 20mA. Assuming I use the 4V standby value and current through a diode squares(?) with voltage, that gives me 400mA @ 4V. That gives me a voltage across my "standby" resistor of 8V and a current of 400mA. Using P=IV, my resistor will see 0.4A * 8V = 3.2 W. Up-size to 5-6 watt for service. Does that sound right? Intuitively, I was expecting a much smaller value.

__________________
Aequam memento rebus in arduis servare mentem.
Register to Reply
3
Power-User

Join Date: Nov 2007
Location: Alberta Canada
Posts: 182
Good Answers: 9
#3
In reply to #2

Re: LED Reactive Lighting Circuit

12/13/2007 5:07 PM

Hi CSM,

I think your math maybe a little off. (or perhaps mine is!) Here is what I came up with.

The votage drop across the LED will always be 2.1 volts (with no apparent resistance), so the voltage the resistor sees is actually 12-2.1 =9.9V

Assuming the current to be 20 mA (0.02A) in the 'ON' position I want to know the value of the presumed internal current limiting resistor in the switch.

V=IR therefore rearrange to get R=V/I; R=9.9/0.02; =495 ohms. The power dissapated by the resistor will be IV; =0.02 x 9.9 = 0.198 watts

You say that at 4V, you get the low level light that you want for standby.

The diode will always drop the 2.1 volts, so the resistor now 'sees' 4 -2.1 =1.9 volts

We already know the value of the resistor from above to be 495 ohms, so the current flow is now I=V/R; =1.9/495; =0.0038A or about 4mA.

Now all we have to do is figure out the resistance to produce this current from the 12V supply.

V=9.9 (cause of the diode voltage drop) I = 0.004. V/I=R 9.9/0.004 = 2,475 ohms. You already have the 495 ohms built in resistor, so if you connect it to the 12V supply with a resistor of about 2,000 ohms when it is "OFF" you should have the current you need. Of course when you turn it "ON" you now bypass the additional resistor to get the higher current.

Power rating for this resistor is IV; 0.004 x 9.9 = 0.04 watts.

The actual resistor values are not that important, just pick the closest common size (2.2kohm?) and connect in parallel with the switch.

Hope this works!

IPG

__________________
Vote for something useful this time, vote to repeal the second law of thermodynamics!
Register to Reply Good Answer (Score 3)
Guru

Join Date: Sep 2007
Location: Reno, NV (USA)
Posts: 608
Good Answers: 66
#4
In reply to #3

Re: LED Reactive Lighting Circuit

12/13/2007 5:43 PM

Thank you!

Those numbers are much more in line with the values I was expecting! My understanding of the LED / internal resistor relationship was imperfect. At least this gives me a starting point to prototype from. Thanks again for your help...I'll let you know what my results are.

__________________
Aequam memento rebus in arduis servare mentem.
Register to Reply
Power-User

Join Date: Nov 2007
Location: Alberta Canada
Posts: 182
Good Answers: 9
#5
In reply to #4

Re: LED Reactive Lighting Circuit

12/13/2007 5:49 PM

Hi CSM,

Please do, unless of course you enbd up blowing yourself up!

It was fun using those rusty braincells again!

IPG

__________________
Vote for something useful this time, vote to repeal the second law of thermodynamics!
Register to Reply
Guru

Join Date: May 2007
Location: Budapest, Hungary, HA5YAR
Posts: 617
Good Answers: 14
#7
In reply to #3

Re: LED Reactive Lighting Circuit

12/14/2007 1:38 AM

You are right.

__________________
Aged man is not old man...
Register to Reply
Guru

Join Date: Sep 2007
Location: Wrong end of the yellow brick road in Oz
Posts: 930
Good Answers: 15
#6

Re: LED Reactive Lighting Circuit

12/13/2007 11:14 PM

Don't think of LED's as Voltage driven, think of as Current driven

R=(Vcc-VLED)/ILED

Most of the LED's I have used were run under recommended current, 12-16mA instead of the recommended 20mA

So, therefore, you can have E-stop running 18-20mA and Other state being 10-12mA

Then using your "Alarm Relay" drop a resistor in parallel to the bias resistor in Alarm conditions, then, you could have that relay pulsing at 1Hz so the E-stop flashes.

__________________
Qn, Whats the differance between a Snake and a Onion? Ans, No one cries when you chop up a Snake
Register to Reply
Power-User

Join Date: Nov 2007
Location: A secret. Bats are a Protected Species.
Posts: 186
Good Answers: 4
#8

Re: LED Reactive Lighting Circuit

12/14/2007 3:38 AM

<What are the consequences of driving a LED at lower voltages?>

Lower current, and therefore lower brightness. To sustain the brightness the current must be increased, by reducing the value of the resistor in series with the LED.

__________________
Innuendo is not Italian for a suppository.
Register to Reply
Guru

Join Date: Sep 2007
Location: Reno, NV (USA)
Posts: 608
Good Answers: 66
#9

Re: LED Reactive Lighting Circuit

12/14/2007 10:55 AM

Thank you all for your feedback.

I stopped by RadioShack on the way home last night and picked up some 2.2 kOhm, 1/4 W resistors and wired the circuit up this morning. (Can you believe that RadioShack doesn't carry a test breadboard anymore?!) Circuit seems to work as designed, but I'll let it run all day to make sure the magic smoke is securely contained.

Zaphod, looks like your maths worked out. Thanks for dusting off the rusty grey matter, I appreciate the guidance. You're right, this was kind of a fun departure from the everyday! Thanks again!

__________________
Aequam memento rebus in arduis servare mentem.
Register to Reply
Power-User

Join Date: Nov 2007
Location: Alberta Canada
Posts: 182
Good Answers: 9
#10
In reply to #9

Re: LED Reactive Lighting Circuit

12/14/2007 11:25 AM

Hi CSM,

Glad it worked out. I guess school wasn't a complete waste of time after all

__________________
Vote for something useful this time, vote to repeal the second law of thermodynamics!
Register to Reply
Register to Reply 10 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

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

Users who posted comments:

CSM Engineer (3); Horrible Old Bat (1); Qqberci (1); Snaketails (1); Zaphod2Headed (4)

Previous in Forum: What Happens to Light Energy?   Next in Forum: Looking for collaborators for study...

Advertisement