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Author Topic: Dimming LED  (Read 1180 times)

Skimmer Fan

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Dimming LED
« on: December 28, 2017, 09:42:17 AM »

Is it possible to dim an LED by just putting a higher value resistor in place of the standard resistor.
White LED on 12 volts with 470 ohm resistor is what normally use.
White LED on 12 volts with 1.5k resistor is what I am thinking of f using. Will this have any detrimental effect on the LED or circuit.
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Skimmer Fan

LJ Crew

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Re: Dimming LED
« Reply #1 on: December 28, 2017, 09:57:22 AM »

No, go ahead it won't hurt either the LED or another circuit. Normal LEDs run at below 20mA. 12 volts and 470 ohms is about 20mA and you could increase the resistor value until you can't see the light from the LED.
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tigertiger

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Re: Dimming LED
« Reply #2 on: December 28, 2017, 12:12:20 PM »

I second that.
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Brian60

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Re: Dimming LED
« Reply #3 on: December 28, 2017, 02:44:37 PM »

From experience you will need a bigger value than 1.5k to make any appreciable difference in the luminocity

Skimmer Fan

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Re: Dimming LED
« Reply #4 on: December 28, 2017, 02:52:32 PM »

Thank you for your comments.
I will acquire some higher value resistors and experiment, after your advice that I should not be damaging anything by putting a larger than normal resistor in, 
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LJ Crew

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Re: Dimming LED
« Reply #5 on: December 28, 2017, 06:28:39 PM »

If you have a number of 470 ohm resistors just join them in series until you find the value you want. The overall resistance is the sum of them. If you want to "fine tune" the resistor value use two in parallel, this will halve the resistance of the two joined in parallel.
Series: Rt=R1+R2
Parallel: 1/Rt=1/R1  +1/R2
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Skimmer Fan

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Re: Dimming LED
« Reply #6 on: December 28, 2017, 08:00:45 PM »

Is it possible to put 8 LEDs in series with one resistor on 12 volts?
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LJ Crew

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Re: Dimming LED
« Reply #7 on: December 28, 2017, 10:01:03 PM »



No, not with 8 LEDs on 12 volts, assuming that you are not using anything exotic. Each LED will have a forward voltage drop of about 2 volts so on a supply of 12 volts you can only use 6 LEDs in theory. You may find that by using a series and parallel combination you could run all eight on 12 volts. However most LEDs have a forward voltage drop of 2.2 volts and to get current to flow the sum of the forward voltage drops must be less than the supply. To run 8 LEDs on a 12 volt supply I would suggest two separate chains of 4 LEDs in series with a 150 ohm resistor.
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cos918

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Re: Dimming LED
« Reply #8 on: December 28, 2017, 10:56:09 PM »

two things . If you are using them very small SMD led . you will struggle to get them to dim . What i had to do is run two in series and then use a very high value resistor ,some thing like 2k I think . The problem is the require naf all current and voltage . 


When running several resistors in series remember the size of the resistor in whatts ,eg run 4 big high powered  through a 1/4w and you might get problems


John
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tigertiger

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Re: Dimming LED
« Reply #9 on: December 29, 2017, 01:23:38 AM »

Check on the forward voltages of the LEDs . I seem to remember that red and green are approximately 2V but white and blue are approximately 3V. This would affect how much resistance you want and relative brightness if you have white mixed with red and green in the same circuit
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malcolmfrary

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Re: Dimming LED
« Reply #10 on: December 29, 2017, 10:07:43 AM »

Another thing about series LEDs.  The point about the forward voltage drop is spot on, but after adding them all together you have to leave a bit of voltage headroom for the resistor to be able to do its job.  The LEDs act like Zener diodes in that they each drop a specific voltage over a wide range of current, but the battery voltage is not constant.  If there is a very low voltage difference, the voltage across the resistor can vary a lot, this will cause the current through the circuit to vary, and thus the brightness.
There are not many ways to damage LEDs - the top one is too much current (i.e. not enough resistance for the voltage) then too much reverse voltage (they don't like it up 'em, to quote Corporal Jones). 
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The Old Fart

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Re: Dimming LED
« Reply #11 on: December 29, 2017, 08:10:16 PM »

check this calculation site,
very usefull.

http://ledcalc.com/

roadrunner440

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Re: Dimming LED
« Reply #12 on: December 30, 2017, 07:42:19 PM »

I have run as many as 5 or 6 smt led's of the 1206 package sizes off 7.4v nicads...the tricky part is the wattage of the resisters..most smt resisters in the 1206 pacage size are at most 1/2 watt but most are 1/4 watt and can get very hot..i ended up having to go to 2505 1 watt resisters for my plcc5050 3led's in 1 package(note these require 3 separate power&ground leads.i used 0.5mm magnet wire  for b+ and slitly smaller on the ground side) but they are 100mv forward at 3.2v and needed 1 watt 47 ohms x's 3 to light the chips up for my deck lamps..i think was worth the effort. but I stress tested(ran for about 4hrs to see hot hot the resisters got...most single or up to 3 of the 1206's were fine with 1/4 watt 1206 resisters..you can use old school resisters on smt led's..and the right resister will prevent.also ebay is where I got all my led/resisters from they also sell assortments of values in 1206 packages for playing around the dimming.also if the led is say 3.2 forward v if you reduce it to say 3.0v on the calculator leaveing your source voltage the same the calculator will give you the value and wattage resister you need.or you could put a variable resister of say 0 to 10k ohms in the ckt but again you will need to pay attention to the wattage
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