Dimming an over bright LED

Discussion in 'LEDs and Optoelectronics' started by sacentre, Feb 17, 2019.

1. sacentre

16
1
Sep 27, 2014
I know that resistance is usually added in series to one side of an LED to limit the current but is is possible to put a resistor of the right value across an LED in parallel when it is already in circuit in order to dim it?

2. Alec_t

2,904
784
Jul 7, 2015
No, unless there is already a series resistor in the supply to the LED.

3. sacentre

16
1
Sep 27, 2014
Thank you Alec.

4. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,418
2,788
Jan 21, 2010
If the led is being driven by a current source then a parallel resistance is the correct approach (unless you can modify the current setting).

5. sacentre

16
1
Sep 27, 2014
Thanks, Steve. Sorry to be dim (I'm not electronics educated) but I'm afraid I don't understand, "if the led is being driven by a current source". I thought any electronic device needed both a voltage and a certain amount of current to drive it. With LEDs, I've only ever seen them used with a given voltage and a series resistor as a current limiter whose value determines the brightness.

6. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,418
2,788
Jan 21, 2010
A voltage source provides a given voltage and the current is determined by the nature of the load.

A current source provides a certain current and the voltage is determined by the load.

Batteries seem like a voltage source, but if the load demands too much current, the voltage falls. The aren't any common current sources, so it's not surprising they seem unusual.

LEDs should be driven by a current rather than a voltage. For an explanation, go to our resources section and look up the resource on LEDs.

Edit: a voltage source followed by a resistor creates a poor-mans current source that may be sufficient in many cases.

And why did you edit away your question?

7. dave9

961
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Mar 5, 2017
Yes you can put a resistor in parallel to dim the LED but there is seldom a good reason to do so since it just wastes power that you wouldn't otherwise waste using just a single resistor in place of the existing resistor, either to limit current to the LED more if existing resistor is in series, or in a current source would be in the driver feedback to limit the output current to the LED more. Usually if you don't want to get that far into it, you'd just put a lower light transmission panel in front (piece of tape if nothing else) or a dome over it or a black sharpie marker to darken it or *whatever*.

If this is the typical complaint I've seen on the internet over the years where you have some widget in a bedroom and a person is convinced (probably wrongly so, that they just notice the LED because they're awake) the reason they can't sleep is a too bright LED from some indicator panel LED putting out a half dozen lumens or less, those usually do use only a series resistor to limit current and that resistor can be swapped out for a higher value resistor to save more power instead of wasting more.

Granted at the drive current for only a few lumens you aren't wasting much if you just throw in a parallel resistor instead, but does it really save any time or hassle? Doubtful, you still have to measure voltage, calculate resistor, already have or order that resistor... same difference as replacing the original with the right value for your needs except for desoldering the original resistor which only takes a few more seconds since you had the soldering iron out anyway to put the new resistor in.

8. hevans1944Hop - AC8NS

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Jun 21, 2012
I would opt for the optical solution: put a dab of ink on the offending puppy.

9. WHONOES

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May 20, 2017
I realise it may be beyond your ability but, the way to vary the intensity of and LED is to use PWM (Pulse Width Modulation) in concert with a fixed voltage supply and current limiting resistor. Such devices are available via E bay for not very much money.