We have a house with an outdoor staircase lit by warm white LED strip lights. However, the lights are "too bright" and we want to lower the intensity of the LED lights. On another loop of the garden lights, the same LED strip lights are lower in intensity because there are more LED lights on the same loop. So, I was wondering what resistor (or other device) would be the smartest to use? A dimmer normally works with 110 V but that is not possible here to use. It has to be something on the low voltage loop so therefore I was thinking adding a resistor or similar to that specific loop. The loop it is currently on is a 5 V loop with 8 LED lights pulling 6 W each (=48 W). Anyone with experience/suggestions?
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3 Answers
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The best and most economical way is to reduce the voltage of the supply with the small orange screw near the terminals. Outdoor hermetically sealed supply don't have this adjustable screw (potentiometer). In this case replacing the supply may be necessary. It can be cheaper or not more expensive than buying a PWM dimmer. PWM dimmer consume more electricity but are better fit for accessible, user friendly, regularly adjustment. For a one time adjustment, setting a lower voltage is better and more climate friendly (LOL). The change in voltage has to be of the order of 10%. If you replace the supply by a supply with 30% less voltage, it will be too much. You can reduce voltage safely. But you can't increase it.
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\$\begingroup\$ "... with the small orange screw near the terminals"!? \$\endgroup\$ Apr 23, 2020 at 22:22
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\$\begingroup\$ @Transistor The screw of the pentiometer is, almost always, orange. \$\endgroup\$– FredledApr 24, 2020 at 9:00
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1\$\begingroup\$ Thx. a bunch. Makes sense ... :-) much appreciate your advice \$\endgroup\$– NilsApr 24, 2020 at 23:29
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Using a resistor won't be the smartest thing to do here. It will dissipate a lot of heat depending upon the dimming requirements.
I suggest using a PWM to dim the LEDs. Something like this:
You can make one yourself if you are patient enough. Alternatively buy it online. Make sure your PWM dimmer works at 5 V.
If you are making one yourself, there are two ways to do it - Using a microcontroller like arduino or plain hardware based using 555 timer.
TIP: If you happen to be laying long cables to run multiple LEDs (or a long LED strip) from a single power supply source, consider using 24 V LEDs. It will give you lesser voltage drop in the cable and your lighting will look more uniform.
answered Sep 26, 2019 at 4:06
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\$\begingroup\$ using PWM to dim LEDs wastes more energy than using a resistor. \$\endgroup\$ Sep 26, 2019 at 10:09
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\$\begingroup\$ Jasen, can you please explain it further. I have a very strong opinion that PWM is better than series resistor in this case but I might be wrong. \$\endgroup\$ Sep 26, 2019 at 10:17
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\$\begingroup\$ LED illuminace is proportonal to current, not to power dissipation. compare 50% pwm with a resistor that halves the current, you'll see that the supply current is the same both ways. except that the PWM crontroller takes some current to run.. \$\endgroup\$ Sep 26, 2019 at 10:20
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1\$\begingroup\$ Jasen, I think OP is asking about dimming strip lights which already have current limiting resistors built in. He wants to further dim the strip. Current draw is 10 amps currently. Lets say you want to set the current to 5 amps by putting 4.7 ohms, you will waste a lot of power - almost 125 watts of power. I think we are not at the same page here. \$\endgroup\$ Sep 26, 2019 at 10:31
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1\$\begingroup\$ Interesting and mind-bending. At least for me. Thanks Jasen and Russell. I have some strip light lying around. I will try this out and make some measurements. \$\endgroup\$ Sep 26, 2019 at 12:55
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if you can adjust the voltage of the supply to the LEDs that will probably be the simplest fix.
If not, adding a resistor is a good idea, picking the right resistor could be tricky. but if your 48W and 5V figures are accurate about 4.7 ohms 2W sounds like a good start.
answered Sep 26, 2019 at 10:19