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Electrical question.

Community Veteran
Posts: 3,826
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Registered: 24-09-2008

Electrical question.

I have a 5m LED strip-flexible light,12 v where 1 meter is rated at 4.8w.
When I plug it into a 12v circuit it glows too bright.
Any ideas on decreasing the brightness?
As an example could I put a resistor in the line to decrease the brightness?
What size resistor would I use to decrease the brightness by about 30%?
13 REPLIES
Community Veteran
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Re: Electrical question.

Could be wrong but I seem to remember reading that LED rated "brightness" is not variable, reducing the voltage would merely take them longer to get to the same brightness.
But could very easily be quite wrong.
LED running lamps in modern cars have to mounted in carefully designed heated (or temperature controlled) mountings to achieve their full brightness. Which is why there is little demand for after-market sets. If you can keep your LED's cold it may reduce the brightness slightly.
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kmilburn
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Re: Electrical question.

Effectively,  the brightness of an LED is fixed,  the primary purpose of resistors with LEDs is to prevent the LED buring itself out.
The primary method used for dimming LEDs is Pulse Width Modulation (PWM),  basically, turning it on and off very fast,  and the ratio between on and off determines how bright it appears.
From a quick search,  dimmers using PWM are available,  but determining if they'll work with your strip would require testing.
Something like this might do the job.
itsme
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Re: Electrical question.

Quote from: kmilburn
Effectively,  the brightness of an LED is fixed,  the primary purpose of resistors with LEDs is to prevent the LED buring itself out.

If you are referring to a single LED then your statement is incorrect. Current flowing through a LED will have a bearing on brightness and the current is dependent on the value of the series resistor.
Community Veteran
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Re: Electrical question.

I'd experiment first of all with a variable voltage DC supply such as maplins etc. sell fairly cheaply.
12v LED's do most definitely dim though; I've installed several in my cars as alternatives to the standard filament bulbs and was quite surprised (initially) to observe that they dimmed in pretty much the same way as the interior lights fade out.
itsme
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Re: Electrical question.

http://www.starscape.co.uk/Images/pdf/30mA%20Flexible%20tape.pdf
Above is a link to a datasheet and from the graphs you will see that relatively intensity do fall off but at temperatures 40C and above. and that relatively intensity is a function of forward currant.
Referring to the data for the 30mA White the strip is made up of 60 LEDs consisting of 20 groups of 3 LEDS. Each group have 30mA flowing through it so giving a total current of 0,6A. To reduce the brightness down to 75% require the current in each group to be reduced to 22mA.  The forward voltage of white LEDs is approximately 3.4V so the in-built series resistor Total Voltage minus the forward voltage divided by current 12-10.2//0.03=60. So each series resistor is 60 ohms with a voltage drop across it of 1.8V. So to reduce the the current down to 22mA require a voltage drop across this resistor to drop to 60 x 0.022 = 1.32. Which mean that the 12 volt supply has to drop 0.48V. So the total current is 20 x 0.022 = 0.44A so the resistor required is 0.48/0.44 = 1.09 ohms
Community Veteran
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Re: Electrical question.

Quote from: Petlew
Could be wrong but I seem to remember reading that LED rated "brightness" is not variable, reducing the voltage would merely take them longer to get to the same brightness.

This is what I don't understand though.. Normal LEDs use around 3V to light up. White LEDs are usually 3V too yet these hand held LED torches use 4.5V - in other words, they're over powered to proiduce extra brightness.
LEDs can work on lower voltages too but they don't glow as brightly.
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itsme
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Re: Electrical question.

LEDs are never connected directly to a power supply. They will have a series resistor and with surface mount resistosr these can be very hard to see as some are the size of grains of sugar.
Community Veteran
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Re: Electrical question.

Just  word of thanks for all the replies Wink
very helpful
Community Veteran
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Re: Electrical question.

You can also bung a few normal diodes (2 to 5 amps rating)  in series with the supply - each diode will drop around 0.7v
A bit crude but effective
wayrail
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Re: Electrical question.

LED brightness definitely IS variable, maybe you are confusing them with other low-energy lighting, such as fluorescent tubes. Voltage drop with diodes generally is only significant when converting from AC to DC. Resistors are much better. You are unlikely to find a resistor as low as 1 ohm, it might be a piece of wire. Current limiting resistors are normally added in series with LEDs, this limits the current, without which they could go bang. For long strips of LEDs, simply connecting them all in series is often self-current-limiting, with all the energy they soak up turning electricity into light. You don't need to alter the fitted resistors, however they're connect internally. Your best bet is to fit a resistor in series, possibly a variable one or a preset for lower cost. 470 ohms would be a safe bet, or a 5 kilo-ohm potentiometer.
Community Veteran
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Re: Electrical question.

A 5k potentiometer would be a bad idea and most ones the original poster would be able to purchase locally would be far too low a wattage and would burn out pretty quickly with this sort of load. 5k is way too high too - you'd need <50 ohms (possible much lower - too early to do the maths)
1 Ohm resistors are incredibly common - I've got hundreds here (I'm an electronics engineer by trade).
You'd be a lot better off grabbing a dashboard dimmer rheostat from an old car and trying that - the older ones are just a low resistance high wattage variable resistor. Newer ones use PWM which could be a bit harder to hook up.
itsme
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Re: Electrical question.

@picbits I agree with your suggestion of using a diode, completely forgot about that option have used it in the past to do the exactly the same thing. Suggest the 1N4004, has it's the industryl must common 1A diode.  1A will be sufficient as @journeys strip is 4.8W giving a maximum current of 0.4A, the example that I gave was for a 7.2W with a current of 0.6A. As for the wattage if a resistor is used as it only dropping 0.5V at approximately 0.5A this give 0.25W
itsme
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Re: Electrical question.

Quote from: wayrail
For long strips of LEDs, simply connecting them all in series is often self-current-limiting, with all the energy they soak up turning electricity into light. You don't need to alter the fitted resistors, however they're connect internally. Your best bet is to fit a resistor in series, possibly a variable one or a preset for lower cost. 470 ohms would be a safe bet, or a 5 kilo-ohm potentiometer.

The forward voltage drop of white LEDs is 3.4V, strip LEDs are groups of 3 LEDs in series which then are connected in parallel. So 3.4 x 3 = 10.2V so with a 12V supply 3 LEDs is the maximum that can be connected in series.