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There never seems to be any talk of using a "LED driver" when using LED light strips in automotive applications. From my research it seems that LED strips have no voltage regulation and only a voltage dropping resistor. This mean that the 14.2 typical peek "full alternator charging voltage" could cause some damage to these. Why does no one seem concerned?

Also I seem to read a lot of double talk about "LED drivers". Shouldn't these just be common DC power supplies that supply a constant voltage of 12V with an amperage capacity that meets or exceeds that defined as required by the LED strip?

And thirdly, why is there not much talk then of the amperage required by a string of "sticky tape" LED strips? Is it because the industry only talks in "LED driver speak" rather than "volt and amp speak"?

****** UPDATE AFTER ANSWER 1 BELOW:

I guess what I am asking, and thanks for this detail by the way, regarding the common "roll of LEDs" on a spool that are made and shipped from China that so many people are selling now, regarding these, what I have read is that they use single resistors for each LED in the strip so something to regulate the voltage would be needed. Now a) is this true, that most of them use individual resistors and b) isn't that really just a constant 12V power supply with an amp rating greater than or equal to that required by the sum of the strips and c) doesn't this mean that the 14.2V commonly reached when an alternator is charging an auto battery at full capacity, that the strip is over powered and will burn out more promptly? Thanks again.

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marked as duplicate by Sunnyskyguy EE75, Voltage Spike, Dmitry Grigoryev, Vladimir Cravero, Sparky256 Aug 7 '18 at 4:40

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • \$\begingroup\$ I've run the LED strips directly from 12 V, 13 V, etc. No need for any kind of current regulation. You don't need a resistor (in fact, that would be bad.) If these are going in a car and they are as cheap as mine (dirt cheap), then just wire them up to your car's supply. (The only protection needed in a car environment is from load dumps which can exceed 100 V. Use a MOV set for 26 V or something, if you are concerned there.) \$\endgroup\$ – jonk Feb 4 '17 at 16:56
  • \$\begingroup\$ A TVS would be faster responding, as protection. \$\endgroup\$ – jonk Feb 4 '17 at 17:02
  • \$\begingroup\$ There are lots of different kinds of LED strips that come on rolls! (some even have a tiny compute next to the LED chip to control the brightness). Can you give an example of the specific kind you are looking at? \$\endgroup\$ – bigjosh Feb 4 '17 at 22:03
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LEDs need a minimum voltage to work, but once they get that minimum voltage they "turn on" and then the amount of current flowing though them controls how bright they are. Too much current and they blow out, so you need to limit that current to the amount that gets the desired brightness level (and is below the maximum).

You'd think you could control that current by changing the voltage, but it is very hard because a very small change in voltage results in a huge change in current. Imagine trying to control the current flowing though a wire by changing the voltage across it - it would be very hard.

So there are two common ways to control the current though the LEDs....

  1. Use a current driver. This is a device that constantly adjusts the voltage to the exact level to generate a specified current. Keep in mind that this voltage is not fixed so it is not like a typical voltage power supply.

  2. Put a resistor in series with the LEDs. Because the voltage drop across a resistor changes with the current, this accomplishes sort of the same thing- as the current though the LEDs rises the voltage dropped across the resistor also goes up which lowers the voltage across the LEDs with lowers the current. These two effects balance out and settle on a limited current level.

The resistor is much simpler, but less accurate (brightness changes with input voltage and temperature) and you also waste power in the resistor. For small amounts of current, people typically pick the resistor, but as you get to larger currents the resistors get big and hot, so a current driver usually makes more sense.

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After much research, I found this inexpensive solution. I have ordered the parts and am going to give it a shot using a heavy duty heat sink. The device is rated for 5A (12v x 5a = 60w) and should handle (theoretically) any of the 5 meter LED strip model calling for 60w or less for the strip to be powered (see second link which gives tables of types of LEDs and wattage needed).

http://www.eleccircuit.com/12v-5a-power-supply-regulator-with-lm1084it-12/

LED strip power requirements in watts

However in the super bright 5050 link, it would not handle more dense than 30/meter, assuming 5 meter length. For that, a more elaborate supply might be needed.

http://www.ledlightsworld.com/page.html?id=38r

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"12V" led strips tend to use a resistor value that aligns with the typical 12 to 14.5 volts of a automotive power system. At 12V, they typically consume 17mA for a 5050 or 3268 package led. At the typical high end of 14 volts of a car on and charging via an alternator, the strip takes 20 mA per segment. 20mA is the typical max ok voltage of a low power led diode, and much lower than its absolute max.

This changes sightly with other led packages.

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You are right with your concerns, but there is no easy solution. Most LED strips are engineered for 12V. They have three LEDs in series with one resistor to do the current limiting for the group of three.

For example for 3 blue LEDs with 3.3V voltage drop each (9.9V total) the resistor gets 2.1V. If you design for 20mA you need a 105 Ohm resistor.

When you feed the same LED strip now 13.8V, the voltage over the resistor raises to 3.9V and with the same 105 Ohm resistor the resulting current is now 37mA, almost twice the current at 12V.

So, for automotive use you should use LED strips designed for 13.8V or use a voltage regulator for the LED strip to limit the voltage to 12V.

One word about the unclear specifications. You buy those thing mostly in places with small margins. Those places want easy or no specs, to be beginner and dummy friendly. Most of the customer buy sort of a modular kit, where the components fit together, without much knowledge. So you get what you pay for, much much specs for not much money...

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  • \$\begingroup\$ 12v strips typically don't run at 20mA at 12v. 16 or 17 at 12v, 20 at 14v. Measure the resistor, or actual current draw when powered. \$\endgroup\$ – Passerby Jun 18 '18 at 14:23
  • \$\begingroup\$ Yes, the strip does not, because it has lots of those three LED circuits in parallel. They typically have 300 LEDs, which gives 100 x 20mA = 2A. This example was intended to show the calculation. \$\endgroup\$ – markus_b Jun 21 '18 at 19:09
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Amazon sells dc to dc buck converters that are small, perhaps 1 inch by 1&1/2 inches in size, and will regulate voltage down via a pot to your specs to limit current. There are 8 dc to dc buck converters rated at 3 amps for 12$. I would not install led 5050, or any led strips in any automotive, RV, boat, etc, without a converter in the supply wires and they are dirt cheap. It’s your labor when the leds begin to yellow and burn out quickly because they are overdriven and at anything above 12 volts you are overdriving them. In a normal automotive environment I agree with a previous poster who said run them below nominal voltage by approximately 10-15% I run all mine at a regulated 11.2 volts.

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