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I'm trying to find a good and cheap way to turn on some led strips with car battery without burning or stressing the leds. I've found out it can be done with a resistor but it's not the best way to do it, since it will get dimmer/brighter depending on the voltage of battery at the time. Also found out it can be done with constant-current led drivers (made with lm317), but the problem is that these regulators have a voltage drop (3v for lm317 I think). So the output voltage will be less than 12v. Will the leds turn on with less than 12v? (It will be around 9v if i'm not wrong.) If not what should I do?

Also this is the driver i'm talking about:

LM317 constant-current driver

I think I'm getting lost ._.

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if you mean 12V led strips, they don't need any drive circuit. they use a current limiting resistor for a few LEDs in a block, and connect the blocks in parallel for various strip lengths. so you are good to connect it the battery. the LED strips will get the allowed current.

A CC mode regulator, variates V in V=RI , in the available voltage range to achieve your desired I (R is constant here obviously).

you can't use the circuit above when the minimum allowed voltage for LEDs (10V) is too near to the maximum available voltage(12.5V). in this case there is no actual voltage range (just 2.5V) that the regulator could variate V in it to achive you desired current which normally is usually set to maximum by you. so it will get eventually dimmer based on the voltage, let alone the dropout; unless you want to be dim and draw currents less than the allowed current.

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  • \$\begingroup\$ Yeah I see the resistors in every segment. I was wondering what they are for. lol. Thanks for the help. \$\endgroup\$ – MohammadSh55 Sep 29 '20 at 21:23
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I've found out it can be done with a resistor but it's not the best way to do it, since it will get dimmer/brighter depending on the voltage of battery at the time.

Yes. If they're 12 V strips then this would be a bit of a problem.

Also found out it can be done with constant-current led drivers (made with LM317), but the problem is that these regulators have a voltage drop (3v for lm317 I think).

The CC driver isn't appropriate as the strips are designed to work at constant voltage - 12 V in this case. If you were to use the LM317 you'd use it in a voltage regulator configuration but, as you say, the voltage drop is too much.

The LM317 is a forty year-old design and things have moved on. Have a look for a low drop-out (LDO) voltage regulator with a 12 V output that will handle the current you require. These are optimised to minimise the drop-out when the the input voltage is low but otherwise work like a regular voltage regulator.

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  • \$\begingroup\$ Thanks for the answer. What 12v LDO regulator do you recommend for this purpose? \$\endgroup\$ – MohammadSh55 Sep 29 '20 at 21:18
  • \$\begingroup\$ We don't make product recommendations on this site. \$\endgroup\$ – Transistor Sep 29 '20 at 21:37

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