# How do I get a constant voltage from car battery

Thread Starter

#### user007

Joined Dec 16, 2011
7
Hi,
Sorry if this has been asked before, I couldn't find the answer.

Intro:
I want to install a strip of LED lights on a car, and need 12V for the lights.
I know that car batteries are 12V most of the time, but they can spike up to 14.5V when starting the car and under certain conditions, and can also go below 12V sometimes.

The problem:
How do I get a constant 12V from the car battery to supply to LED strip?
The LED strip needs around 1.5A of current.

A LM7812 regulator wouldn't work here because of the dropout voltage of the regulator, so what should I do to get a clean 12V?

Please let me know if you need more info.

Thanks all for your replies.

#### absf

Joined Dec 29, 2010
1,942
The problem:
How do I get a constant 12V from the car battery to supply to LED strip?
The LED strip needs around 1.5A of current.
Hi 007, welcome to the forum.

I am not sure if this type of questions is allowed in this forum as it involves adding fixtures to the car. I'll wait for the moderator first before giving out my views.

Allen

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#### Audioguru

Joined Dec 20, 2007
11,251
A car battery is 12V when dead. It charges at 14.4V most of the time.
If the "12V" LED strip is designed for a car then it will work fine from the 14.4V.

#### rfredel

Joined Jan 23, 2011
25
Hello 007 and absf,

I think, that is not only a problem for using in cars. Sometimes you need a constant powersupply for devices.

You need a buck-boos-inverter with a input range from 10V to 16V an a constant output of 12V.

You can find a WEBENCH-Designer on the site of Texas Instruments. The designer will make you some offers and calculate the complete circuit.

http://www.ti.com/

Bset regards

Rainer

PS: Sorry, if my English is not so good

#### takao21203

Joined Apr 28, 2012
3,682
A small resistor would be sufficient eventually but this needs to be tested in place, at first a conservative value and then decrease a little.

In my opinion. Are these big power LEDs? You can use a voltage booster and then set it to 14V for instance. But they can't really output less than input voltage.

Thread Starter

#### user007

Joined Dec 16, 2011
7
A small resistor would be sufficient eventually but this needs to be tested in place, at first a conservative value and then decrease a little.
A resistor is not really helpful here. I want to maintain a 12V supply to LEDs no matter what the car battery voltage is.

Are these big power LEDs?
It's just a standard LED strip (3528 White LED), check it out here - it's the second item in the table down the page.

Thanks

Thread Starter

#### user007

Joined Dec 16, 2011
7
A car battery is 12V when dead. It charges at 14.4V most of the time.
If the "12V" LED strip is designed for a car then it will work fine from the 14.4V.
Interesting, I though car batteries are 12V all the time.
I bought the LED strip from eBay and it just says it needs 12V to turn on the lights. It doesn't say that they're for cars. The seller doesn't even know how much current the LEDs draw.

What should I do now? I still want to install them on the car.

Maybe I'm asking the wrong question in wanting to maintain a constant 12V volts supply to the LED strip.

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#### Tealc

Joined Jun 30, 2011
140
My PWM LED strip controller, which looks like the one you have, happily works at 15v. All that happen is that each LED gets a little bit more current, that's all. In fact mine worked down to 6v but the Green and Blue LEDs switched off because they didn't have enough voltage.

Usually these strips consist of a set of 3 LEDs in series limited by a single resistor for each colour. The number of LEDs per metre determines how close together the LEDs are but there'll still usually be 3 in series.

#### takao21203

Joined Apr 28, 2012
3,682
It's helpful to see the actual LED configuration.
There might be components directly on the strip, and there seems to be a controller as well with 12v input.

Since you use eBay to get these, there is DC/DC booster for 12v/35v max, less than 10 dollar, could be set to 16v or 18V, and then again downconverted to exact 12V. This converter has relatively good efficiency.

Not sure if the controller can tolerate different voltage than 12V, or if you have adjustable power supply to test this.

No, correct a resistor won't be suitable for this LED strip.

here the boost converter, I have one here, used it a few times
http://www.ebay.com/itm/DC-DC-10V-32V-to-12V-35V-150W-Power-Supply-Boost-For-Module-Mobile-Notebook-Car-/221004399645

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#### mcasale

Joined Jul 18, 2011
210
LEDs really run off of current - but you need voltage to generate the current (that's EE101). If the current varies a bit, the brightness will vary a bit, but the eye is not sensitive to small changes. Why do you think it has to be an EXACT 12V?

If a 12V battery is all you have, and everything has to be dead-nuts-right-on, you will need to boost and regulate the voltage to the lights.

Thread Starter

#### user007

Joined Dec 16, 2011
7
My PWM LED strip controller, which looks like the one you have, happily works at 15v. All that happen is that each LED gets a little bit more current, that's all.
It's GREAT to hear that it worked for you. I don't have the controller though, I just have the LED strip and plan to connect it directly to the car battery. Would it still work as well without effecting the life span of the strip?

It's helpful to see the actual LED configuration.
There might be components directly on the strip, and there seems to be a controller as well with 12v input.

Since you use eBay to get these, there is DC/DC booster for 12v/35v max, less than 10 dollar, could be set to 16v or 18V, and then again downconverted to exact 12V. This converter has relatively good efficiency.

Not sure if the controller can tolerate different voltage than 12V, or if you have adjustable power supply to test this.

No, correct a resistor won't be suitable for this LED strip.

here the boost converter, I have one here, used it a few times
http://www.ebay.com/itm/DC-DC-10V-32V-to-12V-35V-150W-Power-Supply-Boost-For-Module-Mobile-Notebook-Car-/221004399645
Thanks. I don't have the controller, just the LED strip.

Any more ideas are welcome, please keep them coming.

#### Audioguru

Joined Dec 20, 2007
11,251
See what happens when you buy cheap Chinese junk on E-Bay?? NO DETAILS!

A photo of the LED strip shows current-limiting resistors on it.

The description in the ad says, "Perfect LED Lights for car and motorbike".

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#### wayneh

Joined Sep 9, 2010
16,102
Get a magnifying glass and read the numbers off the little black rectangle (resistor). That will tell you its ohms value, and give an idea of how protected the LEDs are against over-voltage. It's probably 470Ω or more, which will be labeled 471 (two sig figs plus the number of trailing zeros). I'd not put more than 20mA through the LEDs at 16V unless you know more about the LED specs.

#### Audioguru

Joined Dec 20, 2007
11,251
The No-Name-Brand cheap Chinese LEDs do not have any specs.
The E-Bay ad says they work from the "12V" in a car or motorbike (if you believe what is said).

#### bountyhunter

Joined Sep 7, 2009
2,498
A car battery is 12V when dead. It charges at 14.4V most of the time.
If the "12V" LED strip is designed for a car then it will work fine from the 14.4V.
+1 mkers use the generic term "12V" to spec auto equipment but it is understood it rises to 14V under normal running operation. Don't worry about regulating the voltage.

#### bountyhunter

Joined Sep 7, 2009
2,498
See what happens when you buy cheap Chinese junk on E-Bay?? NO DETAILS!
True, but sometimes the price is irresistible. My PC's external hard drive died when the original power supply failed and I found a replacement on ebay for $4. It actually works. Got a couple of sets of "Tektronix Clone" scope probes for my Oscope for about$5. Brand new and they work. Bought a brand new Croton dig/analog watch that retails for $350 on ebay for$60 (and it's not a knock off!) I also bought my Tek 454 scope off an ebay seller for \$200 and it is like new.

It's always a risk buying off fleabay, but sometimes the stuff is ridiculously cheap and worth the risk..... one thing I would never buy is a motorcycle chain or any other item you bet your life on in normal usage.

#### Tealc

Joined Jun 30, 2011
140
Oh I assumed you had the control box as well.

I've always bought my LED stuff off ebay and haven't had many issues. The only one I did have reason to complain about what an Orange LED strip that was actually more yellow than orange.

I've bought quite a few red and white strips and they seem to always come with higher resistance values than I'd normally look at using.

Thread Starter

#### user007

Joined Dec 16, 2011
7
OK, had a closer look at the built-in resistors and they say 151, which is 150Ω, but something doesn't make sense.
There is only 1 resistor for every 3 LEDs, and I had a look at the datasheet of the same LED but different color (red instead of the white ones I have) and it says the max forward current is 25mA.

So:

V=12V
R=150Ω
I(max)=25mA

If I connect a 12V supply into the LED strip, there will be 12V/150Ω=80mA of current through the LEDs.
That's WAY over the limit (25mA) specified in the datasheet.

I plugged a 13.5V supply into the LED strip and it light up with no problems - but it was only for a short while (< 1 minute).

I'm confused by the above numbers. Anyone got any ideas?

#### BSomer

Joined Dec 28, 2011
433
So:
V=12V
R=150Ω
I(max)=25mA

If I connect a 12V supply into the LED strip, there will be 12V/150Ω=80mA of current through the LEDs.
That's WAY over the limit (25mA) specified in the datasheet.

Not quite, but almost.

You only have part of the information listed here. What is the forward voltage of the LEDs? Your calculations are showing the full supply voltage going across the resistor. The LEDs will take up some of that. So if the LED Vf is 3.2V and you have three of them you get about 9.6V across the LEDs which leaves 2.4V across the resistor. So 2.4V / 150 Ohms = 16mA.

#### Tealc

Joined Jun 30, 2011
140
To be sure you'd need to measure the forward voltage drop across the LEDs. As it stands, with an assumed Vf of 3.2v and a 150ohm resistor if you had a voltage of 14.4v you'd have too much current flowing through the LEDs, assuming again they are 20mA typical, 30mA Max.

14.4v - 9.6v = 4.8v
4.8v / 150 Ohms = 32mA

However if the Vf of your LEDs is higher than 3.2, and it may be as much as 3.6v then everything is fine.

3.6v x 3 = 10.8v
14.4v - 10.8v = 3.6v
3.6v / 150 Ohms = 24mA

I'd just use them and hope for the best. Cheap stuff that fails early can be replaced.