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Running leds from car battery.

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bitem2k

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What is the best way to run devices from a cars battery?
Should i wire into the fuse box?

thanks.
 
bitem2k said:
What is the best way to run devices from a cars battery?
Should i wire into the fuse box?

thanks.

Yes it should be on a fused circuit.
The automotive 12V system is full of transients during startup & load dump. Also consider protecting the LED from reverse voltage from these transients.
 
If you find a hot wire in your car ,such as a cigarette lighter,it usually already has a fuse.If you wire it to your fusebox, for blade fuses, there are special connectors to use.One kind will attach to the leg of a fuse. Another kind will fit into a fuse socket by itself.There are hookups in your fusebox that will wire stuff hot all the time or hot only when your ignition is on.You should always use a fuse ,otherwise your wires could melt and burn up your vehicle.You should browse the auto parts electrical section for the right parts.You need to be more specific to get any better information.If your talking about a 50 amp device then you'll need to wire it to your battery with the correct wire and fuse.
 
50A? A little LED will try to draw 50A if it doesn't have a current-limiting resistor.
 
I just want to put some superbright white leds (3.8v @ 20mA) in the dark regions under the seats in the front.

Would i be able to get away with 1/4 W resistors?

To protect against reverse current what sort of diodes should i be using.
thanks very much.
 
Connect two LEDs in series and in series with a 270 ohm resistor and in series with a 1N4148 diode. When the battery is charging at 13.8V then the current will be 20.4mA. The resistor will dissipate only 112mW so a 1/4W resistor is fine.
 
If batt voltage is 12 - 14 v should I not connect 3 3.8v leds in series with the resistor, with the diode?

Correct me if im wrong but I looked at the 1n4148 spec sheet, at a graph and it says the that the voltage drop that occurs across the diode when 60 mA is flowing is 0.8v

so I a plan to run 3 parallel chains of series leds, resistors, and diodes, this is my calculation (forgive me if this is horrendously wrong).



13.8 (max v) - 3(3.8) - 0.8v = 1.6v

R = 1.6v / 0.02A = 80:eek:hm:

so 3 * series chain =

3(0.02A) = 60mA

is this right?:confused:

thanks mate.:)
 
audioguru said:
50A? A little LED will try to draw 50A if it doesn't have a current-limiting resistor.

Hi AudioGuru. Funny you should mention that. A work colleague of mine mentioned the use of LEDs as current references, and I had to raise an eyebrow at that. I mean I have seen them used as voltage references before using the forward voltage drop across them, but I can't see how they would be good current references.

Have you ever heard of this?

Sorry for hijacking the thread by the way...

Bri
 
The battery voltage doesn't stay at 13.8V. When the charging is slow or is stopped then the battery voltage will drop to 12V, then 3 LEDs in series with the other parts would not have enough voltage so would turn off. 2 LEDs in series and in series with the other parts would still work but will dim a little.

An LED is frequently used as the voltage reference in a constant current source or sink circuit.
 
bitem2k said:
If batt voltage is 12 - 14 v should I not connect 3 3.8v leds in series with the resistor, with the diode?

Correct me if im wrong but I looked at the 1n4148 spec sheet, at a graph and it says the that the voltage drop that occurs across the diode when 60 mA is flowing is 0.8v

so I a plan to run 3 parallel chains of series leds, resistors, and diodes, this is my calculation (forgive me if this is horrendously wrong).



13.8 (max v) - 3(3.8) - 0.8v = 1.6v

R = 1.6v / 0.02A = 80:eek:hm:

so 3 * series chain =

3(0.02A) = 60mA

is this right?:confused:

thanks mate.:)
Never mind.
 
Two or three or any number of things in series all have the same amount of current.
 
Why not do it like this ?

Use as many LEDs in series as you need to get to about 10V. Use a variable voltage regulator to provide that voltage. The car provides enough volts to regulate down to 10 in all cases. The LEDs would have constant brightness and you do not need any resistors or non LED diodes to drop the voltage.
 
3v0 said:
Why not do it like this ?

Use as many LEDs in series as you need to get to about 10V. Use a variable voltage regulator to provide that voltage. The car provides enough volts to regulate down to 10 in all cases. The LEDs would have constant brightness and you do not need any resistors or non LED diodes to drop the voltage.
No.
You can use a voltage regulator for light bulbs, not for LEDs. LEDs need to have their current limited by a series resistor or by a current regulator.
 
I am attempting to understand this.

As the supply voltage approaches the voltage required by the LED the value of the current limiting resistor gets smaller. But we can not have the supply voltage close enough to the LED voltage to do away with the limiting resistor. Is it because LEDs have an exponential current-versus-voltage characteristic ?

While trying to understand this I found a that might be of use to anyone wanting to do series LEDs. In it the author talks about using the LM317 as a current regulator. Fun stuff :rolleyes:
 
An LED is a diode. When the voltage across it increases a tiny amount then its current doubles or triples.
You don't know the exact voltage of your LEDs, it is somewhere in a range of voltages.
 
audioguru said:
Two or three or any number of things in series all have the same amount of current.

I was refering to the sum amount of current used by all 3 parallel chains of leds runnning at 20mA each in series.

i.e One series chain consists of 2 leds, one diode, and one resistor.

I would then create 2 more chains, and then connect all three in parallel to each other.

thanks
 
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