Custom led advice?!

[Lukeypoo]

Right.. I've always undergone my own DIY projects (mostly car related) and my latest project has hit a stand still due to a voltage problem..
I have 150 4v leds that I have used on a set of tsi's I've made (turn signal induction) I'm clueless to as how I go about stepping down from 12v to the 4v needed :L
Any advice would be great advice [/list]

 

[Aragorn84]

you could simply put 3 LED's in series to give a 12v string then parallel these up?

I think ideally you want a proper LED drive circuit though.

 

[ricicle]

Do the LEDs have their own current limiting circuitry or are they package LEDs ie stand alone ?

 

[bernardgreen]

An LED element is current driven.

An LED lamp is often an LED element with a driver that controls the current fed to the LED.

An LED element will develop a voltage across its terminals when current is floowing. This voltage is roughly constant and does not vary with current in the same way as the voltage across a resistive does.

A simple way to drive 4 volt LED elements from 12 volts is to connect 2 in series, giving a over all voltage of 8 volts. Then use a resistor in series with the LEDs to control the current.

Resistor value calculated from ( supply voltage - LED voltage ) divided by current

 

[ban-all-sheds]

A simple way to drive 4 volt LED elements from 12 volts is to connect 2 in series, giving a over all voltage of 8 volts. Then use a resistor in series with the LEDs to control the current.

Why not 3, and no resistor?

 

[Spark123]

Is it a constant 12v DC supply?
What is the project about, I'm guessing a turn signal induction is something to do with a car?

 

[pjcomp]

Probably should be turn signal INDICATOR.

pj

 

[JohnW2]

A simple way to drive 4 volt LED elements from 12 volts is to connect 2 in series, giving a over all voltage of 8 volts. Then use a resistor in series with the LEDs to control the current.

Why not 3, and no resistor?

Because goodness knows what current would flow!

Kind Regards, John

 

[Aragorn84]

So long as you keep under the maximum forward voltage, shouldnt the LED limit current by itself?

3 in series in a car might not work too well though, as when running the voltage will likely be nearer 14v, so likely over the maximum forward voltage.

4 or 5 in series would probably be fine though, they'd be slightly underdriven, but its likely you wouldnt notice much difference in output.

 

[JohnW2]

So long as you keep under the maximum forward voltage, shouldnt the LED limit current by itself?.

No. as with any diode, the forward voltage across the LED will remain largely unchanged over a wide range of currents. If the supply voltage is less than the Vf of the LED, no current will flow, the moment the supply voltage rises above that threshold, the current will essentially be limited only by the resistance of the circuit. If one has no explicit resistor, that will be the internal resistance of the voltage source, which could be very low, leading to very high currents.

Kind Regards, John.

 

[JohnW2]

4 or 5 in series would probably be fine though, they'd be slightly underdriven, but its likely you wouldnt notice much difference in output.

As I've just explained, that wouldn't work at all. If the sum of the forward voltages of the LEDs exceeds the supply voltage, no current will flow at all - so 4 x LEDs each with a Vf of 4V would not work unless the supply voltage was in excess of 16V (and 5 would nead at least 20V). I think you probably need to read up on the voltage/current characteristics of LEDs (or,indeed, diodes in general).

Kind Regards, John

 

[Aragorn84]

This is a datasheet for a "4v" LED (pulled randomly from RS):

http://docs-europe.electrocomponents.com/webdocs/02b0/0900766b802b0b3f.pdf

If you view page 8, you see a chart of current against forward voltage.

Does that mean that if you supplied it with 3.5v (for instance) 20ma would flow, but it wouldnt light?

That doesnt make much sense to me?

 

[JohnW2]

This is a datasheet for a "4v" LED (pulled randomly from RS):http://docs-europe.electrocomponents.com/webdocs/02b0/0900766b802b0b3f.pdf
If you view page 8, you see a chart of current against forward voltage.
Does that mean that if you supplied it with 3.5v (for instance) 20ma would flow, but it wouldnt light?
That doesnt make much sense to me?

I would call that a 3.5V LED, since the typical Vf at 20mA is 3.5V. As you can see from that curve, if the voltage falls below about 3V, the current (hence light output) becomes negligible. - so that's what would happen if you put 4 in series across 12V. If you had 5 in series (2.4V across each of them), you'd probably get no current (or light) at all (the curve doesn't go down far enough to be sure).

Kind Regards, John

 

[ban-all-sheds]

3 in series in a car might not work too well though, as when running the voltage will likely be nearer 14v

You'd think that with all the electronics in cars these days that an infrastructure with a regulated power supply would be provided.

Maybe even two - 12v and 5v

 

[ban-all-sheds]

the moment the supply voltage rises above that threshold, the current will essentially be limited only by the resistance of the circuit. If one has no explicit resistor, that will be the internal resistance of the voltage source, which could be very low, leading to very high currents.

So are you saying that if you took one of these 4v LED units you could not connect it to a low-impedance 4v supply?