Anyone have some basic Electronics understanding?

[Dave S]

Hi Folks

I have a bit of a dilemma, and for the life of me I just can't remember my theory. I have LED's that are plumbed with their individual resistors to run on 12VDC, I want to plug them into a car to make some unique lighting, for the dashboard lighting i'm using LM7812 Voltage stabilisers to maintain a 12V constant, however, I want to use LED's in several other places around the vehicle. Would I need to have the voltage stabilised everywhere where LED's are used, or would simply vehicle voltage be tolerated? An alternator typically charges at around 13.8 to 14.2 VDC, and battery voltage would drop to between 9.2 to 10VDC.

Anyone have any ideas?

[adrianh]

Forget about the LM7812. Just stick a LM334Z 10mA constant current source in series with each string. The LM334Z operates from 1V-40V and will always put 10mA on the LED no matter what. If you drive the LED directly through the LM334Z then you would put a 4.7ohm resistor on the R and V- pins of the device.

If you wanted to push say 20mA or 30mA you would then add a transistor to the circuit so as to limit the amount of current passing through the device and direct the bulk of the current through the transistor. Keep in mind that LEDS notmally run at 30mA and when they are put in series the current requirement remains at 30mA.

Search for these terms on google to find some circuit examples: lm334 led driver circuit

[Dave S]

Thanks Adrian, I was looking at the circuitry from a voltage regulation persepctive, rather than current regulation. I very much a novice when it comes to electronics, maybe you can offer some words of wisdom here. I was looking at voltage reg. so that I can wire up LED's in parrallel, therefore allowing each to be controlled individually, as apposed to series wiring, where only a string of LED's can be controlled individually, am I correct in this assumption?

The biggest problem I see is that the vehicle voltage fluctuates between 9.2VDC and 14.2VDC, I'm not sure if parrallel LED's would be able to tolerate this voltage without regulation? I am looking into the LM344Z that you mentioned and have come up with a few circuits (internet search) that would work for me.

[adrianh]

The trick with leds is to put as many in series as possible for the avaliable (or synthesized) voltage and then to put those series strings in parallel for the available current. (I am not talking about 350mA high power leds)

So, you could put 3 or 4 in series with a current limiter. Each of these strings could then be placed in a different location and power can be drawn directly from the fluctuating supply for each string.
or
a 7812 (1Amp TO220 large case) in a central location and poser each led individually (this is highly ineffient because you will have to put a resistor in front of each led and you will quickly amass a lot of current)
or
a 78L05 (100mA TO92 case - normal little black transistor size) in series with your led and resistor
or
a 78L09 in series with your 3 leds and resistor

You see, there are many ways to "Delaminate a feline".....there is hardly ever one solution to a problem but many, some more efficient that others and some just different but no better per se.

[Dave S]

Thanks Adrian, you have removed some trees, now I see the clearing...