question re: potentiometers for an auto install. looking for knowledge

[jousley]

Hey guys and gals. I did some google research, but didn't find what I was looking for - thought i'd go straight to the source - electronics and not auto.. Here's what i'm looking to do:

I'm going to be connecting 4 of these LED components in series: http://www.superbrightleds.com/more...s-rectangle-flexible-accent-light-module/832/ they will be the red ones - so 20mA.
I am going to connect them to a switch in my f150 - and when the switch is activated, it will go through a potentiometer, giving me the ability to dim them when I want. when the switch is off - no power.. lights out. switch on - they are on - and can be dimmed from full to zero. simple. It's an auto - so 12 V DC

Here's my questions:
1. some sites have indicated that I need a 1/4 Watt resister installed between the lights and the pot... correct? seems curious as I thought the pot is the increasing/decreasing resistor? let me know what i'm missing.
2. What "size?" "ohm" pot do I require? I ask this because I'm not sure that if I wire these up in series means that I have to add the 20mA times 4 (4 lights)? or is it just 20mA total as they are in series? either way, I'm not good with the math. What ohm pot would be sufficient. and this is the most crucial question I have as I have read that if you choose... unwisely... it could be a potential fire hazard.
3. Lastly, for added protection, I would like to add a fuse inline. I assume that inline fuse goes between the auto's power source for the lights (the 12V lighter (always on)), and the switch. please let me know if I have this wrong.

As always I appreciate all knowledge and other ideas that may work with what I'm trying to accomplish. Thanks in advance.... and this is my first post... glad to be here.

J.

 

[davenn]

Hi there jousley
welcome to the forums

there's been many posts like yours lately

you cannot properly dim LEDs from off to full using just a pot

generally to do it properly you need to use PWM ( pulse width modulation)

have a look on this site scroll down a little way to see a circuit for PWM control of a motor then a set of LEDs

cheers
Dave

 

[jousley]

thanks for the link... unfortunately, it's a bit beyond my comprehension. I googled the 555 chip. and was hoping someone could explain it to me operationally. how does one purchase this, completed in a circuit board where I just have to worry about where I need to place the dimmer nozzle in the truck... I may be in over my head.. i'll keep searching... and anyone who can see where i'm at, and explain what I need to know... as always... its appreciated.

 

[(*steve*)]

Sure, get on to eBay and search for "PWM LED dimmer". There's heaps there.

This is one that might be suitable for you. Note that it ONLY does the PWM, it does not limit the current to the LEDs.

Those modules you have *seem* to have integral current limiting (well, series resistors in any case) so you probably don't need anything else.

edit: NOTE: you'll have to connect those modules in series. They're set up for 12V. 4 in series would require 48V.

 

[jousley]

Wait im sorry... in series you say would require a 48V PWM LED dimmer? or did you mean in parallel. sorry for the dumb question.
J.

 

[(*steve*)]

No.

The LED arrays are designed for 12V

Your power supply is 12V

The dimmer works at 12V

What you need to do is connect the LED arrays in parallel so that they all get the full 12V, not a fraction of it.

 

[jousley]

got it... thanks so much for the responses.

 

[jousley]

OK guys - I am almost there. purchased the PWM and want to hook it up to a switch. so the power will come from the battery, to the switch, to the PWM - so I can turn on the led's when I want and then dim away.. Just some questions on the wiring.
The PWM has IN + & -... and OUT + & -. Here's what i'm thinking:
power from the battery goes to the + on the switch. ACC (middle prong) on the switch goes to the IN + on the PWM. Led's + wires go to the OUT + on thePWM, and the Led's - wires go to the OUT - on the PWM.
so does the IN - on the PWM just go to a chassis ground? or do I connect the ground prong from the switch to the PWM IN -? but this doesn't seem to make sense to me.
Please let me know as I"m just about there.... appreciate all info and hope it makes sense.
J.

 

[(*steve*)]

Take a wire from the +ve of your battery (or wherever this comes from -- perhaps the cigarette lighter socket) and connect it to one side of the switch.

Take a wire from the other side of the switch and connect it to IN + on the PWM controller

Take a wire from your -ve -- this may be chassis ground. Connect this wire to IN -

Connect a LED module across the OUT+ an OUT- connections. If they have a red or + marked lead, this goes to OUT +. the black ot - labelled lead goes to OUT -

have the brightness turned all the way down when you power it up an turn the intensity up slowly to ensure everything is OK.

 

[jousley]

ok makes sense.. so power to PWM IN+.. chassis ground to PWM IN-.. and the led's + & - to the PWM OUT + & -...

thanks again steve.