RGB led strip mood lighting - what value pot ?

[Amonra]

I will be using one of each colour ( red, green & blue ) of these: http://www.hebeiltd.com.cn/?p=led.strips - str1 type
to create an RGB mood light sort of thing.

I intend to feed the strips from a 12V SLA battery and was thinking of using a pot for each colour strip to be able to vary the intensity of each colour.

However i have no idea of what value in ohms the pot should be to be able to vary the brightness smoothly and relatively accurately from off to max brightness.

I was guessing a 1K ohm linear pot should do the job but this is just a very rough guess.

any help would be greatly appreciated.

Thanks

 

[Skibane]

Since the specs indicate 120-240 mA of current consumption, you'll need to pick an actual value. Let's call it 200 mA.

Supplied with 12 volts, 200 mA implies that the LED strip has a resistance of 60 ohms (12 / 0.200 = 60).

So, if you put a 60 ohm pot in series with the LED strip, half of the voltage would be dropped across the pot (at its maximum resistance setting), and the other half (6 volts) would be applied to the LED strip - which would make it pretty dim.

With a 100 ohm pot, almost 2/3rds of the battery voltage will be dropped across the pot, with a little more than 4 volts available for the strip - probably enough to completely extinguish it.

So, my suggestion would be 100 ohms. Ideally, your pot should be rated for several watts, in order to keep it running reasonably cool. Wire-wound would probably be a good choice.

 

[Brlux]

Usualy RGB is done using PWM. I think you will have a hard time geting the range of intensity and and precision of contrloe especially near the brighter end of the spectrum with a linear or audio pot. I would check out this website
http://www.rgbled.org/
You can get a rather inexpensive RGB controler with a serial interface for PC controle. You will also need to buy/buid a mosfet driver for being able to switch a higher voltage and current.

For a simple 555 based pot controled PWM check out this Schematic. I have made a slightly modified version of this design which gives a 1.5Khz refresh rate so that it is less noticable to the eye and I am actually able to go from 0-100% intensity. I put a picture of it in this thread.
Though I have never actually seen one I believe this 3 chanel PWM kit from Quickar is of a similar design to the original schematic.

 

[Amonra]

well actually the strips are configured in a series paralell combination and each series set has a small resistor so i dont think that the pot will get such a big load

 

[dk0013]

Check this out:


http://cgi.ebay.com/3-CHANNEL-150-WATT-PWM-RGB-LED-CONTROLLER-KIT_W0QQitemZ260103430292QQihZ016QQcategoryZ66954QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

You have to assemble the parts but its PWM controlled.

If you want to spend some money to have something really cool, go with an Arrow 3 channel controller:

http://www.arrowled.com/products_arrowmax.html

They sell for $100 + shipping.

And Lamina Ceramic Atlas RGB LEDs

http://www.laminaceramics.com/docs/AtlasDatasheet.pdf

 

[AndyTiedye]

Added to LED Fixed Lighting Index

 

[Skibane]

well actually the strips are configured in a series paralell combination and each series set has a small resistor so i dont think that the pot will get such a big load

The SPECS say that your LED strips draw 120-240 mA, which is not a small load - certainly enough to smoke a small pot.

 

[Amonra]

these RGB "modules" are to be mounted to the underside of restaurant tables. i have to build 20 of them and im on a bit of a tight budget so $100 per controller is definitely out of the question. i was looking for something closer to 5 - 8 bucks which is why i opted for the pots.
accuracy is not paramount just as long as i have basic, decent mixing possibilities, no need for 16 million colors.

 

[Ken_McE]

these people also do 12 V LED light strips, and they come potted too:

http://www.acolyteled.com/

 

[Wim Hertog]

Again, the chromoflex will do just fine. One controller (35 euro) can drive all the LED strings as long as you know how to modify the output (few extra FETs, that's all).

I don't work for the company that makes the chromoflex, but I think it's an outstanding product and the people over there are ready to answer any question you might have...

Potentiometers wont work very well as most of them are not designed to control large currents. You'll notice that after some time they'll start to become very "noisy" (=LEDs will flicker, even when the pot is left untouched). You can however use them to bias a transistor and use the transistor to control the current. Slightly more complicated, but also better are 555 based circuits with variable duty cycle.

 

[wakibaki]

I think you may find that this comparatively simple application nevertheless requires some professional-level expertise to solve in a cost-effective fashion.

Why not offer 60 bucks for a 5 dollar circuit design? That would still meet your 8 dollar budget. Perhaps you could get some bids going.

w

 

[Amonra]

i will need a controller for every table as they will be set in different colors, for a total of 20 controllers. therefore i cannot have one controller to control all of them.
i cannot afford expensive controllers as i have a very tight budget for them.
i am not an electronics expert and i have to have them finished in a week.

these are the reasons why i chose to go for the simple 3 pot method, although not as good as some of your suggestions (for which i thank you just the same.)

Therefore unless there is an other equally simple, fast and cost effective method, i only need to know the pot values ( ohms and watts ) for the three pot design in this application .

 

[Amonra]

i received the led strips today and assembled one set to do some tests.
i started with 100 ohm pots, 220 ohm pots and 500 ohm pots and found that none of them dimmed the strips enough.
so i then jumped to 1K ohm pots and found that they did the job and reduced them to almost off. the only thing is that after about 50% brightness, the brightness goes up really fast with very little turn which i expected. nonetheless i can mix about 15 distinct colors and another few shades of them with this setup which is enough for this application.
ill try to post some pics later.

 

[Amonra]

I managed to find a 1K linear pot brand with a relatively smoothish curve which improved the accuracy and colour mixing.
All the tables are now finished and ready to go and i must say that im quite happy with the results here:

http://www.candlepowerforums.com/vb/showthread.php?p=1986094#post1986094