Simple Dimmer Circuit for Led Grid

Hi Guys,
I got a hold of some of these to use for signalling. They are 12V and from testing run at upto 350ma depending on the color.

http://www.ebay.com.au/itm/10X-48-5050-SMD-Warm-White-Amber-Blue-Pink-LED-Light-Panel-T10-Festoon-Ba9s-Dome/390535947510?ssPageName=STRK%3AMEBIDX%3AIT&var=660113366197&_trksid=p2057872.m2749.l2649

The only issue is they are to bright and i need to dim them down a bit.

I’ve considered a few dimming options. The best one is most likely PWM. But i need to keep this as simple as possible as i will be installing 90 of these lights.

Adding a resistor in series could have a dimming effect but obliviously there will be a voltage drop and i’m unsure if it will just cut out if the leds are designed to run at 12 volts.

I found this resource on the subject.

http://www.reuk.co.uk/wordpress/lighting/led-dimmer-circuit/

It shows the use of a 555 timer to achieve this.

Can anyone recommend what they would consider the best approach to this?

Is there a simpler way of using a 555 timer other than whats show in the link?

Is it just wishful thinking that i could solve this by just reducing current with one resistor?

Since i need to install allot of these the less parts and circuit complexity the better. Even if that means using something other than PWM.

Thanks

Adding a resistor in series could have a dimming effect but obliviously there will be a voltage drop and i'm unsure if it will just cut out if the leds are designed to run at 12 volts.

So what are you using for current controll at the moment? You can't just connect these up to 12V and expect them to last very long.

With something like this you need a constant current supply.

Grumpy_Mike: So what are you using for current controll at the moment? You can't just connect these up to 12V and expect them to last very long.

With something like this you need a constant current supply.

So far i have only tested these direct to 12v. From what i read these things have had resistance added though it wasn't the best English - "Low Power Consumption, The power count by theoretical power of LED. Because we add the resistance to protect the LED. So the actual output power is less than of the theoretical power."

From this i was under the impression these are intended to run straight off 12V with no added current control.

OK, I can't find those words in that link, I would be much happier if you could find the resistors on those panels, is their some on the back? I would expect one resistor per run of six LEDs.

If this is the case then yes the circuit using the NE555 would be fine. However given the number of LEDs I would use a FET in place of the transistor because you loose less voltage across it. As the 555 is running off 12V then their is no need for a logic level FET.

Grumpy_Mike: OK, I can't find those words in that link, I would be much happier if you could find the resistors on those panels, is their some on the back? I would expect one resistor per run of six LEDs.

If this is the case then yes the circuit using the NE555 would be fine. However given the number of LEDs I would use a FET in place of the transistor because you loose less voltage across it. As the 555 is running off 12V then their is no need for a logic level FET.

After peeling the back off one i found there is 2 301 resistors per 6 leds. So it must be 300 ohms per 3 leds. I cant find anywhere that gives a straight answer on this so here goes.. Could i add large 5-10w resistor in series to acheive a similar effect? I know that it would be less effecient but would love to know if this is also an option.

Adding more resistance will reduce the brightness for sure :)

What rating you need (both resistance and power) depends on the current you want.

But 300Ohm per 3 leds seem a bit low of a current to start with.

And with one led, do you mean one led housing? Because know, those are 5050 leds which have 3 leds per housing (that's why they have 6 legs).

I think your success will depend on how much you wish to dim them. And how well you LED are matched.

I think for modest dimming (maybe 50%) you might get away with a series resistor or reduced voltage.

For significant dimming you will need PWM. I learned this when installing LED strips on my stairs. I needed to dim them nearly 100:1. At this dimming factor using a resistor or a lower power supply resulted in different diodes showing different brightness. And not being stable over time and temperature.

However, if you only care about the LEDs in one panel, and the LED are very nearly the same you might get away with lower supply or resistor. Or maybe you don't care if all the LED's are exactly the same brightness.

septillion: Adding more resistance will reduce the brightness for sure :)

What rating you need (both resistance and power) depends on the current you want.

But 300Ohm per 3 leds seem a bit low of a current to start with.

And with one led, do you mean one led housing? Because know, those are 5050 leds which have 3 leds per housing (that's why they have 6 legs).

It must be one resistor per 3 led housing then. The faint traces on the board show they appear to be grouped in threes. The overall current draw seems to vary depening on light colour, for example green uses 350ma and red uses 310ma.

JohnRob: I think your success will depend on how much you wish to dim them. And how well you LED are matched.

I think for modest dimming (maybe 50%) you might get away with a series resistor or reduced voltage.

For significant dimming you will need PWM. I learned this when installing LED strips on my stairs. I needed to dim them nearly 100:1. At this dimming factor using a resistor or a lower power supply resulted in different diodes showing different brightness. And not being stable over time and temperature.

However, if you only care about the LEDs in one panel, and the LED are very nearly the same you might get away with lower supply or resistor. Or maybe you don't care if all the LED's are exactly the same brightness.

The amount of dimming needed wouldnt be that much so it may be worth at least trying a resistor or lower voltage. Id estimate around 60-70% brightness is needed. I will have to do some tests then

300Ohms seems right for that. 310mA for all makes 350mA / 16 [groups] = 19,4mA. 12V - (3 [leds in series] x 2V [per led]) = 6V for resistor. 6V / 300Ohm = 20mA aka that matches pretty nice.

So they are not really pushing the leds that much because that's 20mA per housing with 3 leds in parallel. That's just over 6,5mA per led :)

Dropping the overall current for example to 100mA with a resistor would be: 100mA / 16 = 6,25mA per set. The resistor that's already there will then drop 6,24mA * 300Ohm = 1,88V. So the combined drop of the leds and resistor are 3 x 2,0V + 1,88V = 7,9V. That leaves 12V - 7,9V = 4,1V for the extra resistor @ the full 100mA. That would give you 4,1V / 100mA = 41Ohm and 4,1V x 100mA = 0,4W of power in the resistor.

I vote for PWM. If you need to make adjustments to the brightness, it will be much easier to reprogram a microcontroller or turn a potentiometer to adjust the duty cycle than it will be to change a power resistor to match the value you want.