i want to build a ambilight using this tutorial: amblone.com
My 12V RGB 5050 60 Led/m Stripes draw 1,2A per metre. For a TV using 4 channels for colour i need about 3 metres of LED strip. 2x 1m for horizontal / 2x 0,5m vertical.
For this setup i need about 12 Transistors: R G B x 4 channels. As a strip can have 1 metre, its about 1,2 A/RGB = 0,4A per transistor.
Which one should i use? Will BDX 53 do the job? -i can get it quite cheap. I read it should be able to run about atleast double the amount of current I need, this one has a max of 12A and a nice TO220 case for cooling.
As for the baseresistor, which value to use?
I calculated
Ib= Ic (0,4A) / Hfe (750)
then (12V-0,3V)/Ib = 21kOhm
It seems a bit high, did i calculate it right?
what i dont understand, what is the point of having a formula to calculate the baseresistor, when its way too high?
Also I read many people would use a Mosfet instead. Are there any advantages? I d say heatproduction, but then there is the much higher price per piece.
what i dont understand, what is the point of having a formula to calculate the baseresistor, when its way too high?
The formula calculates the maximum value for the resistor. You can use any value less than that.
PS: You did it wrong... it's not "(12V-0,3V)/Ib" - the Arduino pin doesn't put out "12V-0.3V"
lax123:
Also I read many people would use a Mosfet instead. Are there any advantages? I d say heatproduction, but then there is the much higher price per piece.
A MOSFET will stay cooler under heavy loads but they heat up when you switch them. If you're using an Arduino PWM output to drive them they can get quite hot (40mA output can't switch a MOSFET very fast).
If it can handle the load, a BJT is usually better for fast switching. Yours can dissipate 60W so they should be fine (with a heatsink of course).
thanks a lot fungus. Your absolutely right about those 5V at the base instead of 12V from the led supply, TY
About that baseresistor: if I pick 1k would it not increase ardus lifetime compared to 330Ohms, as less current gets drawn and by that less heating takes place?
I read some articles and then calculated that a TO 126 would not heat up more then 75°C at those 4,8Watts i need, so i guess with the metallheatsink on TO 220 i guess i wont need any addional heathinks at all, is that right?
Is it even an addional advantage for pwm that that bdx is a darlington transistor, for increased switching speed, or doesnt it matter?
Using a 1K base resistor for a "typical" NPN BJT with 5V TTL Logic levels is considered to be the de-facto standard as it ideally suited to the most commonly used NPN transistors. Most general purpose NPN's will have a Hfe rating that puts them somewhere between 150-300 or so. (BDX, being a darlington, has even more gain) While a notable exception is devices like the 2N3055 Power transistor with a whopping HFE of 5 at 10 amps, it is typical to "try" using a 1K resistor since it's likely going to have your NPN behaving just fine.
lax123:
About that baseresistor: if I pick 1k would it not increase ardus lifetime compared to 330Ohms, as less current gets drawn and by that less heating takes place?
330 Ohms won't wear it out (it's less than 15mA from the Arduino pin) but pick 1k, 2k, whatever...Darlingtons have plenty of gain.
lax123:
I read some articles and then calculated that a TO 126 would not heat up more then 75°C at those 4,8Watts i need, so i guess with the metallheatsink on TO 220 i guess i wont need any addional heathinks at all, is that right?
That's hard to say, it depends on stuff like airflow, ambient temperature, all sorts of things. You can try it but I think 75°C is still quite hot for continuous use. It might decrease the life of the transistor.
lax123:
Is it even an addional advantage for pwm that that bdx is a darlington transistor, for increased switching speed, or doesnt it matter?
Although the BDX53 will do the job, like any darlington it has a high saturation voltage. The datasheet gives it as 2V max @ Ic=3A and Ib=12mA. It will be lower at 0.4A, but probably still above 1V. So if you use a 12V supply, your LEDs will get less than 11V.
You would do better to use either a medium current transistor such as BC337 or ZTX851, or a mosfet.
Regarding the base resistor, ignore those who say you should calculate the base resistor from the hfe. The hfe given on the datasheet is measured at a higher collector voltage that you want (e.g. 3V for the BDX53, but often as high as 10V). You should drive the transistor at a much higher current than the hfe implies, in order to achieve a low saturation voltage. For example, Vce(sat) for the BDX53 is quoted at Ic=3A, Ib=12mA, which is a ratio of 250:1, much lower than the quoted hfe of 750.
