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Question about hijacking other devices gate transistors.
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Zero999:
I would not advise building the power supply from scratch. I recommend buying a pre-build power supply.

The only part I suggested you make, is the board with three transistors, switching the LEDs, because it's much easier to do, than hacking a board. The transistors don't have to be match. They just need to have sufficient voltage and current ratings for the LED strip. Heck do whatever you feel more comfortable with. If you prefer to hack, then go ahead.

The voltage rating of the LED strip can be determined by looking at it an counting the number of LEDs vs resistors. The blue LEDs have a voltage drop of about 3V, with the resistor dropping the remaining voltage. If there's one resistor per LED, then the strip is designed for 5V operation. 12V and 24V LED strip has, 3 LEDs and 6 LEDs, per resistor

The current of the LEDs on the strip can be determined using Ohm's law.
I = (VS-VF)/R
Where R is the value of the resistor. This can be obtained by looking at the resistors on the LED strip. It's normally the standard surface mount resistor code, with the last digit representing the number of zeros and the preceding digits the value, so 101 is 100 Ohms.
VS is the supply voltage
VF the LED forward voltage. Normally about 3 for green, blue and white and 2 for red. Of course this is multiplied by the number of LEDs in series, so if your strip has thee LEDs, per resistor, that means there are three LEDs in series.
That will give you the current through each series string of LEDs.

Define reasonable brightness and size?

Note that higher voltage LED strip is generally more efficient. 5V strip is poor as 3V goes to the LEDs and 2V is dropped in the resistor, so 40% is wasted. 12V and 24V LED strop, drop 3V, or 4V in the resistors, which is 25% wastage. 24V is generally a bit better because the current is lower, so there's less voltage drop along the strip.

If you're using 5V LED strip, don't worry about killing your PC. It's virtually impossible.

If you're using 12V, or 24V LED strip, it's unlikely you'll kill your PC. The driver transistors are connected to the Arduino, rather than directly to your PC, so the Arduinio is more likely to blow up, if the driver transistors fail. Using resistors in series with the gates of the driver transistors (refer to my schematic a few posts ago) it will provide some protection for the Arduinio, against the driver transistors faling.
msuffidy:
Well in the end thanks for the information. I am moving on to other things now granted the strip continues to work properly. I did assume a sort of attitude that the transistors would be in a steady ready to go state, some designs may have had that rail on a voltage control and I did not at all look out for that. That was not the case here.

The code examples are sort of hard wired for me and not really commented where you would need to know about it.
The main things are the 3 data lines have to be re arranged to the color and line, or the program does when it sends the request. Long story.
Also I like to do this: Find the weakest led of RGB. Make that 100% then find the other 2 that seem to create a white color.
Then a value of 0-255 needs to be multiplied by some number to make it a number out of 1000, where the strong led is 1000. So it is like that.

Ardunio code
http://web.ncf.ca/fs864/pickup/rgb-pwm-v3-puck.ino
Linux calling model
http://web.ncf.ca/fs864/pickup/tether-puck-no7.c
Windows calling model
http://web.ncf.ca/fs864/pickup/tether-7-windows.cpp


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