Boarduino 12v DC + USB 5v power

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gazzat5
 
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Boarduino 12v DC + USB 5v power

Post by gazzat5 »

Hi there

In the course of one of my projects I am powering 12volt LED strip.
I have a nice 12v 1a smps to supply this.

I control the LED's through a transistor connected to a PWM pin - similar to the motor control tutorial shown here:
http://itp.nyu.edu/physcomp/Tutorials/HighCurrentLoads

To simplify the connections I found I could supply 12v to the boarduino via the DC jack and ALSO power the boarduino from a USB source by switching the jumper

Wall wart 12v -> Bourduino DC Jack
USB Power - > Boarduino FTDI connector
Arduino VIN -> LED +
Arduino GND - > Transistor emitter -> Transistor collector -> LED -
Arduino (pwm pin) 9 - > Transistor Base
*BoArduino power jumper set to USB*

While this seems to work fine, i have found the regulator can still heat up even thought the boarduino's 5v should be coming from the usb. I wondered if this was to do with using the boarduino's ground pin through the transistor to the LED strip. (possible difference in ground between 12v and usb? or something? IDK)

Questions:
0.Does this configuration check out, or is there a major problem with it?
1.Is this configuration bad for the hardware? (I have run it this way for a few hours at a time and it seems to run ok but I'm no expert)
2. Why does the regulator run hot?

Thanks for your help :)
Keep up the good work!

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adafruit_support_bill
 
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Re: Boarduino 12v DC + USB 5v power

Post by adafruit_support_bill »

It shouldn't be a problem running it that way. But I don't know why the regulator would be running hot. There should be no load on the regulator with the power jumper set to USB. What happens if you run the +12v direct to the led strip and connect the ground back to the Arduino?

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floresta
 
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Re: Boarduino 12v DC + USB 5v power

Post by floresta »

Arduino VIN -> LED +
Arduino GND - > Transistor emitter -> Transistor collector -> LED -
Arduino (pwm pin) 9 - > Transistor Base
I'm assuming that when you say "Arduino" you mean "Boarduino".

Have you considered using a current limiting resistor in series with your LED? Another resistor to limit the base current wouldn't be a bad idea either.

The excessive collector (LED) current won't affect the voltage regulator on your Boarduino but the excessive base current could be the culprit.

Don

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floresta
 
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Re: Boarduino 12v DC + USB 5v power

Post by floresta »

UPDATE: The current issue (#269) of Circuit Cellar Magazine, which arrived at my house yesterday afternoon, has an article which fully explains why your voltage regulator is overheating. Essentially it says that when running from a 12V supply the regulator can barely handle the devices on the Arduino board itself without overheating.

Don

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adafruit_support_bill
 
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Re: Boarduino 12v DC + USB 5v power

Post by adafruit_support_bill »

when running from a 12V supply the regulator can barely handle the devices on the Arduino board itself without overheating
This I understand. But in the case of the OP, he has set the Boarduino jumper to take +5v from the USB. From the schematic, I believe that should leave the 7805 unloaded.

http://www.ladyada.net/images/boarduino ... inosch.png

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floresta
 
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Re: Boarduino 12v DC + USB 5v power

Post by floresta »

OK, the excessive current due to the lack of resistors should be affecting the 12V supply and the USB supply but not the 7805. With his present setup it is the 1N4001 and pin 9 that are being abused but the 7805 should be unloaded as you mention.

One of the output capacitors could be shorted which would cause the 7805 to get hot. He could disconnect everything except his 12V supply and see if the 7805 gets hot in that situation.

Don

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