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Topic: I think I fried my Arduino (Read 2049 times) previous topic - next topic

Chris7777


You can skip the story and go right to the questions at the bottom if you want.
I was playing around with a TIP-122, base being turned on by the Arduino to pull a 9v battery to power a relay. However the min power req for the relay coil is 9v and the battery was dying so just to test I grabbed a wall plug that outputs 9v 1000ma and rigged it up. To power the relay and it worked. So I attempted to swap the TIP-122 with a 2n2222a just wanted to test if it worked the same way, well I thought the rating on the 2n was 1amp turns out it was closer to 600ma, well it worked for two seconds and them pop and a puff of smoke came out of what looked like the Arduino but might have been the 2n(but it doesn't show any damage) and the Arduino smelled like burning(but no damage). However the Arduino still seems to work fine uploaded another sketch and pin 7 and both grounds I was using still seem to be working. The 2n was not so lucky tho I tested it with a multimeter and it shows a low connection(010 between almost all points) when another good 2n shows 650 only between the base and the emitter or collector. I then tested the power plug and it shows 2.65 amps out, when its labelled 1000mA eeek!

1.   So I have linked to two pictures of the Arduino in hopes someone could look at it and maybe see damage I missed?
        Large(500k) http://farm8.staticflickr.com/7204/7027743079_c9f1e0caaa_b_d.jpg
        Super Large(5.3MBs) http://farm8.staticflickr.com/7204/7027743079_5332951ec6_o_d.jpg

2.   Is it normal for a wall port to be listed as 1000mA and really be able to push 2.5Amps??( tho I did buy it off ebay and I should have checked it first with the meter)

3.   What kind of back voltage/amps can the Arduino handle on the digital pins?

Again Thanks for any help.
Chris

retrolefty

The problem had nothing to do with using the wall wart directly. The problem was you exceeded the max current and/or heat dissipation capacity of the smaller transistor which caused it to short circuit from emitter to collector long enough to explode. The wall wart doesn't 'push current' it's the load that determines the amount of current to flow and no more current flowed then the resistance of the relay coil allowed to flow. This can happen even with transistors 'rated' to flow a certain amount of current if you don't have enough heatsinking on the transistor that allows that 'rated' current to flow. A very important specification for a switching transistor is a graph normally shown in it's datasheet showing its SOA, safe operating area, which at all times must be within regardless of the actual voltage and current values being switched. This is a specification that considers the operating temperatures you are subjecting the device to.

Lefty

Chris7777

Thanks for the input Lefty
I see from this datasheet http://www.fairchildsemi.com/ds/TI/TIP120.pdf
That the tip122 I was using before I melted the 2n2222a has the SOA
but this datasheet for the MPS2222a I was using does not http://www.datasheetcatalog.org/datasheet/motorola/MPS2222A.pdf
or is it called something different in this datasheet.

I should have worded that question about the wall wart better, since its only rated to output 1000mA should that not be the maximum my multimeter can pull from it through its 10A fuse when testing it?

Would the 2n2222 burn out first and not pass any more voltage back to the Arduino's digital out pins? And in fact my Arduino is safe? If so why does it smell like smoke and the 2n doesn't?

Thanks
Chris

retrolefty


Thanks for the input Lefty
I see from this datasheet http://www.fairchildsemi.com/ds/TI/TIP120.pdf
That the tip122 I was using before I melted the 2n2222a has the SOA
but this datasheet for the MPS2222a I was using does not http://www.datasheetcatalog.org/datasheet/motorola/MPS2222A.pdf
or is it called something different in this datasheet.

I should have worded that question about the wall wart better, since its only rated to output 1000mA should that not be the maximum my multimeter can pull from it through its 10A fuse when testing it?

Your multimeter in 10A current mode uses a very low ohm shunt resistor (maybe .1 ohms?) to measure the voltage drop to calculate the current flowing through the meter. Your wall wart is rated for specific output voltage for up to a specific max current, 1 amp in your case. What isn't specified is how your specific wall result will respond if more then 1 amp is tried to be drawn from it. Some just gradually have their output voltage lower as more and more current is drawn, others may blow open a fuse like link in the primary winding making the wall wart useless. In your case current was still having to pass through the relay coil so that was the current limiting for the circuit and the transistor's failure didn't make that be more then normal flow through the relay coil.
Would the 2n2222 burn out first and not pass any more voltage back to the Arduino's digital out pins? And in fact my Arduino is safe? If so why does it smell like smoke and the 2n doesn't?

I'm pretty sure your arduino is OK, but testing the output pin with say a series resistor and led to ground and blinking it should verify it. The failure mode you experience doesn't have to mean the arduino was damaged as there was still the series base resistor to limit current back through the pin. Don't let your nose be your only test equipment, check out the functionality of the output pin so you know one way or another. I wouldn't sweat the problem too much, worst case if you ever do lunch the chip it's just a $6 replacement cost for a new processor chip, and my experience is they are pretty tough chip.

Lefty
Thanks
Chris

smeezekitty

It is not at all uncommon for a wallwart to create more then rated output current.
The current is rated for nominal voltage and ripple at that current.
You can draw more current with reduced voltage and increased ripple and I have done so but it may cause the transformer to develop shorted turns (which causes overheating) if you do it for long periods of time.
As lefty suggested, test your Arduino pins with resistors and LEDS but I think the smoke was just the transistor so very little harm done.
Avoid throwing electronics out as you or someone else might need them for parts or use.
Solid state rectifiers are the only REAL rectifiers.
Resistors for LEDS!

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