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Topic: Exploded my first capacitor! :-P (Read 5285 times) previous topic - next topic

kjkrum

I just blew up a Ladyada mshield (http://www.ladyada.net/make/mshield).  I'm pretty new at this, and I was hoping someone could help me figure out what I did wrong.

Before attempting to run a motor, I first built a circuit with four LEDs connected to the terminals on the shield and grounded through a 300 ohm resistor.  My Uno was powered by USB, and the shield was powered from the Uno.  I wrote a sketch that stepped once every two seconds.  The LEDs blinked in sequence as expected.

Then I disconnected the LEDs from ground and connected them to the motor coils.  The motor is a M42SP-1B (http://alaruex1.free.fr/Documents/Recup-datasheet/moteur_pas_a_pas/m42sp-xx.pdf).  I grounded the motor's common leads through a 2K resistor.  The Uno was still powered by USB.  I removed the "power" jumper from the mshield and connected a 24V wall wart to the power terminals.  I booted the Uno and the mshield's power light came on, which I thought was strange because the jumper was removed.  The sketch was running; the LEDs were blinking faintly, which I also thought a bit odd.  Determined, I plugged in the wall wart.  Almost instantly, C8 exploded.

Fortunately the Uno does not seem to be damaged.  At least, it can still blink its LED on pin 13.

Now that I think about it, I probably should not have connected the LEDs in series with the motor.  I doubt they can handle 120mA for long.  I should have put them in parallel with the motor coils, each with their own current-limiting resistor.  But even if the LEDs failed open or short, I don't understand why that would make the capacitor blow.

Any pointers?

dc42

#1
May 04, 2013, 09:10 am Last Edit: May 04, 2013, 09:13 am by dc42 Reason: 1
The usual reason for electrolytic capacitors to blow up is because you have connected them the wrong way round, or you have exceeded their working voltage. C8 on that motor shield is connected across the motor power supply input to the board and is rated at 16V according to the schematic. So your 24V wall wart was too much for it. The other possibility is that you connected the 24V input the wrong way round.

The L293D chip that the board is based on is good for up to 36V, so it is surprising that they chose a 16V capacitor for that component. Replace it with a higher-rated capacitor, and the board may work again.

Alternatively, send it back. The adafruit page says it can drive motors up to 25V, and with a 16V capacitor, clearly it cannot.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

Grumpy_Mike

Quote
Almost instantly, C8 exploded.

Which C8, shield or arduino? Is there a schematic to the shield I couldn't see on in that link.

Quote
I grounded the motor's common leads through a 2K resistor.

Why?
That sounds wrong.

It is not clear but it sounds like you connected the Arduino to an unpowered shield. - Bad

Quote
I removed the "power" jumper from the mshield

Did that include the ground, if so that could have done it. The arduino would have been parasitically powering the shield and so the ground would have been high. So when you plugged in the wall wart you had a floating ground and so applied 5V + 24V to the shield. This might have been too much for the capacitor. What was the voltage rating of it?

kjkrum

@dc42: You're right.  The surviving C7 capacitor on the shield is marked 16V.  Where are you seeing the schematic?  The parts list actually calls for 25V caps for C7 and C8.  I'll try replacing them.  (I paid $10 for the shield... not worth shipping anywhere.)

@Mike: If I understand correctly, the motors can be powered by +5V from the Arduino (by leaving the jumper on) or from the shield's aux power terminals.  With the jumper removed, I guess the shield would still use the Arduino's 5V for its logic.  The aux power would only power the motors, switched by the H bridges.

Thanks to both of you.

dc42


@dc42: You're right.  The surviving C7 capacitor on the shield is marked 16V.  Where are you seeing the schematic?


I followed your link to http://www.ladyada.net/make/mshield, followed the "Full kit available for purchase..." link on that page, clicked on the Downloads tab on that page and ended up at http://www.ladyada.net/images/mshield/mshieldv1-schem.png.

Better change C7 as well, that one is also connected to the motor power supply. I suggest you use 35V caps rather than 25V, since your 24V wall wart will probably deliver well over 24V when it is lighty loaded.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

Coding Badly

(I paid $10 for the shield... not worth shipping anywhere.)


Where did you buy it?

MarkT

I suspect if you report this to Adafruit they will help you out - I'm sure they will want to
rectify this problem.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

kjkrum

The shield is a Chinese copy.

I have some ceramic disc caps marked "821J", but if I'm reading the right datasheet, they're rated 50V but less than 1uF.  Looks like I'm going to Radio Shack today.

runaway_pancake


I just blew up a Ladyada mshield (http://www.ladyada.net/make/mshield). 



The shield is a Chinese copy.


Gasp!
"Who is like unto the beast? who is able to make war with him?"
When all else fails, check your wiring!

kjkrum


Gasp!


I'm sure the Chinese make all the components of the "official" version, and probably assemble it, too.  (Along with every other electronic device I own.)

Grumpy_Mike

Yes they make the parts but when it comes to putting them into a product they are way behind. I have dealt with Chinese manufacturers and you have to watch them all the time.

In there culture they think it is better to tell you what you want to hear rather than tell you the truth. I have had incorrect inductor values fitted in a product that sold for $25,000 each and they refused to admit that they had done it. In short there quality control ethic sucks. Which is why you got insufficiently voltage rated caps from them.

Coding Badly

The shield is a Chinese copy.


Which makes this a terrible lie...

Quote
I just blew up a Ladyada mshield (http://www.ladyada.net/make/mshield).


You did not blow up something made by Adafruit Industries.

Now that you've given the correct provenance the advice is straight forward: stop wasting your time.  You have already wasted considerably more than $10 of time dealing with what is clearly a defective device.

kjkrum

You have already wasted considerably more than $10 of time dealing with what is clearly a defective device.


Time spent learning is never wasted.  And the board I have is "defective" only in the sense that it was built exactly to the faulty (or, to be generous, perhaps simply outdated) schematic currently published by the designer... whose site, incidentally, directs DIYers to many of the same component importers where I buy my stuff.  I've seen the full gamut of manufacturing quality that comes out of China, and I'm pretty confident that the board I have is built to spec with quality parts and workmanship.  It's my own fault for not catching that it was built to an older spec.

kjkrum

I (quite inexpertly) soldered in some 35V 47uF caps, and everything's fine.

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