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Extruder 12V rail has shorted?

Posted by floe 
Extruder 12V rail has shorted?
July 23, 2010 05:22AM
Hello everyone, I've just tried to test my extruder and I seem to have fried something in the process.

The board was working as expected, giving temperature readings via RS485. I then turned on the heater
and turned up the trimpot on the extruder controller. Immediately, the overcurrent protection of my power
supply (2A @ 12V) cut in, and now I have a resistance of about 3 Ohms between GND and +12V on the
extruder board. The logic parts still seem to run when I power it through the USB-serial cable, however,
it looks like one of the power parts has shorted out. I do have NIF5003 MOSFETs installed, so I'm a bit
surprised about that..

Does anybody have suggestions as to how to proceed?

Floe
Re: Extruder 12V rail has shorted?
July 23, 2010 08:14AM
I think you probably turned the trimpot to more than 2A and fried the motor control chips. The worst a shorted MOSFET would do is cause the heater to be permanently on. It wouldn't take too much current. The motor chips are on the 12V rail though so if shorted they could short the rail.


[www.hydraraptor.blogspot.com]
Re: Extruder 12V rail has shorted?
July 23, 2010 08:54AM
All three MOSFETs are still at around 14 Megaohm across their terminals, so they probably are OK.

However, I forgot to mention that I didn't have the motor connected yet, wanted to try the heater first.
Should I have connected a dummy load instead?

Florian

P.S. One additional observation: when I power the board from USB, the power LED glows dimly. AFAICT that shouldn't happen. Could the voltage regulator be the problem?
Re: Extruder 12V rail has shorted?
July 23, 2010 10:50AM
No you don't need a dummy load if you don't connect the motor, but never connect or disconnect the motor with the power on.

It could be the voltage regulator, not a lot else connected to 12V rail. Odd for it to go though. Check C1 and C2 are the correct polarity.


[www.hydraraptor.blogspot.com]
Re: Extruder 12V rail has shorted?
July 26, 2010 01:24AM
> It could be the voltage regulator, not a lot else connected to 12V rail. Odd for it to go though. Check C1 and C2 are the correct polarity.
Well, from the way it looks now, it's one of the motor control chips after all.. :-( I removed the voltage regulator, but the resistance of 3-4 Ohms between GND and 12V persisted. Some more measuring revealed that it's probably terminal 2B of motor driver 2: this one has close to 0 Ohms to GND and about 4 Ohms to 12V, as opposed to ~15-20 Megaohms for the other three terminals.

I guess I don't have much choice except to remove the motor driver and solder in a new one (the A3949 is pretty hard to get...).

However, I'd like to avoid this happening again. As I said, I didn't even have a motor connected at the time, and I had turned up the trimpot for at most two notches out of 11 (*). Do you have any tips what I should watch out for?

Many thanks,
Florian

(*) "This one goes to eleven." :-)
Re: Extruder 12V rail has shorted?
July 26, 2010 02:01AM
No I can't think of any reason why the chip would blow with no load. Perhaps it was already damaged by ESD, or too hot when soldering.

Are you sure the 0R to ground is inside the chip? A simple explanation would be a solder bridge on the board. The chip would then go pop as soon as you tried to use it with any significant PWM setting.


[www.hydraraptor.blogspot.com]
Re: Extruder 12V rail has shorted?
July 26, 2010 04:35AM
Whatever the original reason was, after removing the chip the extruder board at least talks to the mainboard again. The short is inside the chip, so it's almost definitely fried. I've found a microscopic solder residue between the 2B terminal and GND; maybe that was the original problem. *sigh* Total casualties: one 7805 and one A3949.

So I'm at least back on track to my original goal of testing the heater & thermistor.

Can you give me some hints as to how far/fast to turn the trimpot? What's your setting?

Right now, I can switch the non-PWM MOSFETs (B = fan, A = bed heat) from the Reprap software, although the heater doesn't do anything. When I slowly turn the trimpot up, at around position 2 the MOSFET LEDs A and B light up and the extruder board seems to freeze (i.e., no thermistor readings returned anymore). I can reset the board and turn the trimpot back down and everything is back to normal, however, I don't really know how to debug this right now...

Thanks for your help,
Florian
Re: Extruder 12V rail has shorted?
July 26, 2010 08:56AM
> Can you give me some hints as to how far/fast to
> turn the trimpot? What's your setting?

I don't use these electronics myself, but the max setting should be the ratio which gives 2A or the max current of your motor, which ever is the lowest. You can work it out by the motor voltage at that current. For example if you have a motor which takes 1.5A and has a resistance of 1.8 ohms the voltage would be 2.7V. Since the supply voltage is 12V you want the PWM about 2.7/12 = 22.5%.

In practice you can advance it very slowly until the chips or the motor start to get too hot to touch and then back off a bit.

>
> Right now, I can switch the non-PWM MOSFETs (B =
> fan, A = bed heat) from the Reprap software,
> although the heater doesn't do anything. When I
> slowly turn the trimpot up, at around position 2
> the MOSFET LEDs A and B light up and the extruder
> board seems to freeze (i.e., no thermistor
> readings returned anymore). I can reset the board
> and turn the trimpot back down and everything is
> back to normal, however, I don't really know how
> to debug this right now...

Odd behaviour, the pot should only affect the motor PWM, not the MOSFETs. Perhaps there are other shorts on the board.


[www.hydraraptor.blogspot.com]
Re: Extruder 12V rail has shorted?
July 26, 2010 09:46AM
Thanks for your advice - after some more testing, I think that the power supply is to blame for most problems. It's current-limited to 2A (which I assumed to be enough for testing), but it seems to be prone to voltage dropouts when load spikes appear. I've added a bank of 5 x 1000 uF caps as a quick fix and I managed to successfully heat the barrel to 205 C. Yay!

As for the stepper, I've replaced the fried A3949 by a new one and have gotten the stepper to at least make nasty noises. :-) I suppose that part of the problem here is a too high PWM frequency. Moreover, when I activate the stepper, there's again some flaky behaviour of the other outputs (MOSFETs) and the temperature readings start to fail. I'll try to get a better power supply tomorrow.

Thanks again,
Floe
Re: Extruder 12V rail has shorted?
July 26, 2010 04:25PM
The noise problem is because the PWM frequency is too low. It is normal for it to be ultrasonic so that it can't be heard, but apparently the Arduino library software does not allow that.


[www.hydraraptor.blogspot.com]
Re: Extruder 12V rail has shorted?
July 28, 2010 06:15AM
OK, to conclude this thread: I've got everything working now, all following problems are very likely going to be of a mechanical nature (slipping filament etc.).

There is one extremely weird glitch left on my extruder board: when I start turning the trimpot up from the leftmost position (=GND), the Mega168 crashes. This happens with the extruder software, with a tiny test program I wrote, even with the bootloader! I've replaced the trimpot and checked for short circuits for half a day without finding anything at all. The other analog inputs (e.g. thermistor) seem to work fine.

After lots of unsuccessful debugging, I've luckily found a workaround for this problem by using the ExtrusionPWM setting in the Reprap host software and not touching the trimpot anymore....

Should anybody ever run into a similar problem, I'd be glad to hear about it (and even more about potential solutions :-).

Floe
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