NEW SEGMENT: EEVsmoke #1

[Godzil]

By the way, the X2 capacitor is a paper based capacitor, and as it dry with time they all fail in a similar manner. All of the failing one nowadays are form device from the 80s or really early 90s. It's quite common in vintage Thomson TO7/MO5 monitor to have the X2 filter capacitor to fail like this, my Apple //c monitor had his X2 filter failing in such a way too, it's not critical as it's just a filter cap, but I used that as an occasion to replace all the electrolytic capacitors with newer one and replace the faulty X2 with a non paper X2 one, and another paper caps with a ceramic one.

It works way better now

I still have the failed X2, I should take a picture of it, it did some really nice disgusting magic black things©

(it is that sort of caps: http://uk.farnell.com/kemet/pme271m547mr04/cap-film-paper-47nf-275vac-rad/dp/2332428 )


Dave: also does huge battery failure would fill that segment? (I have a board where a NiCd battery failed in a really uncontrolled manner, it's like if a nuke happened in the device, that's absolutely impressive)

[akos_nemeth]

Washing machine drama

The ST ACS102-6T "Transient protected AC switch"(http://www.st.com/content/ccc/resource/technical/document/datasheet/64/e8/a1/ac/ad/58/41/e2/CD00083453.pdf/files/CD00083453.pdf/jcr:content/translations/en.CD00083453.pdf) on the control panel PCB of an Electrolux front loader washing machine (EWM 2500 platform http://www.4mcculloch.co.uk/images/mediator/559/599527452.pdf) was blown up probably due to a short  around the door interlock (PTC bimetal switch?).

The trace on the PCB between pin 2 of the ACS102-6T (OUT) and R3 is misteriously evaporated (why only this section?). Another interesting feature of this PCB is the rectangular fiducial-like marking (what could be a purpose of it?). Capacitor C3 is not populated on the PCB but there is soldering on the pads, maybe the designer decided not to use it, but the stencil was already made?

Regards,
Ákos

[VinzC]

The story of a mishap...

Here's one of my projects at work, of which I blew up the MCU quite stupidly. The board is a motor controlling unit that uses stall currents to stop the controlled motors. It drives up to 2 motors and has 4 SPST relay outputs. Motors are powered by an IC that I re-used from my predecessors' project. The difference is my board uses miniature OMRON H-bridge relays instead. The absolute maximum current is 23 amps, as per MEGA-FIT connector specs.

The microcontroller is an Atmel ATmega64M1 as shown on the picture. The firmware has several configuration options such as inrush delay, power on delay and threshold (stall) current. There's also a stall delay that is zero by default. The latter delay allows a motor to run on even though the stall current was reached. The configuration can be changed by plugging a USB cable and connecting it to any computer with a VT100 terminal emulator (that's the purpose of the MCP2221).

When motor currents fall within the specs even when stalled, all is well. All motors my unit drives stall at no more than 20 amps... with one exception: our sunroof motor. The latter is a rather old unit that stalls at more than 30 amps! As I didn't have time to upgrade the board for stronger currents, I decided to give it a go.

Unfortunately the rooftop didn't close even with a current limit of 20 amps. I don't know what I had in mind that day but I enabled the stall delay and set it to 1 second... I lost all communications with the unit all of a sudden. I took the board off the car and plugged it to my test bench and then... smoke!

I quickly realized the VND600, which is rated for 25 amps maximum was blown without a hint, which incidentally caused its inputs to be shorted to ground. Had I driven the VND600 inputs with the MCU internal pull-ups instead of setting the outputs directly, I would have spared the microcontroller.

That experience also stresses on the fact you need to be extra careful also designing the firmware as well. I didn't use intermediate resistors to drive the VND600 here, which would not have been a problem if I had enabled the MCU port pull-up circuitry. Also the now obsolete VND600 has no current limitation internals, which modern drivers, like the VND5xx series have.

Call it a lesson learnt ;-) .

[bktemp]

Had I driven the VND600 inputs by with the MCU internal pull-ups instead of setting the outputs directly, I would have spared the microcontroller.

I doubt it was the outputs driving into a short. The output transistors of typical microcontrollers are too weak to let any significant current flow.
More likely when the VND600 failed it put the motor supply voltage onto its input pins and also into the AVR.
Only adding series resistors could have saved the microcontroller.

[VinzC]

More likely when the VND600 failed it put the motor supply voltage onto its input pins and also into the AVR.

That, I can check, thanks for the hint.