Sunday, April 30, 2017

"If normal at first, measure again"

The board didn't work.

Or at least it didn't before.

My previous post was a false positive and everything started to behave really weird, with negative voltages and what not, as soon as something was connected and plugged in. The power switch was controlling a MOSFET, which in turn disconnected or connected all power lines to GND. For some reason the current flow was really wack because of this. For instance, inserting a LED in the 5V line really shouldn't reroute power from the 3.3V line and vice versa. This was a real pain to troubleshoot, but after butchering the board quite badly (#sadface) I ultimately found the problem (#happyface).

The solution simple enough - just bypass the MOSFET by shorting the drain and source. I really wish that I'd found this simple solution before massacring all capacitors. Now things are finally working correctly with the exception that I cannot turn power on and off with the switch, as I've hoped to do. But it's a minor issue and was more of a nice touch anyway, as it wouldn't be used in production.

1) Everything's looking dandy! For real, this time...

A major give-away, which I can't believe I overlooked, was that the status light previously didn't light up. It's directly connected to pin 13, so it should have blinked during boot. The fact that it never once did (before) would have told me that current was going the wrong way - or was insufficient. But I was simply assuming that I was shorting stuff when in reality it's perfectly fine to measure where and how I did. The wonders of stress...

Another issue leading to "premature optimization" was that I'm using an old Chipkit, which of course had some pins enabled that are now used for MOSFET control. This lead to having the light rows being always on for instance, but since they turn off during reset of the Chipkit they work perfectly fine.

Lesson learned? Don't put in untested parts in the schematics on the basis that "it should work, right?". The power switch was a last minute addition and a huge pain the Arizona.

Tuesday, April 25, 2017

Capacitors... Crap-acitors?

After investigating I think that the board actually might be working correctly after all (why do I sound surprised?).

A couple of issues -

* I have only tried to power the board via another ChipKit, since I haven't got a spare PSU. The only one I have is in the machine itself.

* The capacitors are "huge" and store a ton of energy compared to it's current consumption, which is more or less nil in its current state. This led me to think that the circuit was powered when not, and also the other way around - turning them on takes a while and is not instant.

* The SD-card reader draws quite a lot of power, and when it was connected (or possibly due to its connection with the ChipKit). Whenever the SD card was inserted, the 3.3V lane "died", i.e all power went to the SD card. As I said, it could also be that the ChipKit is trying to power itself from the SD card data lines etc.

* When attempting to read voltages I must have created a connection with the multimeter and supplying current instead of simply reading it.

So - I removed the SD card (temporarily) and powered up 3.3V and 5V simultaneously with a 3V battery pack. (not ideal, I know). And viola;

1) The connected power lines are active! #celebration
The LED's are protected with resistors for each voltage, so the 5V LED naturally looks dimmer. The green LED's are also diffused instead of clear, and of less brightness naturally than the power and status LED's.

The board look correct and both lines get power. Removing the SD card only instead of the whole connector also works, so there's no short circuit either.

Flipping the power switch to off yields this:
2) Everything is off!
Side note: The yellow capacitor on the SD is not connected to 3.3V and CLK, but 3.3V and GND. It just looks that way. :)

Which is exactly the way it should be - all lines are off!
When putting a LED in the 5V + GND sockets ("unconnected" test-areas) it properly lights up and turn off when supposed to. Only putting power on either line when the switch is off does not power up the LED over +5V + GND, just like it should.

So I'm thinking the capacitors are the main culprit, along with insufficient power to boot the board properly?

I guess the next step would be to arrange a PSU and proper power and take it from there. I'm still taking baby steps since I don't want to fry any components, worst comes to worst. :)

Trouble in paradise!

Still waiting for the final parts... but...

It seems there might be an issue with the power section.
It would seem that current leak from 12/5V into the 3.3V and more or less turn on the circuit even if there's just a single voltage lane (that should be off) active. It could also be that the debug ChipKit I am using, an old broken one, could be outputting current on the power status connection (which will be an input later) enough to open the gate.

As always with electric and digital troubleshooting, the hardest part is usually finding the error. In this case, most likely a well placed diode will remedy the issue. I will look into this tomorrow, but I'm pretty sure it's a minor detail.

On a side note - 
I'm certain the end result will be worth it, and there's really no other way to do it properly than to remove all the old "gunk" and re-do it piece by piece. Properly.

So close, almost there - only to be torn apart again.
It's a bit scary to dismantle the machine...

Monday, April 17, 2017

Quick update from the land of PCB's!

Almost done with the circuit board!

Just the ATX connectors that needs salvaging from other boards and the power sockets that I've forgot (?) to order. Other than that, it should be ready for a check and conversion from old to new.

I'm especially proud of the big capacitor "UPS" for the ChipKit and the accompanying power sense circuit, meaning that the board will run for a few milliseconds after the power disappear, just long enough to save data to the EEPROM and shutdown things neatly. The watchdog will still kill any lingering solenoids, but it won't hurt to properly turn them off.

1) Almost fulled assembled. Sorry for potato quality picture!