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PostPosted: Mon Aug 03, 2015 9:40 am
by Timtator
Hey guys,

Is there a reason the Parallella board doesn't have a diod?

I unfortunately received a part for my cluster that had the power wires soldered the wrong way around so that the black was the positive.

This then damaged 3 of the boards, which now won't let me power them through the mounting holes.

My dad mentioned that a diod would prevent damage if the positive and negative were applied the wrong way.

I've contacted the company that provided the wrongly soldered part which was the opposite to their instructions.

Hopefully they are willing to cover the cost to replace the 3 boards. I have now invested in a multimeter so I can be more prepared next time.

Hopefully I am the only one who received the incorrect part for their cluster.

Re: Diod?

PostPosted: Mon Aug 03, 2015 1:16 pm
by tnt
There are several ways to protect boards against reverse polarity ... each with their pros and cons.

You can mostly divide them in two class:

Series protections: In its simplest form you have a diode in series with the power line. The down side is that the diode will incur a voltage drop. The very best of them will still drop about 0.3V so first you're wasting 6% of your power _just_ for protection, and second you can now only provide 4.7v to the USB devices attached to the board. The specs says it must be >= 4.5v so you've just eaten 60% of your safety margin in one single device ... (and you still have the ohmic losses and the power switch losses to account for).

Parallel protection: In its simplest form, it's again a diode, or even a zener if you want to protect against over voltage at the same time. It's mounted so that it's reversed bias in normal use and so there is no current flow and no losses in normal operations. When the polarity is reversed, then it becomes forward biased and starts conducting _a_lot_ and the goad here is to basically make either the fuse or the power supply burn-out as quickly as possible. This is destructive in the sense that either the fuse or the power supply ends up dead (hopefully it's the fuse) but with a bit of luck it burned fast enough that the expensive electronic on the board survived and it can be easily repaired. But obviously your diode needs to be _very_ beefy because it needs withstand all that current without dying (because if the diode dies before the fuse does, then nothing protects the board and it's like there was no protection). this makes the diode large and expensive.

All in all it all comes down to trade-off ... the board was designed to cost 99$ and at that price, every $ of BOM cost count !

I'd also argue that's why there is no jumper or no easy way to provide power other than the barrel connector, because the barrel connector is a pretty safe bet to be center-positive (there are others but they're very rare !). If you solder the jumper and power directly, it's a case of "You'd better know what you're doing !"

Re: Diod?

PostPosted: Mon Aug 03, 2015 1:20 pm
by tnt
Two more things:

- you say "won't let me power them through the mounting holes." ... do they allow you to power them some other way ?!?

- Feel free to mount a diode yourself ... instead of the jumper, you soldered, you can put a series diode there if you can tolerate the USB non-compliance of the output voltage.

Re: Diod?

PostPosted: Tue Aug 04, 2015 5:36 am
by Timtator
Ok thanks for explaining that. I figured it would at the very least come down to a cost reduction.

I have no idea the extent of the damage done by the reverse polarities, as I have said I have little knowledge about such things.

The damaged boards will power on using USB, but I don't even have a 2A USB supply to use, I just used my 13000mAh Anker 2A USB battery pack just to see if it would even turn on. This is also not a recommended method of power supply though and I don't have any 2A power supplies for this to be a work around by any means.

The whole reason I bought the cluster kit was to reduce the number of power supplies I would need and as you can imagine I am very upset that the boards have been ruined.

It's just unfortunate that a mistake during manufacturing has somehow caused the wires to be the wrong colours. Mistakes happen though so I understand.

It may be worth the small increase in production costs for the next generation boards for a zener diod to be added as you explained, just to reduce risk of damage as the boards become bigger (more cores) and more expensive.

This certainly isn't how I wanted my project to go. At least I have 1 working board out of the 4 to begin my work on.

Thanks again for answering my question about a diod. Appreciate your time.

Re: Diod?

PostPosted: Tue Aug 04, 2015 5:54 am
by tnt
If they power on via USB, then the only thing that burnt is the fuse, which exact purpose was to protect the board so I guess it worked. You'll just need to replace that.

(It is a surface mount fuse though ...)

Re: Diod?

PostPosted: Tue Aug 04, 2015 8:56 am
by Timtator
Is this easy to achieve?

I'd rather the people who supplied the incorrect part fix it to be honest.

Considering I needed my dad to help me solder, I don't think replacing a fuse is going to be up my ally.