Wrong polarity on Vfield on Mesa cards is bad.....

I just found out...... PCW is correct. Putting the wrong polarity on Vfield will kill your board.......

Not that I readily didn't know that already. I just forgot to test one more time before I connected it all up. And I'm an electronics engineer, so I should know..... But it's so much to think about that something like this is easily forgotten.

Robustly designed as these cards are this is one place I could wish for better protection. It wouldn't be too difficult to put a protection diode in there. These ports are meant to be connected to all kinds of machines. And other factors can mess with these supplies.
And the voltage loss over such a protection diode shouldn't be too much of a problem, alternatively it could be possible to bypass it with a jumper if it is critical. I might put together an external protection filter for my own use when I have bought new cards.....

I don't think I'm the first person to toast my 7i77 (and my 7i84), so if you are considering making hardware updates to these cards this is one improvement I would hope Mesa can consider implementing. The added cost would probably not be that great anyway, diodes are cheap. This is dependent on how much functionality you put in to it obviously. There are loads of functions that could be added, but basic protection against wrong polarity alone should not be too complicated (and not add much to the parts cost). Overvoltage could also be considered, but would probably ad more to cost than basic diode protection.

And for those of you that use this card..... do check the polarity of your Vfield one extra time as this is one thing that WILL toast your card if you get it wrong! I speak from personal experience....

Now to Mesa's new webshop to get new cards..... and after that wait for them to be shipped all the way to Norway.... sigh....

Anders

The field I/O cards do have some reverse polarity protection but its via a shunt
reverse protection diode.

A series diode is not used for a couple of reasons:

1. Too high power dissipation at high output loads (up to ~3A cont on a a 7I77 and 9A on a 7I71 / 7I72)

2. Diode drop (especially at low field voltages)

The shunt diode has a 3A rating and will take 20A for about 1/2 a second so
should blow a power supply fuse

Ok PCW, fair enough.

At that current the dissipation of the diodes would become a problem. And you have a lot more experience with these cards, and I'm sure you have considered all ways possible to try to protect them.

And I'm not in any way accusing you of not doing a good enough job of protecting the cards, this fault is my own fault for not checking. And it can be fixed.

My concerns was more in the line of if the external supply should experience some form of problems that caused the Vfield line to misbehave in a way out of my control I'd like to know that I'd be a bit more safe.

I haven't made up my mind yet, but I think I will try to find some ways to protect my machine a little better. This will be a little more dedicated to my use than what you'd have to build in to the cards, so I have a bit more freedom.

I don't have an issue with diode drop as my voltages are in the high range in the first place, around 25V I think it was. So I have no concerns with the 0.7V or less voltage loss from a series diode.

I don't think my setup will require that much current, so the power dissipation will not be such a problem.
Secondly I will be able to slap on a waaaay overkill diode on there and not be concerned about it whatsoever. I have plenty of TO3-P (and larger) packaged diodes rated for 30-40A continuous and more floating around. So it won't cost me more than the added time to install them. These large diodes also have the benefit of having a lower voltage drop than smaller ones as well.....

The easiest way is just to attach them on to the chassis and use that as heat sink. That way there will be no concerns about dissipation either.

I might also attach some fuses on there just to be sure. This is more to protect the existing hardware. That's not as easy to replace in case something happens.

I hope to have the new cards one of these days. Then I'll hopefully be able to continue from where I stopped last time. And this time I will have no concerns with regards to hooking it up to the mill.

Thanks for the clarification!

If anyone's interested in this I'll try to document it so others can copy it.

Anders

For reverse polarity protection a logic p-mosfet works without serious losses
Is very simple you can add it externally.

A p-channel FET works as long as the voltage doesn't drop into the linear region of Vgs. Then the FET starts dissipating power and since it is not supposed to do so it will quickly desolder itself....

A reverse diode is a good solution.

A logic level p channel mosfet goes to saturation with only 1.5-3V Vsource-gate (depends on mosfet saturation VGS & RDSon)
better than a diode it is not dissipates heat if it is logic level.

I know. I have been using that trick also, with and without hotswap/overvoltage/overcurrent controller. Linear technology makes a couple of nice powerpath controllers, for example.

But still, the 7i77 is made to be installed by professionals or at least people that know what they are doing. It is expected that they know how to connect power to the board.
So, a MOSFET with its associated components in series costs money and a little energy, are just other components that can fail reducing the MTTF, and in 999 out of 1000 cases it does not add any benefits.

The humble antiparallel diode or zener (to add a little transient overvoltage protection) is much cheaper than a MOSFET, is not in the normal power path, has only two large solder joints that have a very low failure rate during assembly and can be inspected well. It usually does the job just as fine, and when it fails from thermal overload it often becomes a short, preventing further board damage and making board repair as easy as replacing the diode.

If I was the hardware engineer at MESA, I would keep the antiparallel diode

Received the new cards and have started figuring out what happened and how to avoid it.

It turns out that It wasn't as simple switching of the polarity of Vfield that killed my cards. From previous experience I remember that the machine doesn't supply any power to the I/O card at power up. It's only after taking it out of e-stop the I/O cards get 24V power.

This has in fact been a major headache for me in the past when trying to fix the original controller. It kept complaining that the parity of the outputs wasn't correct when in fact the parity control circuit wasn't powered (and therefore couldn't report a correct parity) until e-stop circuit was powered. And this error kept me from powering up the e-stop circuit.....
The only way to fix it was to replace the complete controller.....

Anyway, what happened was I switched the connectors for input and output. Another bad design of the original controller is that the input and output use exactly the same 37 pin d-sub connectors. But the input port use pins 33-34 for ground and pin 35-37 for +24V and the output port have this the opposite way. Talk about potential for damage if you swat the two with the original controller...... whoa.... no protection to prevent damage either....

Anyway..... What happened to me is that I accidentally swapped the input and output (I know..... not a good thing to do....). So the output ports on my cards had the +24V connectors tied to ground. Not a bad thing in itself.... if not for the fact that several of the outputs from the mill was turned on (which should be expected).
The fact that these outputs now provided +24V in to several of the outputs of my cards, who had the +Vin tied to earth meant that the output driver chips now saw negative 24V from its outputs to earth.

Apparently I have now found out that that will fry them. And that is clearly seen by the crater on top of one of the NCV7609 output chips on each of the cards.....

The question is whether this means these are the only damaged components. From what I can see on the card these are the only components that were subjected to any voltage during this ordeal.

I have a friend with the needed equipment to solder SMD components. The problem now is to find a place to buy the components. After all I find it worth the risk of trying to fix my cards. In particular the most expensive 7i77 card. But also the 7i84 card should be possible to save if it's only the output devices that are damaged.

The question now then is where to buy these parts. They are made by ON semiconductors, and I have found the data sheets. But I have had no luck finding anyone who have them in stock. Mouser and all other seem to have a stock of 0, and a lead time of 25 weeks.
They are very nice driver chips though, and I'm surprised to see they are so difficult to find.

As it is now it seems to only be one of the two drivers on each of my broken cards that are fried. Looking at it now only some of the outputs were on, and they correspond to the inputs to the fried chips. That makes me hope that the other chip on each of the cards are in fact ok. And one thing I consider it to reuse the chip from my 7i84 card and put that on the 7i77 card to replace the broken chip on that card. That might work.

But if I'm able to buy the parts I'd really like to replace all 4 of them, 2 on each card. They aren't expensive either (a few $ each), so the question now is where to get them. Anyone have a recommendation on where to look?

Anders

Another question that comes to mind.....

I hope I'm correct when I assume it's ok to use the supply from the mill for +24V to power the outputs, even though it's not powered initially?

In order for this to work I need to power up the I/O processor with a separate supply (I was thinking of using the +12V from the computer power) to Vin, pin 5 on TB2 on the 7i77 and TB1 on the 7i84.

I hope I have understood it correctly by doing it that way? What I need to do is move jumper W1 on both cards to the right hand position. I have measured with a DVM that this means there is no longer contact between the Vfield inputs and pin 5 on these connectors when doing that. So I'm confident this is the correct way to do it.

But having already blown up 2 cards I'd like to be absolutely sure by asking here..... It's expensive (and a long wait) to keep popping cards because I connect them the wrong way. So this time I'm asking to be 100% sure.

If this works I have to figure out how to connect my e-stop system..... but that is a different matter.

Anders

Maybe directly from ON Semiconductor? Check the website; there is often a sample/buy button next to the part.