Potential DSC Damage

[EricVanWyk]

Fe_Will -

Could you send me further information on how you had mis-wired the pressure sensor? A link to the sensor's datasheet and a quick schematic of the wiring would be great. I'd like to take a look to see if it possible to prevent this sort of damage in the future.


slijin -

I'm glad to hear that you have resolved the issue, at least enough to compete.

From looking at the video, the good news is that it happens very shortly after power on. This leads me to believe the fault is within the DSC and not the 9403. It might be interesting to try again with the cable between the DSC and the 9403 disconnected.

[Fe_Will]

Originally Posted by EricVanWyk

Fe_Will -

Could you send me further information on how you had mis-wired the pressure sensor? A link to the sensor's datasheet and a quick schematic of the wiring would be great. I'd like to take a look to see if it possible to prevent this sort of damage in the future.

I really don't have either... It was a KoP pressure sensor and the 5V wire was switched with the signal on the sensor side. The initial wiring was done Thursday at the Autodesk Oregon Regional, a hasty fix for sure. At first we thought we had a chassis short. During testing we discovered that the DSC was bad. We replaced it and continued looking for our perceived short. After burning out DSC #2 we started looking at what was going on with that part of the system. A few changes later we mounted #3 and turned it on. Everything was fine until the pneumatic system started charging and then we melted that one.

An alum from another team was able to solder the contacts back onto the best board of the three, we ran with it for two events after rewiring the sensor. The other boards were damaged beyond repair and disposed of. Good thing we had spares and someone who was able to fix the one we had. Unfortunately, our main concern was competing and not further testing the DSC.

[Kevin Sevcik]

Eric,

The pressure sensor is a Nason SM-2B-115R/443. It's just a pressure actuated switch, so it's nothing more complicated than a NC dry contact.

I'm not sure if I understand the mistake Fe_Will made in wiring the sensor. Standard wiring is signal to one side and common to the other, as with a limit switch. Reversing the connector where it plugs into the DSC wouldn't cause any problems. I think he's saying they wired it by chopping the end off a standard PWM cable, and they accidentally wired red and black to the switch instead of white and black. I assume a short from 5V to common would damage the DSC in some fashion, yes?

[EricVanWyk]

Originally Posted by Kevin Sevcik

I assume a short from 5V to common would damage the DSC in some fashion, yes?

Thanks for the clarification.

Usually, it would be fine. The 5V supply will detect the short and shut itself down. Periodically it will wake up and try again. I don't remember the exact numbers, but with a dead short I think it averages out to 100-200mA (don't quote me on that). I've left the supply shorted out for hours at a time on the bench without issue.

There is a "danger zone" where it doesn't draw quite enough current to detect the fault. In a worst case scenario, a specially tuned resistance could draw up to 5 Amps continuous. Maybe the sensor + wire path has exactly the wrong amount of resistance?

I'm still mildly surprised it caused permanent damage. 5 Amps is a lot, but there is a solid connection to the power and ground planes to wick away the heat. I'm wondering if bonus current is being added to the circuit - maybe from the compressor?

[Fe_Will]

Originally Posted by Kevin Sevcik

I think he's saying they wired it by chopping the end off a standard PWM cable, and they accidentally wired red and black to the switch instead of white and black. I assume a short from 5V to common would damage the DSC in some fashion, yes?

Yes it was a PWM cable that was used, is this not common practice?

I can't speak for the DSC, as far as I am concerned it is plug and play. All of the boards had melted resin on some or all of the board. It pooled in some areas and was dry in others in the severe board. In the least damaged case it had a wavy, non-uniform appearance.

[Matt Krass]

Originally Posted by EricVanWyk

I'm still mildly surprised it caused permanent damage. 5 Amps is a lot, but there is a solid connection to the power and ground planes to wick away the heat. I'm wondering if bonus current is being added to the circuit - maybe from the compressor?

Don't you mean Wyk away the heat?

Joking aside, 5A is a fair amount of current, if it was pulling that much for a long time (in the last few days of build our poor robot was pretty much on nonstop for 15 hour stretches), it seems feasible that the board would eventually start suffering damage. The impression I got from this post:

During testing we discovered that the DSC was bad. We replaced it and continued looking for our perceived short. After burning out DSC #2 we started looking at what was going on with that part of the system. A few changes later we mounted #3 and turned it on.

was that there was a fair amount of time between power up and the actual damage, which may have also been done incrementally?

The 5V regulator for the DSC is the TPS5431DDA, correct? As I see here:
http://www.ti.com/lit/ds/symlink/tps5431.pdf
That is a nifty buck converter, nice high speed switching frequency too
It also has a thermal shutdown, but that is at a (typical) value of 162C, which is pretty high, I don't have the figures on the PCB/resin material to estimate how hot it is getting, nor what temperatures it will tolerate, so could you tell me if that kind of failure is feasible with long term exposure to those temperatures?

If so, perhaps using the (currently disconnected) enable line on the converter with a temperature sensor on the board near the regulator and a comparator to provide a much lower thermal shutdown? I can't imagine our typical operations heat the system that much (but, again, I don't have all the numbers here). Just throwing ideas out there.

Matt

[EricVanWyk]

Originally Posted by Fe_Will

Yes it was a PWM cable that was used, is this not common practice?

I can't speak for the DSC, as far as I am concerned it is plug and play. All of the boards had melted resin on some or all of the board. It pooled in some areas and was dry in others in the severe board. In the least damaged case it had a wavy, non-uniform appearance.

Can you send me some high res pictures? I'll PM you my email address.

Originally Posted by Matt Krass

Don't you mean Wyk away the heat?

Groan...

It rhymes as "I like VanWyk", if I can steal the campaign slogan. Re-rhyming that to wick seems a little too... personal.

[Al Skierkiewicz]

Fe,
We need some pictures but I suspect there is more to this story. The PWM cable alone cannot support enough current to melt anything on the DSC. It would burn and smoke long before anything on the DSC let alone melt the solder on the pins. You are using a Spike to drive the compressor and not trying to drive it direct from the DSC correct?