Let the smoke out. Requesting input on repair

[denney]

Hello all,

I built up the semi-assembled OpenSprinkler v2.1u and expansion module back in April 2014 and have had a great experience since then.

This year, I was adding more valves and inadvertently had a short between common and a valve signal wire when I re-applied 24VAC power.
The short was present on the expansion module but ended up backfeeding to the main unit.
I didn’t get power disconnected before smoking both the expansion unit and, I believe, the main unit. The short has been remedied but I don’t get any response from the OpenSprinkler display. I still have 24VAC from the power source, so the supply seems to be ok.

Given this scenario and someone familiar with the circuit diagram, is it easy to know which component(s) would have been fried? I’m hoping that I can replace only these components and not have to replace the whole unit. Also happy to ohm out any components to help in the troubleshooting process.

If it’s not obvious as to what would have fried, I can study the schematic more closely. (I don’t have it on hand but I believe I’ve seen it before).
Just thought I’d check as this might be a common (pun?) doofus mistake.

Thanks much,
–Denney

[Mike]

I’m not a hardware expert, but I’d start with looking at the fuse (if there is one) and then looking for any visibly damaged components.

Mike

[denney]

Thanks Mike! I’ll open it up tonight and see how things look. It’d be great if I blew fuse(s) but I’m not terribly hopeful as the units actually smoked.
Unless it was a fuse that would smoke when blowing 🙂

[denney]

Got to open things up this weekend. Just wanted to reply back in the event it might help someone else.

First, I was wrong about it backfeeding; I shorted/burnt the triac for zone1 of the expansion board. This seems to have only burnt the fuse on the main unit (which sourced power to the expansion board…no backfeeding).

I applied USB power to the main unit and it tries to start up, so that’s a good sign.

I shorted out the fuse and ohmed out the 24VAC pins…got something in the 6-7 Mohm range, so there are no shorts there. This was done with the expansion board connected but with the valves/outputs disconnected. Reading should be retaken once the fuse is properly replaced and outputs are hooked back up, prior to applying 24VAC.

I placed a digikey order for some spare fuses and triacs. I suspect I could just avoid zone1 on the expansion board but it shouldn’t be too hard to replace the burnt triac.
Triac digikey partno: 1740-1007-1-ND
Fuse digikey partno: F4985-ND

Attached is a photo of the expansion board and burnt triac.

I’ll try to report back in 1-2 weeks after things are (hopefully) back up and running to confirm the above.

Attachments:

1. 

   expansion-board-cooked-triac.jpg
   

[Ray]

Looks like zone 1 traic is damaged. It’s fairly easy to replace with a soldering iron. Of course if you have a hot air gun that’s the best to replace surface mount components. If you don’t have a hot air gun, you can use a diagonal cutter to carefully cut the three legs on the right hand side, then use a soldering iron to remove each leg and the main body. The solder a new triac.

The replacement parts you ordered are fine. In particular, the triac is BT1308W. A suitable substitute is Z0103MN. It’s also possible to solder a through-hole version (MAC97A6 or MAC97A8 are both suitable substitutes) — just have to figure out the correct corresponding pins and solder through-hole legs to the surface mount pads.

[denney]

Thank you for the reply, Ray!
I got the parts replaced last night (my family is lucky enough to have a nice iron which makes it easier).

The triac on the expansion board has the 3 pins as well as a larger pin on top…perhaps a mechanism for sinking heat?
It looks like the larger pin and the center pin are connected in the part. (page3 of the datasheet)

When we removed the burnt triac, it appeared that the pad for the center of the 3 pins was gone/vaporized. However, the pad for the larger mount/heatsink pin appeared to be fine. The connection/trace to the output pin also looks to be intact. So I think even with the smaller/secondary pad gone, it should still be fine?

In my case, I really only need ~12 outputs, so I could just avoid output1 of the expansion board if it doesn’t work.

I hope to get it hooked up sometime this weekend to try things out.

[Ray]

On the triac, the large pin on the top and the middle pin on the three-pin side are internally connected. So if the large pin pad is still fine, then it should be good. If the other PCB traces are gone, you will probably have to figure out where those traces are connected to, and use a wire to repair the connection.

[Author]

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