Fuse Protection Issue, Burned Triac

[John K]

I just had one of my newest units burn out a zone, I believe the part is called a Triac that burns out, I’ve had this happen before. The solenoid valve tied to that zone still works when powered directly to a 24v power supply. I would like some help sorting out why the damage to the controller happened despite several fuses being used.

From my understanding, the 24V power supply feeds through a 2A fuse before feeding into the system, its built into the controller. I believe the Triac is rated for 800mA (please correct me if I am wrong). I use a 24V power supply that is rated to output up to 1660mA.

For one of the 2 wires from the power supply to the controller (right before orange block), I installed a 1.5A inline fuse, in hopes of extra protection.

The wire I use has 6 zones and a common, typical 18/7 irrigation wire. I used an inline fuse rated at 1A on the common wire, which is of course after the solenoid, heading back the controller. I have 50-60 zones on this controller, so maybe 10 different fused common wires in this manner, which all tie by together post fuse and then head into the controller. My thinking is if one of the solenoids short, it would blow the fuse and the 6 zones tied to that common wire would no longer work until the fuse is fixed. No damage to the controller and I would just need to replace the one bad solenoid then all would work again.

Where have I gone wrong in this thinking? I believe the inline fuse after the solenoid is not ideal, I should place one for each zone right after the controller port and before the solenoid….is that it? Or should this still work?

Is the fuse rated to high? Should I be using 800mA fuses inline to each zone or group of (tied to one common wire)? Would that protect the Triac from burning? Also, the 1.5A fuse after the power supply shouldn’t be an issue if there is a 2A fuse inside the controller anyway…right?

OR
Is this a situation where the Triac burned out for another reason not related to shorting and fuse placements….if so how to I sort this out?

My goal with the fuses is to prevent any damage to the controller and still allow me to run 3-6 stations at a time.

[Ray]

To start, is this an OpenSprinkler or OpenSprinkler Pi? They have slightly different designs regarding where the fuse is placed etc.

[John K]

Sorry left out a few details. The unit is the newest one, OS3.2 AC. The burn out occurred on the first extension board, not the controller itself but I assume it works out the same regardless if its on the main controller or on an extension.

[Ray]

Yes, the fuse is connected inline with the COM wire so all solenoids will go through the fuse since all are connected to the COM wire. I can’t explain the burn out. The circuit already has a number of protections in place: each zone has a RC snubber circuit to absorb sudden voltage/current changes, it’s protected by a TVS diode for transient high voltage, and there is an inline fuse on the COM wire. The fuse rating should be higher than 800mA because all triacs can sustain a quite high impulse current (up to several amps) in the event of transient high current. Generally, in the event of shorting, a very high amount of current would flow through the fuse, hopefully burning it out to protect the circuit. So exactly how the traic is burned, I don’t know. Is the expander completely unusable or is it that just the first zone is not?

[John K]

Hey Ray,

Thank you for the information. Would there be a benefit to to moving the inline fuse to before the solenoid and/or changing to a lower rated fuse for each solenoid? I add them for extra protection and if I have them placed wrong I would like to fix that regardless of this situation, this is where my limited knowledge shows 🙂

The expander works except for the one zone, I’ve sent these in for repair before, looked it over the burn out seems minor, enough to stop working but not a lot of physical damage as I have seen in the past.

[Ray]

Have a fuse for each zone would take up a lot of space. I don’t know if you’ve seen the existing fuse, it’s big. The current enclosure has no space for such fuse per zone. But you can certainly add per-zone fuse yourself.

[Author]

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