Fire risk for small push-pull solenoid on timer?

[Scribbler]

This is a very basic question, as I am not a techie!

I am interested in some basic information about how push-pull solenoids work. I understand that they are built as an electro-magnet, so that a small, brief electric current from a timer will trip the switch.

I was wondering first, how much power they draw when they go off? I understand that the voltage is very small.

Second, I was wondering how much fire hazard they create?

I have a timed feeder for my horse, which was built by a handyman friend with push-pull solenoids attached to timers. The solenoids run for a minute, dropping a tray of hay to the horse’s stall. A number of people have these at my barn. Here is a photo. You put hay on each drop tray, and so the horse gets fed four times over each 24 hours. You have to re-set the whole thing once a day.

http://s988.photobucket.com/user/Overom … g.html?o=1

We were having discussion at the barn as to whether these pose much of a fire hazard (huge fear for horse people) and whether the solenoids need to be enclosed to keep them free of hay. Heat or a spark could cause a fire in hay, and as you see from the design, hay can end up touching the solenoids.

I’ve been using this feeder for almost five years, and the solenoids do not get hot while they are running. Each solenoid runs for one minute in 24 hours, then the current is shut off by the timer. The only place that there is a “live” connection is the coupling that runs from the timer to the solenoid.

We were also having a discussion whether the voltage draw from the solenoids could cause a fuse to blow on the barn circuit board. Usually, no two feeders are firing at the same time. Also, if the voltage is very low, then I can’t imagine it would be more draw than a light bulb, for instance? If this is a problem, we could make sure no two timers fire at the same time.

Any fairly clear, simple on-line sources to back up your answer would also be great, to help me explain things to other barn members who don’t really understand how things work

[ghedipunk]

The solenoids really are as simple as an electromagnet with a plunger inside, and are low voltage.

To answer the question about their power draw, based on the 5V solenoid sold at SparkFun, they pull 1.1A at 5V, so 5.5W. For the 36V solenoid, it’s up to 1.85A, so up to about 67W.

To your question about fire hazard in an environment saturated with highly flammable material, the short answer is that, unless there’s frayed or damaged wiring, this setup poses no fire risk, but read on for ways to verify this.

There are two times when sparks can be produced by electronic equipment: When a circuit is being made, and when a circuit is being broken. (Well, there are also Tesla coils, but if you had one of those, you’d know it, and your stable would have burned down the day it was turned on.)

At voltages below 1,000V, it is unlikely for there to be a spark while making a circuit, and unless you’re working with some exotic gases, impossible under 327V. Based on your pictures, it looks like you’re running at 120V at the wall outlet, and that’s the highest voltages you’re seeing in your setup.

When a circuit is mechanically broken, though, there’s another chance for a spark, and there can be a spark from extremely low voltages, even from 9V batteries.

There is no spot for a circuit to be broken (mechanically or not) within a solenoid… no switches of any type. My suggestion to prove it to your peers is to buy a solenoid yourself, take it apart, and show it around, showing that there’s no point where wires sometimes come together and sometimes don’t.

The only place where a circuit is regularly made and broken is in the timers themselves, and since you’re using timers with digital displays, I’d stake my own life on them being completely spark proof, because there are no mechanical switches in them.

Again, best proof is to take one apart and inspect it. A mechanical switch should be trivial to find, and would be a solenoid. If you’re truly paranoid (and kudos to you if you are; there are lives on the line, since horses can’t open their pens on their own in an emergency), then to double check, go to a quiet room, remove the cover, and set the timer to go off as soon as you can for a full on then off cycle. Then turn off the lights and wait. If you hear a click when it turns on, or see a spark when it turns off, it’s not safe.

However, it is nearly absolutely certain that they’re using a semiconductor as a switch. These are not mechanical switches and can never produce sparks when the circuit is broken.