Electrical Safety

[Bryan]

On Hack-A-Day, here is the link on electrical safety. A good refresher on how one can electrocute themselves. Getting shocked from the neutral to ground, had to think about that one for a bit.

http://hackaday.com/2017/03/01/the-importance-of-electrical-safety/

https://www.allaboutcircuits.com/textbook/direct-current/chpt-3/importance-electrical-safety/

[brucehoult]

It can be done safely, but I just prefer to stay away from 240V as much as possible!

In 2011 I made an Arduino-controlled "smart thermostat" system for home, controlling a 2400W oil column heater (and later a separate fan, used when the heater was needed on hi average power).

I used the predecessor product of these:

https://www.jaycar.com.au/wireless-3-outlet-mains-controller/p/MS6142

At first I hacked into the supplied remote and attached wires from the Arduino across the manual push buttons. Later I reverse-engineered the connections those buttons made and used the datasheet for the chip inside to work out what codes were sent. Then I programmed a $2 chinese 433 MHz module to send the same codes.

The only problem with these is the control is open loop. You don't know if it really did it. I found that on average about one command in 1000 was lost to interference or whatever.

My code made a decision to possibly turn the heater on or off once every 30 seconds. The thermal mass of the heater averaged this out very nicely. Even if no change was needed I sent the previous command again anyway.

Well, the other problem is the devices aren't intended to be switched that frequently. I found they lasted about two winters (mid April to mid November) before becoming unreliable. Probably something on the order of half a million switching operations. Certainly well over 100k.

[Bryan]

How many things can you point out wrong with that example (refer to the original site) and assumptions it makes?

I am sure there is some errors and different opinions on the article, but I thought it was a good primer and education on the dangers of electricity. Can never know enough when it comes to working on mains electricity. Event the poor bird gets zapped once in awhile <g>

[james_s]

You need to use a solid state relay to control the heater, those little remote outlet controls have mechanical relays with a limited cycle life.

[brucehoult]

You need to use a solid state relay to control the heater, those little remote outlet controls have mechanical relays with a limited cycle life.

I know :-)

To be honest, they lasted a lot longer than I expected. And they're cheap enough (~$10) that I didn't care that I had to throw the most actively switched one out every two years. It would be different if I was selling the setup as a product. I also wouldn't accept open-loop control in that case.

Ideal would be if someone made remote control 240V switches with a solid state relay and an nrf2401 or whatever the equivalent is now (I haven't looked for five years). Or maybe a TI CC2533 (2.4 GHz radio with a built in 8051 you can program). I seem to remember more recently reading about a cheap Chinese wifi chip with Xtensa CPU inside?

But no one does.

There are various things sold as part of a system, whether X10 or some more recent "home automation" thing, but they are always ridiculous prices like US$50 or more for one.

I bought some mains remote controlled switches from China for very cheap. Like $2. They were advertised as 240V, 10A (and labelled as such on the device too). I tried one and it got very very hot quite quickly with my heater attached to it. I opened it up ... inside was a 115V 2A relay!!! Jeez. You might get away with that for controlling a lamp or fan or something. But not a 2400W heater.