Main Site Documentation

Easier to make generic power switch for appliances?


#1

I’ve got the daunting task of trying to connect relays to a humidifier, a space heater, and a fan. I have been thinking I need to take the switches out and wire them to my relays, but wouldn’t it be easier to mage generic power strips that I could turn on and off?

Anyone done it this way?


#2

Welcome aboard Carl :slight_smile:

For many reasons, Yes it is a better idea to hack into a cheap power strip instead of hacking your appliances.

I had an application in mind and I was going to go to a hardware store and buy some metal standard boxes and plugs and put the relay circuitry inside. This keep everything fire-safe.

Of course, working with AC power is not a joke. So if you are not 100% sure then hire an electrician.


#3

I actually started working on something similar late last year with the idea of fitting inside a standard power strip. The hard part was fitting everything inside the enclosure. I bought two or three different power strips from Lowes but none had enough free space. If you want to build your own enclosure then it’s certainly an easy enough project but I was trying to keep it in the factory enclosure. I ended up abandoning it and going with a different demo.

Is this Carl from .NET Rocks?


#4

Is this Carl from .NET Rocks?

It is.

I found a combination switch/outlet at Home Despot. That looks like the ticket.

Carl


#5

Yes, Gus, I’m definitely going to hire an electrician. :slight_smile:


#6

I’ve got a powertailswitch device that I intend to use to do this level of switching. That’s a “commercial” approach that can be a good way to affect this. Another way that you could approach this is using the RF or IR controlled power point - intercept the RF/IR that the remote normally issues, and have your device pump it back out. Both may be easier (safer) than opening a device/enclosure


#7

Welcome to the community! I love the show.

BTW, this project was the main inspiration for my build except I was trying to use beefier 10A relays.


#8

Guys -

I’ve had good luck adding relays inside an enclosed wall mount box for 2 and 4 plug outlets. I’ve found some pretty “deep” plastic boxes with enough room at my local Ace hardware. I hacked the end of an outdoor extension cord and ended up with my own custom relay controlled solution. I use the beefcake kit from SparkFun, https://www.sparkfun.com/products/11042 . I’ll post a picture later on. I can now check the pool water temperature and turn the filter on and off remotely. Looking for a cheap free chlorine level sensor to add next.


#9

I’ve never understood why people still use mechanical relays. Here’s an SSR that can switch 25 amps of current at 250V and operates on 3-30 volts. Panel mount with screw contacts. $5. Done.


#10

Pardon my ignorance … but what’s the benefit (other than perhaps size) of a SSR over a mechanical?


#11

What is the advantage of a transistor over a vacuum tube? :slight_smile:

Size, power, speed and cost.


#12

They’re basically opto-isolated transistors, so they require very little drive current and have no nasty inductive kick-back. Because of this, you can directly drive them from a microcontroller (even at 3.3V).

They can be PWMed too, as long as you de-rate them a bit so they don’t get too hot.

I wouldn’t say they’re particularly small or low-cost. The main advantage is that they “just work” and they’re extremely reliable.


#13

The big benefit SSRs bring though is in extended use. Over time, a mechanical relay’s contacts will pit with the make-and-break actions, potentially leading to a “welding” together of the contacts. None of this is an issue in an SSR.


#14

As SSR is great where you need high frequency or many switching cycles (usually go together!)

An SSR needs heatsinking and much more space than a mechanical relay. SSRs are very sensitive to over-voltage and sustained over-current.

A mechanical relay is a very compact way to switch a large amount of current. Mechanical devices (contactors) are the only reliable way to switch multiple phases or circuits within a known time. They generally don’t require special cooling considerations (almost all the heat comes form the coil).

Mechanical relays are the only way to achieve functional interlocking (for example motor reversing).

The failure modes for both are different, as well.

In my experience, cheap relays out perform cheap SSRs when reliability is important (except for high frequencies). For high speed, good SSRs outperform mechanical relays every time.