While trying to build my reflow oven I needed a way to control the power to the heating elements so that I can set the oven’s temperature. Most DIY designs use a solid-state relay – which I am to – but connect it directly to the elements. I didn’t want to do that as I felt this was more complicated than necessary but instead wanted a solution similar to that used on the EZ Make Oven where the power is controlled by a separate outlet.
Adafruit sell one but unfortunately for me here in the UK it’s only rated to 120VAC while we use 230VAC for our mains supply. The different socket types would not have been a big issue as I could have always wired up a US plug to my oven.
So I had to make my own controllable outlet but this turned out to be quite straightforward using a handful of off-the-shelf parts.
The design
I’ve chosen the SSR-40DA solid-state relay. It’s rated at 24-380VAC and 40A which provides plenty of headroom for my 230VAC 1200W mini oven. It can be driven by a wide range of voltages – 3-32VDC – so it can be connected directly to a Raspberry Pi (3.3V GPIOs) or Arduino (5V GPIOs).
I’m using a very simple design where the Live feed is interrupted by the contacts on the solid-state relay.
There is no flyback diode as the heating elements are resistive and the solid-state relay only switches on a zero crossing so there shouldn’t be any large current loops. If this was a commercial design this type of protection would needed.
I managed to fit everything into a dual surface patress box with one side housing the relay and the other the socket.
The mains feed was connected to the relay using M5 insulated ring crimps and a set of flying leads with Mini-PV connectors were wired to the input terminals. If you want to find out more information about crimping Mini-PV connectors then I can highly recommend Matt Millman’s site.
I had to use an insulated in-line crimp on the Neutral feed as I accidentally cut that wire too short. Once everything was assembled it turned out to be a compact little device.
It’s alive!
I’ve put together a short video where I’m using my outlet to control a table lamp using the GPIOs on a Raspberry Pi. A simple script was thrown together using GPIO Zero.
The video below shows a close-up of the solid-state relay as it switches the lamp on and off where you can see the corresponding LED indicator also illuminate.
All in all the parts for this whole project came in at under £20 and given how good it looks I’m very happy with this.
