I've tinkered together various bits of arduino circuitry over the past couple of years, including a fermentation fridge and a (currently) a mash-temperature controller. They're currently a mess of solderless breadboards and things cabletied into toolboxes - all a bit precarious and easily knocked out of connection. So the time has come to unify into one box! I've bought a 30cm/40cm project box from maplin, and I'm planning out various 'planes' of circuitry. The idea is to have one LCD/Keypad control panel on the front to do everything with (including switching between fermentation and mash control - they're never on at the same time).
My first layer of design is the plane that will input 240v and provide three types of output...
240v through two SSRs. The maximum load here should be 10a (a 2.4kw kettle element)
12v to the microcontroller plane - which will power the arduino brain.
Variable 0 to 12v to a pump to vary its speed (max load, 1.2a)
I'm currently planning the plane as per the attachment. Can anyone offer any advice/observations?
What you should do is build a 5 VDC regulator system to run the Arduino (bypassing the on-board regulator via the +5V pin), instead of using 12 VDC at the barrel plug. This is because the on-board regulator of the Arduino will need to dump 7 volts as heat; it will get very warm, and while it will work, I personally think that you should have a larger heatsink than just the rather wimpy PCB of the Arduino. Using an off-board regulator with a larger heatsink will work better and keep the heat away from the Arduino (look at the spec sheet for the 7805 on how to build such a regulation circuit - alternatively, consult the schematic for the Arduino where a similar circuit can be found). Alternatively, and better, you can use a switching DC-DC regulator - they are much more expensive than a simple 7805, but worth it; you likely won't need a heatsink at all, then.
Finally, whatever you do on the SSR side of things, make sure you well isolate the 240 VAC away from the logic level inputs; put some kind of plastic or insulation or something over the screw contacts (if any). It is actually best to have them in a completely separate box. Depending on the loads on the SSRs, you may need to mount them to heatsinks as well (remember to apply heatsink paste/grease as well!). Also, you will want properly rated MOVs on the outputs of the SSRs to prevent any spikes (especially if they are controlling motors) from blowing the SSRs when switching.
Thanks for your advice, makes sense to me. I've redesigned (see below) with a dc-dc dropdown inline (I'm assuming I can feed this to the vin pin rather than to a barrel connector?). I've got heatsinks ready for my SSRs, but hadn't read about using MOVs so will do (I will, just to be on the safe side, but just for my own education: how vital are they if you're not using a motor?). The addition of the extra ssr is just so i don't have to swap plugs - in reality, only one of these (plus the arduino and 12v supply) is ever active at one point. The 240v supply will exist in the same project box as the logic circuits, but I'm planning to mount them on entirely separate 'planes' so they only connect at the header pins. Other than these changes, does it look ok - I might start building!
markpeace:
Thanks for your advice, makes sense to me. I've redesigned (see below) with a dc-dc dropdown inline (I'm assuming I can feed this to the vin pin rather than to a barrel connector?).
Not Vin - see the schematic:
You want to feed regulated (it -must- be well regulated!) 5 VDC power to the +5V pin, which bypasses all on-board regulation of the Arduino; Vin still goes thru the on-board voltage regulator - it is basically meant to be used when you don't want to use the barrel jack (or USB power), but you want to still use the on-board regulator.
markpeace:
I've got heatsinks ready for my SSRs, but hadn't read about using MOVs so will do (I will, just to be on the safe side, but just for my own education: how vital are they if you're not using a motor?).
Here's a pretty good document on SSRs and details:
They are mainly meant for inductive loads - so if you are controlling anything like a motor or a transformer, you'll probably want one. But - you may want to check the datasheet for your SSR first - some SSRs have such "snubber" circuitry built-in (and some use a much better device called a "transorb")...
markpeace:
The addition of the extra ssr is just so i don't have to swap plugs - in reality, only one of these (plus the arduino and 12v supply) is ever active at one point. The 240v supply will exist in the same project box as the logic circuits, but I'm planning to mount them on entirely separate 'planes' so they only connect at the header pins. Other than these changes, does it look ok - I might start building!
As a block diagram it looks ok to me; as long as you make it safe from shorts or other issues between the high and low voltage sections, it should be alright...
