I'm wondering if there is a better option out there for switching AC loads with a logic level signal, than the generic relay board you see for sale everywhere? I have a timber kiln setup of which I have 4 components switched, the compressor pulling about 2A 240V and the heater about 9.5A 240v. The heater relay failed first, you can see I've coloured it black and rewired circuit to the next relay labelled H. Since then, both the compressor relay and the replacement heater relay have failed too.
By fail I mean the contacts are stuck closed when the relay should be open. (Components are running with no power to relay coil)
Both are within the 10A rating, the compressor is only 20% of the rated load so I guess the components I'm using are just junk. Without getting into switching larger relays with transistors or smaller relays is there a turn key solution that is going to work for me?
Contact welding is because of CLOSURE, the INRUSH current is the hidded relay killer. Use an SSR or an industrial CONTACTOR instead of a relay.
SSR's are great but they bleed 10mA or so when OFF -- enough to give you a solid tingle. A contactor however functions exactly as a small relay but has contacts that won't easily fry and galvanic isolation when open.
30A 240VAC CONTACTOR
This one has a 120vac coil, but are available in low voltage dc coils as well.
Compressors have motors, motors have stall currents well above their operating current. I would expect your compressor to draw a lot more than 2A when it starts, possibly more than 10A. As for the rated current of a (cheap) relay contact, don't trust it, if not actually a lie it will certainly be optimistic.
When you look at the low price of a relay module such as that one, it seems evident that the components are likely to be of inferior quality. I don't regard those ubiquitous blue relays as of any use except for experimenting with.
Folks here have disassembled them and been horrified at how tiny the contracts are. You're going to need better hardware as suggested above if you want this thing to last.
BTW, this is an Arduino forum, your controller looks remarkably like a Pi
I wondered if someone was going to pull me up on the pi xD. I'm an Arduino enthusiast, this project just suited a pi better. I could've described a hypothetical situation using an Arduino with the same problems but there you go haha.
So, I guess I knew about all these solutions but they are all going to require me to build some circuitry to support them. I'm not uncapable but time poor at the moment. There's no such thing as a relay board with fets for the coils, flyback diodes, rated for 10A etc etc? Can an SSR be switched directly from a gpio? I knew they leaked a little hence avoided but, the leakage probably isn't a big issue.
I'd love to drop something in there and carry on with the next load. Is there a MOSFET equivalent of a relay board for microcontrollers that I could use to switch individual contactors?
For anyone interested this is the end of the last cycle, the pi hosts an instance of node-red which does all the functionality and provides the interface. You can see the heater has stuck on, the vents purge a few degrees above the set point (that page not shown) hence the sawtooth curve as the humidity plummets. It's had a bucket of water thrown in it and turned off which is the nice curve at the end there.
The results have been stunning - until the relays started sticking
I've seen a number of MOSFET modules from the usual suspects although mostly they only seem to have one or two per board. Still, you could get a few and drive one of the DC contactors that @madmark2150 mentioned.
Use SSR's. You can drive them directly from any DO line. You can easly control 40A or more. Check out omrom, opto22, or mouser for pn sh24d25. 3-32v in, 12-280vac control, massive amps.
Just to update here, I ended up switching out the relay board for Opto22 SSRs, they have a 240Dxx series I got 10s for everything else and a 25 for the heater. On the bench it draws 2.1mA at 3.3v. cool gadgets.
These are the contacts of the little blue relays for anyone interested. The second photo I have bent the armature up to show the mating contact. Very small indeed!
