# Question: Is a relay board suitable a replacement for SSR?



## Navin (12/8/17)

Hey all, i'm designing a hefty upgrade for my BCS controlled panel and was wondering if anyone can see a problem with my scope of works. Every system i've looked at seems to utilise solid state relays for switching the elements that have heatsinks mounted externally for effectiveness. Relay boards, if they can cope with the heat will be less costly overall and eliminate the need to cut a hole in my control box to mount a heat sink. Is it likely that these relay boards will produce less heat than SSRs? or is that breaking the law of thermodynamics.

I was also hoping to utilise a 24v circuit for safety reasons (and possibly utilise the 24v for some solenoid ball valves later down the track). 

2 march pumps
2 2400w elements (hlt, hex) at this stage

Switching is planned to occur as follows:

BCS- toggle +5vdc outputs to Relay Board 1
Relay Board 1- upon receiving +5vdc toggles +24vdc outputs
pilot lights, alarms, switches (manual on- off - bcs auto) on this 24vdc circuit providing low voltage to the controls.​Relay Board 2- upon receiving +24vdc switches ~240vac to pumps and elements.

Alternatively Relay Board 1 is rated at 10a so could i just run everything ~240vac from it and eliminate the +24vdc circuit and Relay Board 2, but each element will draw 10a so even with a 10a inline breaker would probably recieve a lot of nuisance trips, hence the second board. I could still keep low voltage to the switches by putting them inline with the bcs +5vdc output but the pilot lights and alarm would need to be on the ~240vac circuit in this case. 

The KISS principle keeps popping into my brain and i feel like i'm over-complicating it. I'm up for any ideas on how i can incorporate a 24v circuit in another way, but is it worth the hassle? I looked at contactors instead of relay board 2 but that would be more costly and all the cheaper ones i could find require ~24vac not +24vdc.

Relay Board 1- rated 10A
http://www.ebay.com.au/itm/DIN-Rail...ule-OMRON-10A-Relay-24V-Coil-G-/372037135613?
Relay Board 2- rated 16A
http://www.ebay.com.au/itm/24V-DC-R...-Relay-Interface-Module-G2R-1-E/271854720129?

Cheers for any input you have.

(Disclaimer: I know i need to have the final product checked by my sparkie.)


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## Camo6 (12/8/17)

I imagine the service life of those relays would be fairly short if you switched them rapidly for decent lengths of time. That's the beauty of an SSR.
Instead of mounting external heatsinks you could always use cheaper heatsinks internally and vent the casing with an exhaust fan.
Personally, I think complicating the build with various supply voltages is a bit unnecessary but sometimes that's part of the fun of designing your own system.
If you build it to standards with proper circuit and supplementary protection and not cheap out on switchgear, there's no reason you couldn't utilise low voltage ac supply for contactor coils and switches. I bet every light switch in your home still has 230V behind it.


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## krusty_oz (25/8/17)

I don't think you can drive those relays straight from the BCS.
Only reason for an 'intermediate' (assuming relay has sufficient mains insulation) is where you need more current than available from the controller output to switch the relay (G2R needs 100mA at 5v). 

Can't find info on BCS website but seen another page suggesting BCScan only supply 20mA port, no problem for an SSR that needs a few mA but not enough to drive a a relay, let alone 4. 
You can get more current by using a transistor or FET. (Google switch relay with transistor, plus understand you need to need to size the components to ensure the transistor or FET is fully on or off). Will also need a couple capacitors, snubber diode, etc to manage voltage transients when triggering the coil (particularly if using same power supply as BCS). also Check the BCS power supply for its output and see if has sufficient output for the load of 4relays and any indicators

Can't go wrong using quality relays like the Omron G2R. Personally I'd make sure supplier is reputable, if in doubt an electronics supplier like RS or element14 (so you are not getting fakes) will not let you down (they also have the DIN rail sockets). Note using the socket (like pictured) or terminal lug options limit to 10a model, if you want to mount them on circuit board you have 16a option, just size the track and terminals properly.
. These are tested at 120operations/min and have mechanical lifetime into the millions of operatio
ns.


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## Boxcar (25/8/17)

SSR = optically isolated trigger with TRIAC switching. Silent. Nothing to wear out. 

YMMV.


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## garage_life (15/10/17)

I generally won't weigh in on the SSR / relay argument as there are so many variables. On the contrary, this is a great video, torture testing one of those ebay multi relay boards kicking around, quite funny and educational. I recommend this channel to anyone interested in home gamer electronics, machining, general fuckery. Hope it helps somewhat.


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## huez (15/10/17)

I use a version of that relay board from that seller in my bcs setup, but for pumps only. Actually i run a lead out and control fridges and heaters with them as well. Stick with ssr for the elements. I mounted mine on heatsinks in the enclosure and added vents. You can pick up decent ssr heatsinks off aliexpress.


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## husky (15/10/17)

On my electrical engineers advice I have oversized my ssr’s And mounted on Din rail heat sinks in an enclosed panel. Using 50A SSR’s for 20A loads because I don’t want to vent the panel. 
I would use ssr’s for PWM.


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## Mardoo (15/10/17)

That answers a build question I had. Cheers!


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## garage_life (16/10/17)

huez said:


> I use a version of that relay board from that seller in my bcs setup, but for pumps only. Actually i run a lead out and control fridges and heaters with them as well. Stick with ssr for the elements. I mounted mine on heatsinks in the enclosure and added vents. You can pick up decent ssr heatsinks off aliexpress.


Yeah, anything with PID control that's going to be switching frequently i'd go SSR. It'd be a real PITA if a relay sticks or wigs out in the middle of a mash etc. You get pretty decent life with AC switching as the voltage potential is fluctuating 50 times a second between 240 and 0 where as DC is constantly at X volts and its preferable to negative switch to reduce arcing and voltage spikes.


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## garage_life (16/10/17)

husky said:


> On my electrical engineers advice I have oversized my ssr’s And mounted on Din rail heat sinks in an enclosed panel. Using 50A SSR’s for 20A loads because I don’t want to vent the panel.
> I would use ssr’s for PWM.


Rule of thumb for Chinesium SSR is to use one double the required amperage as most of the specs are overstated unless a garanteed genuine part, whatever they are


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## TheWiggman (16/10/17)

From my limited exposure to the current ratings of SSRs the closer you are to the recommended rating the more heat it will generate under load. I have a 15A heater and run 40A SSRs. I've melted two 25A SSRs but that could also have something to do with them being a Chinese ripoffs of the Chinese products (yes, there are counterfeits of the Chinese FOTEKs out there).

Personally - being of the mechanical ilk - I do my best to avoid mechanical components because mechanical stuff wears. The relays will absolutely work but the SSRs outlast them for cycles. As Camo and husky say for PWM cycling you're better off with the solid state stuff as long as you can keep it cool. Also, being mechanically based, when electrical things cark it it's traditional and simple to lay blame on the electrical folk, making oneself feel less responsible when things go wrong which is core to the values of a healthy maintenance team.


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