Relay with heater (toaster)

Hi,

I'm currently making a 3D food printer (chocolate and wafer!) which incorporates a second heater, in addition to the heated bed, heat the food from above and below. We're taking the heating element for this from a toaster.

The heating element will be mains powered (240V) and connected via a relay to the D09 of the RAMPS 1.4.

I'm quite the electronics noob, so I really am not sure about how to use relays. My queries are:
1) Given that I am unsure the thermal mass will be enough for adequate heat retention, would bang-bang mess up a mechanical relay?
2) If yes, should I/is it possible to use a solid state relay with PWM?
3)Do I need to be worried about the input current to the relay?
3) Does anyone know of a 12v DC input, 240V, 5-10A output relay that would fit the bill?

Thanks a lot !

I would not try pwm the mains, for few reasons. One, its a sinusoid voltage so when you try pwm it, you dont really know what you get, you could get the zero point or you could get the peak 308v, so this is mainly why it is not done like that because the results are not confident. To change its average usually its used a triac which is fired at different angles, basically means it let only a certain part of the sine to go through it, and this way it gets reproductible and reliable results. Not techinically pwm, but still sort of a proportional control. Other reason like that, the transients at 308v and 5-10A are something to be avoided, so its better to use a zero voltage switching system. This is usually done with an zero voltage optocoupler that commands a triac, and this way it only opens or closes when the sine goes through zero, this is best way because transients are avoided. There are plenty of diy schematics on the web for it. This means bang-bang control (on-off) while the effective switch waits for the sine to go zero. Among other things i control my bed this way, being mains powered, and i am quite happy with the system, never had any issues. The opto+triac would be cheaper than an already made ssr, while being internally the same thing, but again with opto+triac you would have some more work to do. If you want to get a ssr, you get this already made, but i think it would be preferable to be "zero voltage" crossing or zvs (~switching) or some other names like that. The ones which are "non-zero" or "random" switching will produce transients, again not a good thing.

Thanks a lot for your response, NoobMan

I've had a look at the optocoupler and triac schematics and I'm still finding them a little confusing for the time being; I think I'll stick with an SSR. Does this one fit the bill?

The toaster I'm using is powered at 900W so at 230V should draw ~3A. Unless I've misunderstood the datasheet, that means I can run it without a heatsink, below 40^oC?

Lastly, I would be grateful to know if there are any other protective measures I should take to prevent damaging the relay or the RAMPS board?

Thank you!

Your opinion on that is as good as mine. At a glance i think datasheet checks out. For input side i saw something like 7mA or 15mA max. You can try driving it directly with a resistor limiter and see how that works. It has internal optocoupler hence it provides galvanic isolation from mains so in that sense is ok. But consumption wise, probably better to use it like a normal relay, to have a low side mosfet and that mosfet to be driven by the arduino pin, that would probably be a bit wiser - to minimize the impact on the mcu.

Check the schematics of soldering stations, there even are open Hardware designs, they are also opto/triac/555/opamp driven I'd also say that's the way you should go..

Btw, cool idea with the food printer... Is it for a commercial project? Sounds linke you open up a cool foodstore ;-)

Thank you both Going to be trying it out in the next few days so hopefully nothing goes drastically wrong!

Ah, thanks Yellobello - it's a slightly ill-conceived university project