SSR driving heating element for reflow oven

[Kdog44]

I got a toaster oven, hopping that I would be able to turn it into a reflow oven for a academic design project. Here is the oven I bought: http://www.walmart.ca/en/ip/hamilton-beach-4-slice-toaster-oven/6000096947407
Is it safe to drive the heating elements inside directly with an SSR and mains voltage (120 VAC 60 Hz)? The heating elements each have a resistance of about 30 ohms. Thanks.     

[Psi]

Entirely depends on the SSR in question.

120V/30R = 4A
I assume it has two elements, one top and one bottom.
So you need to switch 8A.

You want a SSR that can handle at least 16A to keep everything well within spec. (and of course it needs to handle mains voltage)
Keep in mind you may need to heatsink the SSR.

Also, make sure you know what type of relay you need, DC / AC / zero crossing etc..

[Kdog44]

So the type of heating element in the oven can take mains voltage? And yes the SSR that I am using is rated for 25 AMP 250 VAC. 

[Psi]

I don't know that exact oven model but yes 99.9% of toaster ovens are just a mechanical thermostat which switches mains on/off to the heating coil.
(If you post a picture of what's inside we can confirm it is doing this.)

So yes, you just feed mains into the coil and it gets hot.
Usually they wire the two heating coils in series for 230V countries and parallel for 120V countries. (Its easier to change the wiring for different voltages than to use a different coil)

[Kdog44]

So if the coils/tubes are being driven with 120 VAC, then what function does the thermostat/temp adjust do? Is it some sort of current limiter?

[SeanB]

Thermostat is a simple control. It has a tube filled with gas inside the oven, and a pressure actuated switch in the control. As the oven heats up the gas expands, and this then opens the switch and turns the elements off. then the oven starts to cool and after it has cooled about 5C it has cooled the gas enough that the pressure drops to allow it to turn the heaters on again.

If modifying the oven leave the original thermostat inside and turn it to around 400C, so that if your controller does fail it will not heat the elements and the oven to red heat in a few minutes. You just break the one lead to the thermostat ( the thermostat is separate to the switch that selects grill or bake) and install the SSR in that lead.

[Kdog44]

I already modified the oven. Instead I have the 'live' 120VAC line in series with the elements which is in series with the one of the output terminals on the SSR. The other terminal is terminated at the 'neutral' 120 VAC line. I ran a test with the oven before I modified it and measured the max temperature the cooking chamber went up too. It was 280 degrees Celsius. So now when I am controlling the oven with a PWM signal driving the DC side of the SSR, I make sure that the oven chamber is always below 280 degrees Celsius. Is that still safe? or do I need the manufacturers thermostat in line with the elements.

[Psi]

If you connected the elements up to 120V and left it like that permanently the oven would get extremely hot. Well over its rated temp, it could even catch on fire.

The heating power of the elements is a lot more than actually needed to get the oven up to cooking temp. The reason for this is so the oven heats up quickly to the desired temp.
Because of the thermostat this excessive heating power isn't a problem. The oven simply heats up very fast and then turns off (exactly what you want in an oven)

Your SSR coupled with a temp sensor and whatever control system your using (arduino etc) replaces the thermostat control with your own control.
This puts you in charge of turning off the SSR when the temp is hot enough.

As far as safety goes. There should be a thermal fuse in the oven already. This is a failsafe in case the thermostat gets stuck on. Its a normally closed switch that opens if the temp gets above its 'trip' temp rating.
You should find it bolted to the oven somewhere. They usually look like a metal cylinder about 10mm in diameter and 10mm long with some mounting holes on the side.
You should definitely keep the thermal fuse in circuit in case your control electronics fails and holds the SSR on.

[Kdog44]

I could not find a individual component in the circuit that could be a thermal fuse. Maybe the thermal fuse was built into the thermostat unit since it was at the same temperature that the cooking chamber was at. I have been using a K thermo-couple with my DDM while running tests to make sure the temperature doesn't go oven 240 degrees Celsius

[Zero999]

If it doesn't already have a thermal fuse, you should add one, especially if you plan to leave it unattended.

With regards to PWM: what frequency are you using? I hope you're aware it needs to be a much lower frequency than the mains, in order to work properly.

[Kdog44]

My PWM frequency will be less then 1Hz.

When you say:

I hope you're aware it needs to be a much lower frequency than the mains, in order to work properly.

Are you referring that if the frequency of the PWM frequency is greater then the mains frequency, the heating elements will only get bits and pieces of the voltages in the sinusoidal signal, causing it to not heat up evenly? 

Also would a thermal fuse like this one work? http://www.digikey.ca/product-detail/en/SDF%20DF240S/317-1142-ND/1014771
The datasheet says that the fuse will act as an open if the ambient temperature goes above 240 degress C. The max temperature the oven will go is 220 degrees C. Do I need more wiggle room then 20 degress C? It is rated for 250V and is rated for the same current as the main fuse in my circuit.

[Psi]

I would probably use 280C trip for a device that runs at max 220C.

To get the entire oven up to 220C you will get hot spots at random points so you need some wiggle room.

[Zero999]

My PWM frequency will be less then 1Hz.

When you say:

I hope you're aware it needs to be a much lower frequency than the mains, in order to work properly.

Are you referring that if the frequency of the PWM frequency is greater then the mains frequency, the heating elements will only get bits and pieces of the voltages in the sinusoidal signal, causing it to not heat up evenly? 

No, it's because AC solid state relays normally have a TRIAC output and zero crossing which means they only turn off when the current drops to nearly zero and on when the voltage across the terminals is near zero, so the minimum on/off time can't be less than half a mains cycle.

If the SSR has a MOSFET output then it should theoretically possible to PWM at higher frequencies but in reality SSRs tend to be very slow to switch which will increase the power dissipation markedly.

Also would a thermal fuse like this one work? http://www.digikey.ca/product-detail/en/SDF%20DF240S/317-1142-ND/1014771
The datasheet says that the fuse will act as an open if the ambient temperature goes above 240 degress C. The max temperature the oven will go is 220 degrees C. Do I need more wiggle room then 20 degress C? It is rated for 250V and is rated for the same current as the main fuse in my circuit.

Yes that's the sort of thing you need but be careful soldering it as it may blow. I prefer to crimp thermal fuses than solder.

[Kdog44]

Also would a thermal fuse like this one work? http://www.digikey.ca/product-detail/en/SDF%20DF240S/317-1142-ND/1014771
The datasheet says that the fuse will act as an open if the ambient temperature goes above 240 degress C. The max temperature the oven will go is 220 degrees C. Do I need more wiggle room then 20 degress C? It is rated for 250V and is rated for the same current as the main fuse in my circuit.

Yes that's the sort of thing you need but be careful soldering it as it may blow. I prefer to crimp thermal fuses than solder.

What type of crimps would you recommend?

[drakke]

Have you seen this?

http://www.whizoo.com/reflowoven

[Psi]

I wonder if there's any digital toaster ovens with hidden menus where you can enable solder reflow mode 

[Kdog44]

Yes I have. That article actually gave me my original interest in this topic.

[Kdog44]

Have you seen this?

http://www.whizoo.com/reflowoven

Yes I have. That article is what gave me my original interest in this topic.