Reflow oven AC cable splicing insulation and thermocouple readings

Question

I'm building a reflow oven. I know what I'm doing, I just want to verify my plans to make sure I'm not doing anything stupid before I start.

The idea is to have an arduino (powered externally) control a relay which is spliced into the neutral wire of an AC cable connected to a toaster. I chose to splice the neutral cable and not the live wire for obvious reasons (that way if you touch something exposed by accident, you're less likely to be shocked). Meanwhile I plan on putting all of the splicing under several layers of heatshrink, glue gun, and electrical tape, and then inside of a small plastic project box for extra protection.

I'm just mainly wondering if the insulation I'm using is safe enough.

Also, should the thermocouple be touching the metal grill that the pcb board to be soldered sits on? should it be in the air above the pcb board? I imagine the safest bet is to take a small pcb board that's not being soldered and attach the thermocouple to that one and then place it next to the pcb board being soldered (in order to best simulate the temperature on the actual cooking pcb board).

thanks for your help!

EDIT:

After getting several answers, I realized I made a huge mistake in trying the neutral wire. Thanks guys. I don't know what I was thinking. (I was so concerned about the insulation failing on the connection to the relay that I forgot that the heating elements weren't particularly isolated).

Also, I will definitely go with with a double-pole relay (well, I'll just get a second single-pole relay and connect their inputs together).

Answer 1

Using the relay to disconnect the neutral line doesn't really sound like you know what you're doing. You're exactly wrong in the safety department. In a perfect world, it's the hot that you want to switch. With only neutral switched, the heating coil is always energized, waiting to shock you.

Additionally, relying on a polarized electrical cord to try to make sure it's the hot you're switching has its own problems. You're better off using a double pole relay to fully disconnect the toaster from the electrical supply. Use a relay that is designed specifically to switch both legs of a power supply.

Also don't need to heat-shrink, glue, or electrical tape the wires, that's overkill and amateur hour. Hiding things in an attempt to increase safety is a sure way to decrease safety.

Use the project box, and mount your circuitry (including relay) on perf board. Simply solder the relay inline on the hot wire. Keep your low voltage relay control wiring (and arduino circuitry) physically separate from the relay and line voltage power wires. Do not let low voltage and high voltage come near each other, except at the relay, and even then, those come from different directions. Physically secure the power wires so they can't accidentally be pulled out. Be neat.

Your heating coils should be shielded from your pcb by a heat spreader, and your thermocouple should be next to the pcb, and not be touching anything. It's the air that heats up the pcb, and the air that heats up the thermocouple. You trying to bake the pcb, not broil it.

IMPORTANT SAFETY NOTE:
The electrical power that comes out of your outlets is extremely dangerous. It's not the fun-times easygoing stuff of 15 volts and below. It will surprise you, and given the opportunity, it will kill you.

From your question, you really don't seem to know what you're doing. I'm not trying to insult you, I just don't want you to die. Advice from the internet is not going to make this any safer. Sit down with someone in the real world who has worked PROFESSIONALLY with household electrical circuits and have them look over what you are doing.

Please be safe.

Answer 2

You should have your relay on the hot side. Otherwise your heating elements will be at 120V relative to the grounded chassis when off.

Your typical toaster oven will have enough space inside to safely house your relay.

If you are concerned about electrical insulation, you should have a qualified electrician inspect your work.

With do it your self reflow, achieving a consistent temperature across the PCBA can be challenging. You may find some areas of your board will overheat, damaging the parts, will other spots will have unmelted solder paste.