Methods for Parallel-to-Series Switching of Purely Resistive Loads at 240VRMS

[Mezzer266]

Hi Everyone,

I'm working on project that will use heating elements to keep work pieces at temperature. I was going to have 2 elements in total (exact resistance still up in the air) and just going to switch them on and of with TRIACs, one for each element since the elements would be in parallel. After thinking about it more though, it would be advantageous to be able to switch between these two elements being in parallel to being in series with each other. The voltage used is 240VRMS @ 60Hz and approximately 15 to 20A.

My go to solution is to simply use a SPDT relay connected to V240p, V1, and V3 (see PSTriac image), but this is my first endeavor with TRIACs and I thought it might be fun to try to get this SPDT action with some more TRIACs. My first attempt was to put two more TRIACs in the circuit, one on each leg of what would be the relay: U3 = V240p--V3 and U4 = V1--V3. I thought it through though and I think the voltage applied at MT1 and MT2 of U3 and U4 would mess with the triggering of each, I don't think it would be from it staying latched, however, I could certainly be incorrect since I'm not familiar with this component.Additionally, I removed U2 to try and correct the issue but I'm still seeing the same problems as before. I also thought about using PMOS switches or NMOS with a high side driver but I'm not sure if there is a nicer solution.

So essentially, my question is if there is a way to get this switching action with TRIACs? If not, what could be an all solid state solution you'd consider? I should also note that this is just a little project for me and although I realize something like a relay would solve my problem I'm curious to know what other solutions could be out there besides the standard. In the photo I have excluded the isolated zero cross controller but it will be in the final design.

Mezz

[Seekonk]

Pretty dangerous.  A triac can false trigger at any time forming a big short. As the triac latches timing the firing is critical. I was called in for a guy who was switching motor direction with triacs and they kept shorting out. He ended up just putting in a relay rather than time delays. Proportionally triggering the heaters is the way to go.

[Benta]

If you want to complicate things, you've found the way. 

For temperature control, I'd go for phase angle control with a single triac. To make it better, a temperature sensor for feedback.

But I'm a KISS guy.

[ogden]

If you want to complicate things, you've found the way. 

Exactly. Very important is thermal inertia. - How fast at max power thermal overshoot of system will be reached? If it is long time like couple of seconds sec then why bother at all. One powerful switch which is changing it's state no faster than once per second and no worries

[jbb]

I suggest a) some kind of backup thermal cutout (TRIACS tend to fail short circuit) and b) just doing full-wave switching of a TRIAC rather than phase angle control.

Phase angle control offers smoother adjustment of heating power but also has more voltage and current involved with the TRIAC turn on, which will generate more electrical noise.