www.TK560.com

anakinjay · Newbie Joined: 22 Dec 2006 Posts: 12

Ok,

Finally getting ready to order the parts for my oven...

I'm trying to make a two circuit version so I can spread it across two breakers...

So (110v 1200 watt) x 2

using these calculations,
http://tinyurl.com/yws7bq

I believe it says I'd need to have 3 inches of coiled nichrome wire and 5 terminals for each circuit.

Is that correct?

What about the other little ceramic things... does it matter how many? or is that just more for keeping them in place?

To squeeze two circuits into there, how would you guys best recommend I layout the spirals? 3.5 spirals per circuit? Should I double up the 1" outter spiral with another one close to it from the other circuit?

And there shouldn't be anything wrong with me running two extension cords to two different breakers... right?

Sorry if these seem like dumb questions, but I just want to get everything right.. I've got the tank and the pump, and after the oven is situated, it seems like it's all downhill from there Smile

Thanks!

cod · Posted: Wed Oct 31, 2007 6:47 pm Post subject:

no, i dont think you are reading the right box. I see 53.778 coiled inches if you are going for 1200 watts,110v,4 segments.

thats a very long length of coiled wire.
I did something similar(2 circuits, 2 extension cords which is fine as long as the cords are rated for the amps),

But I reduced each of the 2 circuits to 2 segments(instead of 4)- I don't have the math in front of me here at work, but you can reduce the amount of coiled nichrome you need by reducing the number of segments by half(which increases the amps that the segments must carry), I seem to remember that is what I did. Then you only need a tap in the middle to make the 2 segments- so I ended up with 2 circuits of 2 segments.

I plug each of these circuits into their own GFCI protected outlets, mounted in boxes, then running to 2 separate breakers. Handy for those who don't have a lot of shop power (keeps 2 circuits under 15 amp break point)

It doesn't matter how many non-terminal ceramic posts you have. Go for coil stability.

there are 2 ways I can think of to handle a dual circuit spiral. I chose to have the inner and outer as separate circuits, which works fine. You could also run them parallel from the center to the outside, but you might have to be a bit more careful that there's no contact across the circuits.
_________________
<.o'>

anakinjay · Newbie Joined: 22 Dec 2006 Posts: 12

Thanks!

Thats exactly the kind of help I needed!

I see here:
http://tinyurl.com/yws7bq

(now that I'm reading the boxes right)
that if I reduce the segments down to 2 and have two circuits (so I have 4 segments total on two breakers) that my total coil needed goes from 53 to about 26. (13 x 2)

that seems much.. much easier!

Thanks again! I think I'm ready to begin now Smile

cod · Posted: Thu Nov 01, 2007 9:05 am Post subject:

drcrash

A standard 2x2 TJ oven has 4 segments in parallel on one circuit. If you want a 2-circuit oven you can put 2 on each of two circuits, or 3 on one circuit and 1 on the other circuit, without changing the segment layout at all. As long as you supply 110-120V to each segment, nothing changes except where you're drawing the current from.

(That said, I think it's a good idea to change the segment layout for a 2 x 2 oven a bit, making each of the outer 2 squares a full segment, and stretching the coils further for the inner two segments. That will give you more heat around the edges and less in the middle, which I'm pretty sure is an improvement, and it's actually simpler, but you need to stretch the segments different amounts.)

If you put 3 on one circuit and one on the other, you may be able to run your vacuum system off the same circuit as the one that only has one heating segment on it. If you have 20-amp breakers, I *think* that should work if, and it simplifies things... you don't need to use a third circuit for the vacuum system, or have to worry about switching the oven off when you engage the vacuum. (And turning it back on, and having it cool in between.) It won't work with 15-amp breakers, though.

If each segment draws 5.45 (at 110) to 5.95 (at 120) amps, the three-segment part will draw 16.35 to 17.85 amps. 16.35 should be okay, but I'm not sure about the 17.85... I've read conflicting things about whether you should run more than about 80 percent of the circuit's rated amps continuously, and that's close to 90 percent.

On the other (20-amp) circuit, if you're drawing under 6 amps with the 1 segment, you've got another 14 or so you can use to run a vacuum system, like a 12-amp vacuum cleaner, with a little bit of head room. Since the vacuum cleaner doesn't run continuously, that's probably okay, but if you have both a vacuum cleaner and a vacuum pump you may need to worry about running a vacuum pump and a vacuum cleaner simultaneously along with a heating element on the same circuit.

One of the annoying things about this is that if you wire your oven hotter than standard, which I think is probably a good idea, you start hitting the limits of two 20-amp circuits; you probably need a third for the vacuum system if you have both a vacuum pump and a vacuum cleaner. (Or a monster of a vacuum pump.) Likewise, if you only have 15-amp circuits, you probably need to use three circuits, or will have to switch all or part of the oven off sometimes.

(One reason why I came up with my double-spiral design was so that in a pinch, I could turn the oven halfway down when I run the vacuum cleaner, rather than fully off, and still get even heat keeping the oven warm. You could also do that with a power diode. Running another heavy duty extension cord is simpler, if you have 3 circuits to work with, but it's clumsy.)
_________________
Paul (a.k.a. Dr. Crash)

Tired of buying cheap plastic crap? Now you can make your own! www.VacuumFormerPlans.com

anakinjay · Newbie Joined: 22 Dec 2006 Posts: 12

Damn dr. crash!

I didn't think about the vaccum pump.

I'm not using a vac, but I'm using one of those 4.5cfm surplus pumps you posted about. Do you think those use more than 5 amps? I have no idea how good the wiring in my house is, so I've been trying to keep the amp's below 15 on each circuit just so I don't burn my house down Razz

(it's pretty old)

would be nice if I could run the pump & 2 segments on one, and 2 segments on the other.

looks like I need a little more planning...

drcrash

drcrash · Posted: Tue Nov 06, 2007 12:22 pm Post subject:

By the way, if you're using hardibacker, don't bother with the ceramic insulators. The hardibacker itself is an electrical insulator, and unless you put the coils on standoffs, they end up resting on the Hardibacker anyway.

I have no idea why TJ used ceramic insulators, but they don't really do anything except keep the wires or bolts that hold them from being electrically live. (Nick Bryson's design doesn't use them, and neither does Doug Walsh's Hobby Vac design.)

You should ensure that people's fingers etc. don't get into the back side of the oven anyway, because even if you do use ceramic insulators, the end connections are electrically live.

Cotter pins work just as well for attachment points for the nichrome. Just use a little bit of stainless wire or extra nichrome wire to hold the heating coil to the cotter pin.

That should save you tens of dollars on your oven, which are better spent on something else.
_________________
Paul (a.k.a. Dr. Crash)

Tired of buying cheap plastic crap? Now you can make your own! www.VacuumFormerPlans.com