Heat Treat Oven Wiring Schematic

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jtdesigns

jtdesigns

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Thanks to Jim and Stacy's input I have used my Autocad to create a wiring schematic for a heat treat oven that has the following features.

240VAC
Auber Instruments PID
Dual SSR
Fused Line Leads
Fused PID
Power On Switch
Power On LED
Heat On Switch
Heat On LED

I did this because I have to everything laid out for me so I don't screw things up. For those in the know, please double check my schematic and maybe the powers that be will include this as a sticky so others like me that need it spelled out to the letter can take on this project and lmit the chances of getting zapped.

Jeff

Click image below:

View attachment Heat Treat Oven.pdf
 
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Yes much better presentation than the one I drew up. As mentioned in our phone call the main power led is a little redundant as when the main switch is on the PID lights up. Like the addition of LED on the feed to the SSR that tells you power should be going to the elements.
 
Two relays are used because both legs of 208/220 are "hot". A single relay would turn the element on/off, but if anything conductive that was grounded touched the elements it would a) turn the element on and b) become a shock hazaed.
 
The schematic shows a fuse in each power leg of the 220 before the switch and one between the switch and the PID, What size breaker needs to be used for this circuit, what size fuses should be used in the 220 power input before the switch, what size fuse between the switch & the PID? Thanks for any help.

Dwayne
 
Those values will depend on the calculated current draw of the coils.
The two fuses should be rated around 25% above the coil draw per leg. They should be slow-blow type fuses.
The PID fuse should be slightly above the PID max current draw...usually you just use a 1 amp fuse here. It should be a fast acting fuse.
The breaker feeding the oven should be at least 50% bigger than the oven draw. The wiring to the oven socket needs to be up to the task,too. It would not hurt for it to be larger than the required code size. Long continuous high current draw is more of a wire over-heating problem than short time draw devices like grinder motors.

For instance, if the coil draws 10 amps per leg, use 15 amp slow blow fuses in the mains, a 1 amp fast acting fuse on the PID, and feed the oven from a 20 amp 240VAC breaker. 10/3cG ( 10 gauge, three wire, with ground) wire would be good for the feed to the socket.
 
Stacy, You said the two fuses should be rated around 25% above the coil draw per leg, are you talking about the SSR's, the ones I put in mine are rated at 40 amp load current. Is this what your calling the coil draw? The main breaker is 30 amp, I went exactly according to the schematic that Jeff shows above. Same size wire & everything. The only problem was the fuses. He didn't list the size of them, I hope I didn't ruin anything, I put 10 amp fuses in all three places & plugged it up & flipped the power switch, something popped & I unplugged it. I haven't had the chance to take it apart to see what blew yet. I hope it was just the fuses. The fuse block I used for the power input was heavy duty and the fuses are probably 1 1/2" long & 1/2" diameter but they're still just 10amp. The fuse for the PID is a 10 amp, small glass fuse. I appreciate any help.

Dwayne
 
No, the SSRs are just an electronic switch. They only carry the current the coil draws. The 40 amp number is the max current they can handle with a heat sink ( before they burn up). I usually figure the SSR at half the rating. The heating coil will probably draw a total current of around 20 amps ( or less), which is about 10 amps per leg ( or less). The breakers will protect the house wiring if there is a complete short. The fuses will protect the coils if there is a temporary short. The PID fuse will protect the PID if there is a SSR failure, or other control circuit short.
 
Thank you for your reply Stacy. I'm seeing it now, when you said coils I was lost, I don't know why I couldn't see you were talking about the heating elements. Thanks again for your help. God bless you & yours,

Dwayne
 
SSR output ( most have both). You want a DC voltage sufficient to actuate the SSR coming from the PID. Most SSRs use 3-15VDC as the activation voltage. Most PIDs have 0-10VDC output.
 
Pardon my ignorance, L1 and Ld are the black and red 220v wires? What happens with the third (white) wire? I am re-wiring an old oven with updated controls, we use the oven to heat foam for thermal molding, and I shorted out my old analog controller, I'd like to replace the old system and drive the old oven with these controllers.
 
I have the exact same question. I am very interested in finding out since I am building a 220 oven
 
In the schematic, it has a toggle switch wired with the door switch. What exactly am I looking for?
 
Kieran Klein said:
I have the exact same question. I am very interested in finding out since I am building a 220 oven
Click to expand...

The white wire is neutral, you would use it to ground the metal casing and to wire 120volt devices in the oven, in my case the pid and the dc converter to run the cooling fan are ran off L1 and the white wire, all wires are connected to a mini bus so there is no possible way for any wire to come lose and short circuit the whole thing
 
The toggle switch in the line that goes through the door switch is a plain 1amp SPST toggle switch. It will carry no real current, as the voltage and current through it are the control voltage to the SSR. It is the "HEAT ON" switch. The SPST door switch is a momentary contact switch of some type. These are often called limit switches or micro switches. It breaks the control circuit if the door opens....thus shutting off power to the coils.
 
John Katt said:
The white wire is neutral, you would use it to ground the metal casing and to wire 120volt devices in the oven, in my case the pid and the dc converter to run the cooling fan are ran off L1 and the white wire, all wires are connected to a mini bus so there is no possible way for any wire to come lose and short circuit the whole thing
Click to expand...

Ok that makes sense. I am sure I will come back to this thread about 30 more times before the whole project is done. Thanks!
 
The simple way to describe the 220VAC circuit for the heater coils is:
L1 goes to SSR1, through the coils, back to SSR2, and back to L2. If the path is any different you have something wrong. All other wiring is control circuits. They carry no large amount of current.

Make sure all circuits are right and someone who knows his/her stuff takes a look before plugging it in.
 
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