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Oven Controls Build

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  • Oven Controls Build

    When Caswell recently updated their forum software the original Oven Controls Build lost much of the verbiage and photo's that were included. I've been able to recover most of the build documentation and pic's so I'll attempt to re-document the control panel build. I'll be adding sections of this as I go so it may take a couple days to get it completed.

    I've also added some information of topics that are frequently addressed. If you're only concerned with the Oven Control Build you can skip down to that discussion.

    Legal Disclaimer:

    Warning – All wiring projects involving AC voltage are inherently dangerous and can cause serious injury or death if done improperly. You assume all responsibility for the electrical work that you perform and the dangers involved therein. If you are at all unsure about completing any aspect of this or any other wiring project, consult a qualified electrical contractor to perform the service(s) for you.

    Always follow electrical code requirements specific for your location. Before undertaking any electrical project contact your local electrical authority and your insurance company to ensure that you comply with all policies, regulations and authorities concerning this work.

    Electrical Service Considerations:

    I've seen many physical oven builds started without first looking to insure the availability of a suitable power source. While there are several wattage calculators available a simple wattage to cubic foot comparison is all that's necessary. IMO something in the vicinity of 150 watts per cubic foot is optimal, assuming good construction of the oven shell and good insulation.

    Single Phase Service:

    Most homes, home based garages and small business locations will probably be serviced with single phase power. Voltages in the U.S. are 120V phase to neutral and 240V phase to phase. It's very important that you look closely to determine what your service panel or sub panel amperage ratings are. You need to have adequate capacity and the availability to add a double pole breaker. The oven circuit needs to be a 3 wire with ground if you wish to run any components such as a light and/or re-circulating fan. Under no circumstance is it acceptable to use the ground as a neutral in existing or new 240V 2 wire with ground circuits commonly used for clothes driers.

    Three Phase Service:

    While 3P power is a welcome addition to any business/industrial site, there are a couple of issues to be noted. Using 3P power has no advantage in billing for a resistive heat application(as used on ovens). Wattage is the billable factor so using 3P power for
    an oven does not result in any dollar savings. The efficiency of 3P motors themselves is the driving force for power consumption savings in installations using large HP motors.

    If you have access to 3P service it can certainly be incorporated into the oven build. While there are true 3P heating elements available they tend to be rather pricey so use of single phase household oven elements is usually the acceptable alternative. It's best to balance the element loads over the 3 phases so wire your elements in this manner. If the 3 phases are identified as A, B & C, then wire the first element using feeds from the A & B source, the second element from the B & C source and the 3rd from A & C. If you do not have elements in combinations of 3 just balance the load as best you can. The transformer will be less efficient but everything still works.

    3P voltages come in many varieties, with common voltages being 120/208, 120/240 and 277/480. If using a 208V 3P service elements rated at 240/250 volts will heat but just at a 25% reduction of wattage(lower input voltage equals less wattage output). If using a 120/240 3P service the elements will produce their stated wattage. Using a 277/480V 3P service is possible but you would need elements rated at this voltage. Placing 480V on elements rated for 240/250 volts would be a hazard and result in very short life expectancies of the elements. If you are not familiar with 3P service and it's nuances contact a qualified electrician to assist you.

    Component Purchasing Considerations:

    When purchasing, the first consideration is to make sure all components are compatible. Before purchasing components determine what capabilities you desire in the operation of your oven. Some considerations are:

    Do you need ramp/soak capabilities from the PID controller which would allow you to control the rate of temperature rise in the oven ?
    Do you desire a built-in timer function that will turn off the oven elements after the cure cycle is complete?
    Do you want internal lights and or a recirculating fan, high temperature or timer expiration alarms?
    Do you prefer an SSR or Contactor relay to control the oven elements?
    Have you determined the wattage necessary for the planned oven size?

    These are all questions you need to answer before purchasing components and starting construction of the control unit. In the build portion of this document I discuss only the specific components I've used. You are certainly welcome to modify the controls any way you wish, but I cannot assist with that effort.

    PIDs - Most of the lower priced PIDs, but not all, are capable of driving either an SSR or contactor and many have 2 sets of output contacts for driving a relay and adding alarms such as for high temperature conditions. Use caution in purchasing the lowest priced Asian offerings as often they provide little to no documentation. I suggest reading through their manuals prior to purchase.

    Ramp/Soak controllers or ramp to set point models of the Watlows such as the 93's, SD's and the newer EZ-Zone are all industrial strength professional controllers. The Watlows are all configurable in many designations, so before purchasing one of these models be certain that it is configured to suit your requirements. One nice advantage of using a Watlow is that they are made in the USA and can be repaired if they malfunction, unlike the Asian offerings.

    Most PIDs, regardless of the model, can support at minimum (2) SSRs or (4) contactors if multiple relays are necessary for your configuration.

    SSR’s and contactors - The decision of which type relay to use is a personal choice imo. Some sort of relay is necessary as the PID controller internal circuitry is not capable of directly switching the amperage loads of a typical oven heating element(s). In this build I am using an SSR so the below discussion concerning contactors is informational only.

    SSRs by design are capable of switching multiple times per second and have a switching life usually in the millions of cycles. They have no moving parts and accomplish the switching by use of semiconductors. The switching process generates heat so a good heat sink is necessary(don't forget to add dialectic grease between the ssr and heatsink). Computer cpu heatsink/fan combinations work well for cooling but these fans will require a 12VDC power source(discussed more in the build section). They can fail either open or closed so use of a high temperature alarm in the pid should be considered.

    SSR’s typically switch only one leg of a 240 volt power requirement for elements so be certain to disable the electrical mains power supply prior to working on any of the oven components. SSRs can be found in either AC or DC input voltage models, with the DC version being the most common.

    Contactors are electro-mechanical relays that open and close contacts using an electromagnet coil. Their switching produces an audible click/clack sound and their life expectancy is normally in the hundred thousand cycles. Contrary to commonly mentioned characteristics, contactors can fail closed(relay contacts weld themselves together) so use of a pid's high temp alarm is always a good consideration.

    Contactor models can be found with input coil voltage requirements in many variations of AC or DC voltages so be certain to get one that matches your intended power supply. A 120VAC coil contactor is probably the most commonly used type in Powder Coating ovens.

    Contactors also come in various pole or contact sets. For a single phase 240 volt oven a 2 pole is most commonly used. If you have a higher amperage load requirement the element loads can be broken up and switched over a 4 pole contactor of a lesser amperage rating than what would be required if using a 2 pole model. For 3 phase applications look for a 3 pole model of the appropriate voltage.

    When using a contactor, the PID parameter controlling cycle time must be increased to slow the on/off cycling process or the contactors life expectancy will be significantly reduced.

    Timer, toggles, alarms, etc. - In the build section I will discuss a number of components I have found to work well. This is not an all inclusive list and there are many choices of components. At the very end of the build I will list some sources I’ve found for various components.
    Last edited by ed_denu; 09-09-2015, 08:05 AM.

  • #2
    Construction of the Control Box:

    The following documents the actual construction of a control panel build for a powder coat oven. I’m using a Watlow Model 93 PID, Omron Timer, Fotek SSR, alarms, switches, etc. Some of the steps are obvious, but I hope this step by step procedure will provide a guideline for anyone considering their own oven control build.

    After you have acquired the components for the oven build lay them out and determine the size necessary for the physical housing. If you have access to a drawing program these can be very useful in creating a scaled drawing for layout of the control box.

    I’ve chosen to place the physical controls(PID, timer, switches, etc.) on the side of the control box rather than the lid or door. There are two advantages I see in this choice. By placing them on the side they are visible from the oven front so side access to the controls is not necessary. The other advantage is that if these components are not placed on the door, then there is no danger the wiring will be pinched by opening/closing the door.

    The square cutouts for the PID and similar instruments can be made using a Dremel with small cutoff wheels. Round openings are most easily made using step drills or Greenlee type punches.
    Click image for larger version

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    Last edited by ed_denu; 09-09-2015, 08:40 AM.


    • #3
      Re: Oven Controls Build

      Once the openings are cut, fit the components into place. Then layout the internal components giving thought to the wire routing. The recycled control box I found didn’t have an internal mounting plate so I constructed one from sheet metal and mounted it to the control box using standoffs.

      Click image for larger version

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      I like to use appropriate colored wiring. I use multi-strand red and black for the hot feeds, white for neutral, and blue for all low voltage DC wiring. Wiring internal to the control box can be 14 or 16 gauge for all pid and timer power, light, fan wiring, etc. Once wiring exits the box into conduit runs then NEC rules apply for wire gauge and maximum allowable conductors within a conduit. Wire gauge for the circuit wiring powering the elements thru ssr’s and contactors needs to be sized appropriate to the element load. Larger gauge wire is marked with colored tape.

      I use crimp terminals for all wiring being placed under terminals. The multistrand wire does not work well under screw terminals without terminal ends attached. The power block connections do not require terminals. Smaller switches and indicator lights work best by having the wire soldered to the connector tabs.

      Power to the box is first routed thru the Master Disconnect at the top of the control box. A master disconnect is important imo as it removes all power to the oven when not in use. Alternately you could use a plug and receptacle or use the service panel breaker as a disconnect.

      When I started this construction the disconnect switch panel mount kits weren’t available so I improvised and made my own panel mount(see picture below). The output from the disconnect routes to the power distribution block. From there, the various power connections can be made. I like the power blocks versus terminal strips as they typically take less space within the control box. I’ve added a main power indicator light to alert me that the main power is enabled to the controls. The flexible power cord is a Service Cord of the appropriate wire gauge from McMaster Carr.
      Click image for larger version

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      Last edited by ed_denu; 05-24-2010, 09:30 PM.


      • #4
        I’m using two fuses. A small ½ amp fuse is used to protect the Watlow pid and Omron timer. The other is a 30A fuse adding additional protection for the element load. My load is around 22A so the 30A fuse should be adequate. The oven is connected to a 30A circuit wired with 10 gauge and a double pole 240 volt breaker.

        The light/alarm buzzers I’m using have 120VAC inputs. One is used for the timer expiration event and the other is a high temperature alarm controlled by the PID.

        The toggles for the light and fan have internal lights incorporated into them. It shows you at a glance if the light and/or fan are enabled. Click image for larger version

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        Last edited by ed_denu; 09-09-2015, 08:13 AM.


        • #5
          The PID I’m using is a Watlow model 93AA-1CD0-00RR. It’s a ramp to set point model for use with an SSR and has an Output2 electromechanical relay that will be used to drive the high temperature alarm. I’m using a salvaged computer cpu heat sink with fan to keep the SSR cool. I drilled 2 small holes in the heat sink base to mount the SSR. Make sure to apply the dielectric grease before mounting the SSR. The SSR/heat sink components are mounted on a pvc plastic base to isolate it from the control box.

          Click image for larger version

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          Last edited by ed_denu; 09-09-2015, 08:15 AM.


          • #6
            The heat sink cooling fan requires a 12VDC power source. When I built this control box several years ago I used a Radio Shack step-down transformer and a small regulated rectifier power supply built on a bread board to convert the transformer’s AC current to DC. A better option now might be one of the AC to DC power supply boards sold on ebay. Search for an AC/DC power supply with AC input of 100-230V and output of 12V DC. Amperage rating isn't very important as the fan's requirement will be minimal.
            Click image for larger version

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            Last edited by ed_denu; 09-09-2015, 08:17 AM.


            • #7
              Click image for larger version

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              The timer I'm using is an Omron H3CA-8 model. It is a countdown timer capable of timing from 0 to 0000 seconds/minutes/days. These timers come in various models(seconds, minutes, days) so be sure to get one that meets your requirements. It has (2) sets of output relay contacts which is a requirement of the design function. One set of contacts is used to drive the DC input signal from the PID to the SSR(this allows the timer to turn off the element heating function) and the second is used to sound an alarm at the expiration of the timed event. If you wish to modify the wiring to use another type timer that's fine, but I can't offer any assistance in doing so. Note that the Auber timers I've looked at only have a single set of output contacts so they will NOT work with this wiring diagram. The ebay offerings such as the B2E models I believe will work as they offer dual output relay capability.

              The (on/off/on DPDT) switch adjacent to the Omron timer allows for manual or timed mode. In the off position the elements are disabled but the PID is energized for parameter changes and temperature readouts. In the manual mode, the PID engages the SSR and will manage to the set point. You can then monitor part metal temperature prior to starting the timed cure cycle. Switching to the timer position begins the timed cure cycle based on the timer’s setting. At expiration of the timer, the elements are disabled and the audible alarm sounds. Switching the toggle to the off position silences the alarm.

              Toggle/Timer operational notes: The toggle is wired so that in the off position the PID remains powered on, but has no control over the elements. If the toggle is switched to either manual or timed mode, then the DC power feed from the PID is fed thru the Timer Relay 1 NC contacts to the SSR and the PID can manage to the Set Temperature. The Timer relay contacts change relationship(NO/NC) at expiration of the timer, therefore when the timer expires, the Relay 1 switches to the NO position and the DC power feed from the PID to the SSR is opened. The high temperature alarm functions in a similar manner as it is connected to the NO contacts, so when the Timer expires the Relay 2 contacts change from NC to the NO position and the alarm sounds.
              Last edited by ed_denu; 09-09-2015, 08:24 AM.


              • #8

                Oven Controls Build

                I’ve also added an ammeter to the controls. It’s certainly not a requirement but it can give me amperage reading at a glance to verify all elements are working. The ammeter comes with a shunt that needs to be included in the element circuit. The meter only functions when the momentary switch wired into the circuit is pushed. It’s not practical to have the ammeter wired for constant readout as it would be flashing rapidly between zero and the high amperage readings once the set temperature is approached and the PID begins cycling the elements on/off.

                Here is a pic of the finished product.
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                I hope this information is helpful to anyone contemplating a control penal build.
                Last edited by ed_denu; 09-09-2015, 08:44 AM.


                • #9
                  Re: Oven Controls Build

                  The forum software only appears to allow 2 pictures per post. Here are a couple of additional pictures of the component layout and the beginning of the wiring.

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                  • #10
                    Re: Oven Controls Build

                    Here is a picture of the ammeter with shunt and a picture of the base and mounting bracket for the Omron. The Watlow also requires a mounting bracket to hold the PID within the cutout. The base and mounting brackets can often be found with the units, or can be ordered separately.

                    Click image for larger version

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                    • #11
                      Re: Oven Controls Build

                      Thanks, lots of great info as usual


                      • #12
                        Re: Oven Controls Build

                        Here is the schematic:

                        Oven wiring mine Model (2).pdf
                        Last edited by ed_denu; 09-09-2015, 08:50 AM.


                        • #13
                          I have not updated this list so the components listed here may or may not be available.

                          Here is a list or resources for anyone wishing to construct their own oven electrical control box. Listed by component with comments.


                          Contactor 4 pole 40A 120 volt coil – If you need something greater than 40A then this is an option versus buying (2) separate 40A contactors:

                          NEW Definite Purpose Contactor SA-4P-40A-120V - eBay (item 280272331804 end time May-07-10 08:05:26 PDT)

                          Auber SSR’s and Contactors. SSR’s rated to 100A, contactors rated to 40A

                          Solid State Relays, Contactor :, Temperature control solutions for home and industry

                          Famous Parts – cutler Hammer contactors

                          C25BNF240A Contactor, 40 Amps, 2 Poles, 120 Coil Voltage

                          Omega SSR’s – both AC and DC input voltages available

                          Solid State Relays

                          FOTEK ebay SSR

                          NEW FOTEK SOLID STATE RELAY 40amp SSR-3/32VDC-24/38040A - eBay (item 400052781537 end time Jun-22-10 16:00:14 PDT)

                          Terminal Block:

                          PB1043 Products

                          Terminal blocks single, order as needed:

                          Power Distribution Blocks | Add-A-Pole Power Distribution Blocks


                          ECX2071-127R Products

                          Switches for lights, fan, pid:

                          New SPST On/Off Panel Mount Toggle Switch 125V 4A - eBay (item 350270475715 end time Jun-23-10 16:20:49 PDT)

                          10, 250v 125v Snap-in OFF/ON Green Lighted Switch,G9H - eBay (item 220512504229 end time Jun-15-10 23:12:04 PDT)

                          Fuse holder:

                          Lot (3) 3AG Electrical Fuse Holder Panel Mount DIY NEW - eBay (item 360240442830 end time Apr-02-10 14:38:19 PDT)

                          Hi temp terminals:

                          High Temperature Terminals | High-Temp Crimp Terminals

                          100 High Temperature Ring Terminal 12-10 Wire #10 Stud - eBay (item 390140074262 end time Jun-05-10 07:16:32 PDT)

                          Digital timers:

                          Auber timer – one set of contacts

                          MULTIFUNCTION TIMER, COUNTER, TACHOMETER [ASL-51] - $33.50 :, Temperature control solutions for home and industry

                          Omron H3CA-8 – (2) sets of output contacts
                          Last edited by ed_denu; 09-09-2015, 08:29 AM.


                          • #14
                            Re: Oven Controls Build

                            Wow Ed, this is some great information. I dropped you an email. Thanks for all your help.


                            • #15
                              Re: Oven Controls Build

                              Originally posted by ed_denu View Post
                              When I started this construction the disconnect switch panel mount kits weren?t available so I improvised and made my own panel mount(see picture below). The output from the disconnect routes to the power distribution block. From there, the various power connections can be made. I like the power blocks versus terminal strips as they typically take less space within the control box. I?ve added a main power indicator light to alert me that the main power is enabled to the controls. The flexible power cord is a Service Cord of the appropriate wire gauge from McMaster Carr.
                              So where can we find a disconnect switch. I assume that your incoming power is connected to this and then goes on from there.