Here is how they deal with it.
I cannot locate the post but all of this is 100% from the great mind of Mr Cashen. I have it cut and pasted as a word doc in my knifemaking tech info folder.
Cashen PID SSR tutorial
In an effort to help those who are challenged by trying to connect a PID and its associated circuitry to a device, I will give the procedure and setup to run a toaster oven as a tempering oven. The same hookup can run a variety of things (solenoid valve on a forge, salt pot, BBQ smoker, kiln, etc.), with the appropriate thermocouple and SSR. If you only want it to read temperature, you just use the thermocouple as a thermometer, and plug in no load device.
Some components can be found in thrift stores, other from electronics suppliers, and most everything can be purchased on Ebay. Search Ebay for PID, Thermocouple K, SSR, Thermocouple plug connector, 120V cooling fan.
The Oven:
Any toaster oven will do fine. It only needs to be able to set the oven on HIGH or have a temperature setting knob. You are going to permanently set it on high or the highest setting anyway. It needs nothing modified beyond a small hole drilled to allow the thermocouple to stick inside. Some folks wrap a blanket of insulation around the oven to get a little more efficiency. That probably isnt a bad idea, but use a proper insulation. A piece of insuwool is NOT a good idea. You want a jacket insulation, which controls the loose fibers, like a water heater wrap from the hardware or plumbing shop. Wrap the entire toaster, including the back, and tape the jacket with duct tape. Leave a flap to drop over the front door. This will greatly help the oven regulate temperature, and hold a somewhat higher temperature.
The PID:
Most PID controllers are pretty much the same. You want 120VAC input and the ability to take type K thermocouples. Nearly every one I have seen does that.
Here is a basic unit from a supplier that many smiths buy from:
http://cgi.ebay.com/UNIVERSAL-PID-TE...742.m153.l1262
The Thermocouple:
The Thermocouple that they often give you with those controllers is pretty useless for most of our purposes, so you will need a new one. The temperature range needs to match the desired use. If it is going into an electric toaster oven, it just needs to be a type K that is long enough ( 4-5) and rated at 1000F. (There is a + and - wire, BTW, make sure you hook them up right or the display will read minus.) Some come with wires and some have just bare ends . You can use a thermocouple mounting block, or connect the thermocouple wire directly to the bare ends. You need to use specific thermocouple wire for connecting the probe to the PID.A good idea is to use a plug in probe and put a corresponding socket on the thermocouple wire. That way ,changing probes or moving the controller to a different device can be easily done.
Here is a basic 5 medium temperature thermocouple:
http://cgi.ebay.com/Thermocouple-2-K...1|240:1318
A higher temperature and more robust probe is like this:
http://cgi.ebay.com/Thermocouple-2-K...1|240:1318
Plug in Probe:
http://cgi.ebay.com/Thermocouple-2-K...1|240:1318
The SSR:
The SSR is actuated by the output voltage from the SSR control circuit on the PID. Most PID output voltage is 8VDC, some are 12VDC.
The SSR units usually take 3-48VDC input, and control 48-480VAC .The amperage needs to be enough to run the device being controller, plus a bit. Most folks just use a 25Amp SSR. You will need a heat sink on the SSR to keep it from burning out ( they only will take 3-5 amps otherwise). A small fan, like from a computer , should be used to cool the heat sink. Here is a basic SSR and heat sink:
http://cgi.ebay.com/Solid-State-Rela...742.m153.l1262
120VAC Fan:
http://cgi.ebay.com/COOLING-FAN-120V...1|240:1318
To hook all this up:
Enclosure:
Select or make a suitable cabinet for the controller. While you can mount it all on a board, open circuitry with 120VAC is a bad idea. A piece of plywood , or Plexiglas, with a plastic bread storage container on the back (to cover the wires) will work if you dont want anything fancier.
On the PID -
Cut the cabinet/mounting board to fit the PID. It inserts into the hole and snaps in. Yours may have come with a case or mounting plate of some sort, so mount it as needed.
Connect the thermocouple to the + and - contact screws. Observe the polarity of the probe.
Connect two 16 gauge wires, about 12 long, to the AC POWER screws. A piece of lamp cord does fine.
Connect two 18gauge SSR control wires to the SSR screws ( about a foot long). They are + and -.Use red and black wires. There may be some jumper wires or clips to connect as the directions will tell you.
On the SSR:
Mount the SSR on the heat sink as described in its instructions ( it may recommend using heat sink compound, which is a good idea). Make sure the cabinet or enclosure has sufficient air flow to aid cooling.
Connect the red/black wires from the PID to the corresponding CONTROL contacts on the SSR.
Connect the black wire on the 3 piece of extension cord to one of the LOAD contacts on the SSR. Connect a 14 gauge, 12 long black AC wire to the other LOAD contact (see POWER below).
Mount the SSR over the fan ,so the air blows through the fins and cools the SSR.
The fan connects to the 120VAC . (see POWER below)
POWER:
You will need a 9 foot , 15 amp rated extension cord. Cut it off about 3 feet from the female (socket) end. There will be several things connected to these cords, so set up a power buss (multi connection strips) is simplest and safest, but wire nuts will work. Connection wires should be white and black and of 14 gauge . Securely fasten the male and female power cord pieces to the case with cable clamps of some fashion.
On the panel, install a SPST power switch and a 15 amp fuse. Connect the black wire from the male power cord (the 6 foot long piece with a plug) to the switch and the switch to the fuse.
There is a common wire (white) and a load wire(black) and a ground wire (green).
Install a black AC wire from the fuse to the load power buss. At this buss the connections are made to:
The load contact on the SSR, The PID, and The fan.
The white wire from the power cord goes to a common buss at which the connections are made to :
The white wire on the Female power cord end (3 foot piece with a socket), The PID, The fan.
The green wires form the two cords are connected together.
The toaster oven ( or controlled device) plugs into the female cord. The male cord will plug into the wall socket.
Insert the thermocouple into the oven chamber. Affix as needed. If the thermocouple is installed just below the rack/grate/shelf, it can be affixed with a twist of bare wire. That way it reads the temperature close to the blade being tempered.
For a really accurate reading, drill a 3 to 4 deep hole in the end of a ½-¾ thick plate of aluminum or steel. The hole should just accommodate the probe. The piece of metal should be the same size as the rack in the oven .This plate will replace the rack. The probe will insert into the hole. The blade will rest on the plate. Drill the probe hole in the side of the oven first. Then mark the plate and drill the hole in it to assure alignment .
You only want the probe in the oven, not its wires. Use a longer probe if needed for larger ovens. The tip of the probe is what reads the temperature, so as long as it is several inches in the oven chamber, it will be fine.
The operation of the PID is simple and the instructions give the setup and programming. A good supplier will offer support help. The basic operation is to set the desired temperature (set point or set value) and turn the control program on. The toaster oven is permanently set at its highest setting, and will be turned on and off by the controller and the power switch. The controller will allow the power to flow to the oven until the temperature is reached. Then it will cycle on and off to maintain that setting. The PID will auto tune itself to learn how to hold the temperature evenly. There are a lot of strange words ,like hysterisis, in the instructions, but once set up it is pretty simple to operate.
