Building a lab-grade Temperature Controller

[Baffled]

Mods: If this would be better in another sub-forum, please move it. I didn't know where to post it.

I've had a couple of requests for a basic build of an effective, home-made PID (Proportional Integral Derivative) temperature controller for shop use. These things are phenomenally useful for any shop operation that requires a liquid (or a gas or electric furnace) temperature to be maintained with great accuracy, with little or no overshoot.

Applications would include:

Parkerizing
Anodizing
Bluing
Degreasing/cleaning tanks
Furnaces - convert a cheap bench muffle into an accurate unit capable of heat treating metal
Electroplating baths
Keeping your mash warm in wintertime,
etc.

The idea is to create a monolithic box that you plug into 120/240 1P mains, and into this box, you plug in (or connect) the electric heating device. The unit controls the output to the heating device using some cosmic PID math, which in simple terms means it is intelligent, not just an ON-OFF thermostat device. The latter can overshoot a set value in a big way, whereas PID sneaks up on it, minimizes overshoot, and intelligently and accurately maintains it.

The heart of the unit is the solid-state controller itself. 20 years ago, these things would cost $400 or more. Today, you can find them for $50 or so. eBay is full of them. Even the cheap Chinese jobs seem to work very well. A handful of additional parts is all it'll take.

My example is from Omega Engineering. Omega has everything you need for this build except for basic stuff like wiring. But they can be pricey.
http://www.omega.com/temperature/tsc.ht ... 7Aod5CIA3g

Exact Model depicted, currently $99:
http://www.omega.com/pptst/CN4000_Series.html



The front of these varies, but they all do the same job. This is a 1/4 DIN-sized model... I like big and easy to use and wire. Smaller will leave you more space in your enclosure.

Controller summary: Look for a PID Temp Controller that has 100-240VAC input, accepts BOTH thermocouple AND RTD inputs, and for the controlling output, you want a DC (or AC) SSR (solid state relay) driver.

Most controllers are set up on these lines.

More to follow...

[Libertarian_Geek]

...Sexy stuff
(if you're a geek like me)

[Baffled]

On the back are the terminals. To finish a controller like this, you'll need the following components:

1) A suitable enclosure. I bought an empty terminal box ov heavy steel from Home Depot. These are nice because they are both strong, and they have knock-outs for wiring. Note the yellow plugs and jacks - these are for thermocouple inputs. More on that later.



2) A Solid State Relay, SSR. This is second only to the controller as the "heart" of the unit. The controller, when it decides it needs to power the external heating device, simply outputs a DC voltage to its terminals. These are attached to an appropriate SSR that accepts a DC input, and energizes (closes) an AC output. Think of these as being two halves that don't interact, like a mechanical relay.

The one I purchased was a 50 amp job. There is no limit as to the size of it; it could be a 1,000 amp SSR. On the picture, you can see the values for both the input and output sides... the input side accepts 3 to 32 VDC, and the output of the controller I used is 12 VDC. Most controllers output 5 to 32VDC; some output AC, so you must match the controller to the SSR and ensure compatibility.



Also, think safety. I picked up a fuse block and some heavy fuses (25A) for the output side, and I have also fused the controller side with a 500 mA electronics fuse.

[Baffled]

The size of the SSR: Amperage is roughy determined by the simple formula W= V x A, where

W = wattage
V = voltage
A = amperage, current

So a 50 amp SSR using 120VAC can theoretically handle 6,000 watts. Note that most household 120VAC lines are fused for 20 amps, so if you think you are going to be tripping breakers, wire your unit for 240VAC.

Keep in mind that the controller can be separate from the load side. I can power the controller side with 120V, and the load side with 240V. It's up to you. Regardless, going from 30 amp SSR to a 50 amp SSR is not much $$, so why not go big? It'll run cooler.

Still, a heat sink is a good idea. To go cheap, buy a big SSR and bolt the metal backing with some heat sink grease right to the case. I decided to buy a heat sink for mine:



Drill, tap, secure as needed. These SSR's CAN get hot. And be advised, when an SSR fails, it failes to the closed position, meaning the heater element is powered ON. Keep the SSR cool and happy.

What else do we need?

3) Temperature sensors. For low temperatures, a PT100 probe is best. These come in either 2-wire varieties, and the more accurate 3-wire. Most controllers can accept either type.



Thermocouples, also work, with the better controllers allowing any type of thermocouple. I like and use type K. I won't go into the theory of these too much, but I can answer basic questions. Be aware that a PT100 probe uses normal copper wiring, so you can extend a jack and wire it with ease, but a thermocouple uses special wire, and the thermocouple leads must be connected directly to the controller using the TC wires themselves, that came with the probe; or, you can buy TC wire of the correct type, and use appropriate jacks to create an extension if you want. The jacks themselves must be of the correct type such as type K. Omega Engineering has scads of info on thermocouples. If you use one, I do recommend a jack, because you might have probes permanently installed in units like furnaces, and the jack lets you move the controller around.

PT100 probes and type K thermocouples are dirt-common on eBay, with the former being more expensive. Look for those coated in an appropriate sheathing material. I greatly prefer teflon (PTFE) for use at <200 C or so. PTFE shrugs off everything. For furnaces, you need a type K thermocouple with an inconel or ceramic sheathe.

[Baffled]

4) Buy a Master on/off switch of good quality - I used a beefy toggle. I also added a 120VAC lamp which can be seen upper left. This is optional, but I am very glad I did. This lamp is wired so that when the high-current heater output is energized, the lamp is ON. It not only warns you that the terminals are hot, you can easily see how the device approaches and holds a temperature.

What else? Basic stuff. Wiring, terminals, crimping tools, the normal hardware stuff most guys have. Remember that the wiring needs to be sized appropriate for the current. Pretty much everything on the controller side can be wired with lighter stuff, like 16 to 20 gauge.

For the heater side, you will need heavier wire. It depends on the current you expect. Look up ampacity of various copper wires, and if in doubt, buy heavier wire. I used mostly 10 gauge, but I rarely power more than 1500 to 2000 watts.

Decide how you want the inputs and outputs to be routed into the enclosure. I decided to go with a "big boy" approach, and created an exposed terminal strip that I bolted to an aluminum angle which in turn was bolted to the enclosure. These are LIVE when the heater circuit is ON. If in doubt, do a better job than I did, and keep terminals secured from fingers. Not yet seen on the left side of this Al bracket, I drilled and tapped a couple of holes for heater-side grounding (green) wires. Be sure you have good continuity between the bracket, the case, all grounds in general.



First, of course, you have to cut the enclosure. Before you begin, have all your parts on hand, and decide how they will be mounted. The picture in the previous post shows the enclosure partly cut. This sort of steel is a PITA, and I put my mill to use, but you can also use drills, files, a jig-saw with a metal cutting blade, whatever.



Be sure no jagged edges can hurt the wiring.

[Baffled]

Mount the hardware before you begin wiring.



In this next picture, I have the heat sink mounted above the controller; the SSR is not visible. Next to the controller is the heavy fuse block for the output side of the SSR. When I say "ouput" in these postings, I refer to the heavy current side of the SSR that powers the heater. A much lighter fuse block is lower right... this is fused in line to the 120VAC that powers the controller; IIRC I used a 1/2 amp quick blow electronic fuse. These controllers draw just a few milliamps, hardly any power at all.

I mounted almost ALL of the hardware on the lid of the box. This is a lot easier than reaching deep inside the box, and I can simply pull the lid off for maintenance.

The wiring begins... I am looking for the actual circuit schematic which I made when I created this thing. If I can find it, I'll post it. If not, I'll draw another one and post it later.




Next is a better view of this small fuse block and the master switch. Keep in mind that the master switch powers ONLY the controller. The controller enables the SSR to close and output the heater current. Think of the SSR as the switch for the heater.



Give thought as to how you want to wire your heater to the controller. This thing can power ANY AC heater device so long as the current is within the limits of the SSR and the wiring. I mentioned I used exposed terminals for the heater output, which means if I have a device with a plug, I have to cut the plug, expose the wires, and screw them onto the terminals. To be a bit more flexible (and elegant), later on, I took a separate little metal junction box, installed in it a quality pair of female sockets just like you find in your wall, and in turn, I wired that box to the controller with a short section of heavy insulated wire.

Keep in mind that you will probably, at some point, want to power more than one heater at a time, so either build this into your unit, or leave space on your terminal strips so you can mount 2 (or more) heaters running in parallel.

There's nothing stopping one from installing female sockets right inside your box. In that case, you'd just plug your heater into your box.

I need to stop here and locate that damned schematic... stand by!

[Libertarian_Geek]

Applications would include:

Parkerizing
Anodizing
Bluing
Degreasing/cleaning tanks
Furnaces - convert a cheap bench muffle into an accurate unit capable of heat treating metal
Electroplating baths
Keeping your mash warm in wintertime,
etc.

These are also useful for brewing (not just distilling) tasty beverages...

[bakerjw]

Just get one of these and some sensors.



It's what I do for a living.

[Baffled]

Got the schematic scanned.

Downloadable PDF
http://www.5bears.com/perc/tempcont.pdf

JPEG:


The thing to take away from this is that I designed this circuit so that the control itself AND the heavy power to the external heater is run off of one power source. This would be very easy to modify so that the heaters can use a separate power line, like a 240V outlet, and in that case, the SSR switches that particular line on and off as needed.

There's not much more, I'll finish this later for those that care. Just basic operation, etc.

[Baffled]

Just get one of these and some sensors.

It's what I do for a living.

Yeah, but where's the fun (and cheapness) in that?

[Dr.K]

I'm currently building a heat treating electric oven, so I'm following this perfectly!

3 months ago, I would have had a puzzled look upon my face, .

Sofar, my silencers haven't needed "heat treating" but the study of metallurgy from this hobby has led to me picking up bladesmithing as another hobby, and I have been enjoying that also, not to mention it doesn't cost me $200 for each blade.

Anyhow, I think it fits perfectly here.

edit:

And just to add, I took an electrician class when I was a teenager, and learned electricity (the math, the assembly, and the wiring diagrams). That bit of knowledge has come in so very handy over the years, it's one of those skills I'm really proud of.

[JohnK454]

Just get one of these and some sensors.



It's what I do for a living.

LOL! My home shop has a Allen-Bradley 5/40 PLC system for controlling all sorts of stuff, including my Moly Resin curing oven. Rack, panelview, thermocouple module, etc. I did industial controls for a living myself and squirreled away old surplus equipment.

[Fulmen]

Nice project, I might do something similar myself one of these days. I have a PID-regulator on on my sizer so I can use Rooster Red lube (hard), now I'm planning on making a molten salt-bath for tempering and the student economy doesn't allow too much investments. The sizer sees only infrequent use, so why not make a movable unit out of it? Your unit seems simple and well designed, I'm sure you'll be happy with it.

[Baffled]

Time to wrap this thing up.

On the schematic, take care to note that some of the wires are drawn heavily - these wires at a minimum need to be able to carry the full current of your heater(s). Since many controllers accept 100 to 240VAC, the entire unit can be wired to 240 V, giving you the same wattage with half the current, allowing for smaller wires.

I tried to do a neat job of the internal wiring, using terminals, and bundling and tieing the wire as necessary. The terminal strip is cheesy. I could have done a better job.

I've seen guys build these with stuff hot glued to a pine board, and that'll work too, but you've got a rat's nest of wires and more exposed terminals than something in a box.

Operation - every controller is different. All of them must be programmed, and the user manual is needed to do this. During programming, you can set things like:

- Type of temperature input... PT100, Type J, Type K, etc
- Hi/Low alarms
- Units like metric vs imperial
- ramps, soaks, all good stuff for heat treating metal
- min and max for SET value
- etc.



This particular model is easy to use. After it's programmed, simply turn it on, and use the up and down arrow keys to enter the SV, Set Value. That's the temp you want the device to hold. PV = Process Value, the temp it is reading from the probe. Note the indicator lamp is on... I'm asking for 65 C and the PV is 31.6, so pour in the heat.



The system can be tested by simply plugging a desk lamp into the unit, setting a SV close to ambient, and either warming the probe in your hand, or cooling it off. The lamp should turn on and off appropriately; ON obviously indicating power to the heater.

The next step is a true test. Attach a good high-wattage immersible aquarium heater, or in this case, I have a cheap cartridge heater in a copper tube. Place it and the probe in a container of cold water, set a value like 90 C, and turn it on.

Watch the PID action; it's interesting. The unit adds a few seconds of heat to the water, then turns off. It waits a moment and sees how quickly the temp rises. If it's heating 1 pint of water, the rapid temp rise is noted. If it's trying to heat 20 gallons, the temp rise is slight. The system figures out if it needs to really pour on the coals, or go gentler.

For example, with 20 gallons of cold water, it quickly figures out "I've got a hell of a job here" and soon the unit is on 100% of the time. Less water, it cycles. As it approaches the SV, it begins to back off on the duty cycle, and the indicator bulb begins to flicker more rapidly on, off, on, off. It intelligently figures out what duty cycle is needed to maintain the SV, with little or no overshoot. Accuracy is excellent. If I need 95 C in a park bath, it'll hold that within a degree or better.

This device has seen a lot of action for various shop projects. Heaters I've used:

- 2000 W PTFE coated industrial immersion heater
- A little 50W aquarium heater
- a 1500W SS hot water tank heater element for park baths
- Various cartridge heaters
- Even an electric stove element, just like in a kitchen stove

Figures them all out just fine. If a heater runs on the unit's AC voltage, you're good to go. In fact, there's rarely a problem running a 240V resistance heater element on 120V, it just delivers less heat.

Hope this helps people.

[bakerjw]

It's great to see others who know the value of PIDs. Since I work with these units, scrap ones occasionally come available. I have a whole drawer of Allen Bradley SLC500s that I need to get cleaned up and tested since they got replaced with Siemens units.

And I will be using a PID loop combined with my GPS in my boat to do an autopilot for my kicker motor.