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Author: Subject: Crock Pot plating bath
Swede
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Crock Pot plating bath

Thanks to some inspiration from Rosco Bodine among others, I have successfully hacked a $10 crock pot and turned it into a precision heating bath capable of holding +/- 1 degree C. I realize this is nothing new or innovative, but perhaps the pictures and the description will help someone along. Background: I am a lead dioxide (among others) experimenter. I have been using expensive polypropylene tanks to do the bulk of the work, and an immersion heater provides the heat. The immersion heater is an industrial cartridge heater encased in copper pipe, soldered shut at the business end. It works... but the Cu is not always compatible with the bath; it corrodes, and worst of all, it occupies precious volume in the system. Note: To do this job well, you need a true heat controller, especially one with a PID algorithm. These are available NEW for about$40 to $60 on eBay and other places. I detailed how I turned a nice one into a full-up heat controller in a blog: http://www.apcforum.net/forums/blog/swede/index.php?view=sho... When you set up your heat controller, do so in a way that allows you to swap heating elements AND temperature probes easily. Mine uses "Dean's Ultra" plugs for the heating element(s). If you are using a thermocouple rather than an RTD (I recommend the latter), be aware than thermocouples require special compatible extension wires and connectors, whereas RTD's simply require decent, high-quality Cu wire. First step. Buy a crock pot. At Wal Mart, they range in price from$10 to $75. This specimen is a 2 quart heater, and costs a whopping ten bucks. They are very simply put together... a pressure washer on a shaft on the bottom keeps the whole thing together. It had to be freed with a dremel by grinding the individual fingers of the pressure washer off, but it popped free with little fuss. At that point, the entire stack came apart vertically. [img]http://www.5bears.com/chem1/crock2a.jpg[/img And this is what is inside: Free the aluminum central chamber from the rest of the device. The heating element is a spring-loaded band that is a snug fit around the aluminum pot. Aluminum conducts heat so well, when that band heats up, the entire aluminum pot also heats nicely, and the Pyrex or stoneware insert absorbs the heat. Almost all cheap crock pots have two settings (besides off); low, and high. I discovered with an ohmmeter, and by analyzing the rotary switch , that there are two separate heating elements inside the band, one of a lower resistance, the other higher, with the latter, of course, being the "warming" element. When the unit is on low, only the high-resistance (low wattage) element is energized; when on high, BOTH of them are on. Since we are using a good temp controller, there is no need to keep the elements separate electrically, so they were pig-tied together, and a crimp was used to secure. The three lines consisted of the "hot" AC line (which is smooth on a zip cord like this one), and the two heater element wires. The original fiber protective sheathe was reinstalled over the crimp. There will be no ON/OFF switch, since the controller takes care of that, although if you wanted to, one could be added at this stage, or you could make use of the original with a bit of rewiring. At the other end of the heater band, there is only a single wire... somewhere internally, the two elements are tied together. This single wire is pig-tied (and crimped) to the neutral line. On an AC zip cord, the neutral line has a "ribbed" sheath and also corresponds to the larger prong on the plug. I carefully (respecting the live voltages) plugged the unit in, and it began to warm. That's all I needed to know at that point. It was time to put it back together. The original wiring had some useless wire loops that were supposed to keep the wiring off the heater band. I did a bit of re-engineering with stainless steel wire, and formed some supports to do a better job of it. The idea is to prevent the excess wiring from contacting the aluminum pot on the inside of the unit. The heater band is an interesting setup... being spring-loaded, it can be moved on the aluminum pot, and if a more rapid heating is desired, buy a second$10 crock pot, and add the heater band to the first. This will also give you a spare stoneware crock.

$10 is an incredible deal for what you get... a heat-resistant crock, heating element (moveable) and lid. I was paying more than$10 for a simple cartridge heater. Coming n a variety of sizes, the lowly crock pot, combined with a heat controller, is an awesome combination.

To secure the crock pot back to its original form, I simply split the shafting at the bottom, bent the "petals" over, and drizzled epoxy all around.

Time to test! I rewired the plug to a female Dean's Ultra to match the output of the heater controller, added cold water, set the controller to 80 degrees C, and let rip. Heatup was quite slow but it was steady, and about an hour later, my two quarts of water stabilized at 80 degrees. The PID logic in the controller did what it had to do to keep it there. A killer plating bath (or a warming bath for a number of processes) for $10, minus the controller. The controller is the "big part" of the system. The controllers coming out of China seem to work fine, and combined with a SSR (Solid State Relay) will do what you need. Here is an example: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=33030...$60 or so will get you going. It does not have to be fancy like the one I made... hot glue the components to a board, watch out for voltages, and functionally it will be the same!

This two quart unit will see duty plating small test anodes with lead dioxide.

Edited to add: I am off to Wal-Mart to pick up ANOTHER 3 quart Rival crock pot. These are about \$15, and have an actual nut at the bottom; come apart really easily. I am going to cannibalize the heater band and make a 3 quart unit with a double-heater. The controller I made has two outputs... I am going to put this one together so that I can use one or both of the heating bands, labeled "UPPER" and "LOWER" (or both) as the situation demands. A heavier zip cord will be used as well.

[Edited on 21-1-2009 by Swede]
hissingnoise
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Nice piece of work there, Swede!
Though I'm not familiar with crock-pots, I would have to question your use of epoxy to seal the bottom hole.
During use, electrolyte is likely to insinuate beneath the glue, creep to the central hole, and result in a cell-leak.
How quickly this happens will depend on cell-temperature and duration of run---it could happen within hours of switch-on.
I don't know enough about these utensils to come up with a remedy---but the epoxy is highly likely to be problematical. . .
watson.fawkes
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 Quote: Originally posted by hissingnoise During use, electrolyte is likely to insinuate beneath the glue, creep to the central hole, and result in a cell-leak.
The electrolyte is entirely contained within the ceramic pot. If electrolyte is touching the working parts, you've got a cracked pot and much larger problems than epoxy material incompatibility.
watson.fawkes
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 Quote: Originally posted by Swede Since we are using a good temp controller, there is no need to keep the elements separate electrically, so they were pig-tied together, and a crimp was used to secure.
Have you measured the duty cycle of the heaters near the set point? I'd also be curious about the duration of a typical duty period near the set point.

The reason I ask is that you'll get tighter temperature regulation when the heater power input is more closely matched to the power loss to ambient cooling. What I don't know is whether this matters in practice. The main limitations on accuracy are going to be signal jitter in the temperature sensor and thermal mass (resistance) between the heater and sensor. Lowering the superheating power would reduce the overshoot before the controller cuts heat, but I don't know if that's a problem.

What I do want to acknowledge up front, to avoid talking about a straw man, is that it's certainly easier with an off-the-shelf controller to use a single heating element.
hissingnoise
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Thanks for clearing that up, w.f; I based my ramblings on the design of deep-fat-fryers.
I should take more interest in kitchen things rather than just the meals prepared there.
Sorry Swede---false alarm.
Keep up the good work!
jokull
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Hi Swede.

I just have to say that this work fits excellent for several purposes, I'd like to know how hot it can be runned. I suppose you setted 80ºC because it's your working temperature, am I right?
Rosco Bodine
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Hardware hackers rule.

Betty Crocker wandered into my lab .....

( I shagged her )
Magpie
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Thanks Swede for your detailed post. I already have a PID, thermocouple, and solid state relay, so the only thing left is a trip to Wallymart.

A couple of experiments I have planned will require such a bath for extended heating of reactants. In one case I have to keep the bath at 170-180C for 7 hours, another at 185C for 2 hours. Do you think these crock pots can reach/take those temperatures?
Rosco Bodine
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No, you won't get that kind of temperature from a crock,
you will need a laboratory mantle and glassware for that,
or a deep fryer sort of unit. The crock may not even reach the boiling point uncovered, due to evaporative cooling. The most powerful crock available I think is
a 7 quart 400 watt unit. The crocks are actually an air bath heating of the ceramic liner by an inner aluminum shell around which is wrapped "heating cords" having a
sleeved fiberglass insulation, and a coil spring tensions
the ends of the sleeved heating element which is looped
around the aluminum liner shell, to secure the heating element in place snugly against the shell. The shell gets
hot and heats the small air gap between the inside of the
shell and the outside of the crock liner. A lot of heat
is lost through the outside case which holds the inner shell
coaxially, so wrapping the outside of the inner heating shell with a couple of layers of fiberglass cloth might raise the heat range capability a bit, or putting a heat transfer paste
or oil in the airgap might help raise the operating limit also.

[Edited on 21-1-2009 by Rosco Bodine]
Swede
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Thanks for the kind comments and thoughts.

I attempted a "doubled up" (2 heater bands rather than 1 on a 3 quart unit) and the heat was excessive. I let it run for about 10 minutes. There was a VERY faint smell of burning, either the paint on the exterior shell, or perhaps some wiring insulation. I took it apart and removed the second heater band. For the temps I need (60 to 80C) it was not needed.

The spring-loaded heater band has some potential for other uses. The wattage density is not too high, which is good. Any heat-resistant vessel of the correct diameter *might* be used to accept the heat, but watch out for cracking with glassware. The price is certainly right.

I doubt a stock crock pot is going to be able to do much more than 100 degrees C without a lot of modification... but I also believe such modification might be possible. I would use a sheet of that asbestos-mat substitute sold for protecting water pipes while soldering in situ, cut appropriately to protect the wiring from the Al pot which will become VERY hot indeed. There's not a lot of space between the inner aluminum pot and the outer liner, which is just sheet steel, painted. A piece of stainless steel sheet, thin enough, would be an excellent substitute. Although now that I think about it, the shell probably IS sheet stainless, but painted. The inside of the shell is unpainted. For a high temperature attempt, I'd strip the paint from the shell, or it may end up charring in places.

Watson.Fawkes - at 75 C, stabilized, I would guess the duty cycle of the heater band to be about 40%. The heat controller I built has an AC indicator light that tells me when the output is energized, and it is pretty interesting to watch the PID find and maintain the correct temp. If anyone builds up a heat controller with a solid state relay, add an indicator light - it is a useful addition to the system.

With the way I have it programmed now, it always seems to overshoot by about 2 degrees. It then slowly allows the bath to drop in temp, and once it nails the temp, it stays nailed very well. If I had a very sensitive process to heat, I'd set the SV (Set value) to two degrees low, maybe 3, then when the system is relatively stable, bring the SV up slowly, a few tenths at a time.

Each situation is different. Some setups leak heat a lot faster than others, while others are better insulated. Others have more mass to heat, some less. The controller figures it all out and does a fine job.

[Edited on 22-1-2009 by Swede]
not_important
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While this makes a very neat appearing device, the work isn't needed as you can simply turn the crock's heating switch to full and connect the original power cord to the new PID controller provided your temperature sensor is a probe type.

An electric deep fat fryer can be used similarly for higher temperatures, maybe up to 200 C.
Rosco Bodine
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@Swede

There is an idea which I have been thinking about regarding
a cell design using three crockpots as an electrolyte recirculation pH control system which operates by gravity flow and requires no pump The idea is based on the principle that a liquid in one container connected by a siphon tube with a liquid in a second container is a system where the liquid will seek to equalize the level in each container through the siphon tube. If you raise one container so
its liquid level is higher than the other, it will drain liquid
through the siphon until the liquid in the lower container
rises to match the level of the liquid that is draining from the raised container.

Now to exploit that principle what I am thinking about
is an assembly something like the old beam scale where
the weighing pans are suspended by three chains from the
balance beam. But for our apparatus instead of a weighing
pan , we have a crock pot 80% filled with electrolyte , suspended by the three chains. And between the two
suspended crockpots, a center stationary crockpot rests
maybe two and a half to three inches above the supporting tabletop so that when the beam is horizontal all three of
the crockpots are at the same level, all being 80% full of
electrolyte. Two 3/8" copper tubes are bent into a hairpin
inverted U-shape like croquet wickets, which bridge the gap
on either side of the stationary center crockpot to each of
the crockpots suspended on either side, and the copper
siphon tubes are filled with electrolyte. For the circulation
of electrolyte, all one has to do is attach a small slow speed
timing motor with a bellcrank and an operating rod to one end of the balance beam so that it slowly see-saw motions
up and down and the siphon action will do the rest.

Neutralizing material can be placed in a layer in the bottoms
of the movable crockpots and loose fiberfill polyester filter
material can be gently wadded and stuffed into the open
end of the tube or a small belled chamber at the end, to
provide filtration. The height of the hairpin and the low flow
rate will itself act as a sediment filter of sorts particularly
if there is an enlarged length of section in the tube where
the flow velocity slows so that any particles carried by the current against gravity, lose their lift and hover there in the
reduced velocity flow, until dissolved.

These copper tubes can also serve as cathodes for the
center stationary crockpot which is used as the plating cell.

To help prevent heat loss, the air exposed portions of the
tubes could be wrapped with woven fiberglass ribbon, and
if necessary they could even be electrically heated with
heating cord or band heaters applied to the enlarged
sections which were mentioned before. The enlarged
sections of the tube would be similar to the enlarged
section on a volumetric pipet.

So what do you think, hydraulic engineers, would this
work like I think it would, or is old Rosco "off his rocker"

I anticipate using a microswitch or two and probably a
delay relay for a horizontal position pause , and setting
the horizontal also as the hard stop position, so the
thing stays calibrated and self-correcting in operation, as well as shutting off in the equalized horizontal position.

I believe this sort of contraption would accomplish the same control as is illustrated in the Gibson patents, and a lot more simply in terms of plumbing and hardware. And this should make it possible also to produce massive PbO2 anodes if that is desired.

I think I just nailed it as far as that industrial process technology on an improvised scale at lowest cost goes

And of course the center "crockpot" which is the actual
plating cell doesn't have to be a crockpot at all, but could be
a much deeper container like a reaction kettle for example,
and I was simply using the description of three crockpots
identical and all in a row to illustrate the principle and simplest implementation. You could plate a twenty pound anode with this setup, by feeding it litharge and keeping the current going for days.

[Edited on 23-1-2009 by Rosco Bodine]
Swede
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 Quote: Originally posted by not_important While this makes a very neat appearing device, the work isn't needed as you can simply turn the crock's heating switch to full and connect the original power cord to the new PID controller provided your temperature sensor is a probe type. An electric deep fat fryer can be used similarly for higher temperatures, maybe up to 200 C.

HAHAHA! I thought of that AFTER I had butchered three crock pots, and I wondered how long it would take before someone figured out something so simple, yet I didn't think of it.

You are absolutely correct... turn it on high, alter or strip the plug, and attach your heater controller. The only thing you MIGHT miss out on is if the high setting does not use all of the heater(s) inside the unit. On the other hand, by hacking it and ensuring such is the case, you may be pushing the crock pot beyond its design limits.

It's still a cheap, handy source of a heat-resistant glassware, lid, heater band, etc. The 2 quart size is especially handy for prototype investigations.

Rosco, I like the way you're thinking. I don't think it could be made any cheaper, short of a nearly free peristaltic pump that could flow the used electrolyte over a neutralizing bed. If the main crock is large enough, you wouldn't even have to heat the neutralizer bed if it is small enough. If it's too large, you may end up with excess cooling and crystallization, but a bit of insulation will probably be all that is needed.

If anyone made such a rig, a photo is an absolute requirement. Three crock pots on a balance rig! The setup would have to be quite sturdy, as even a loaded 2 quart crock isn't super light.

The main crock could be 4 or 5 liters, while the ancillary crocks could be 2 liter jobs. Got to have safety backups - the thought of spilled lead nitrate is unpleasant.

I have become a bit paranoid about working with these lead salts, so much so that I bought two bottles of oral EDTA (chelating agent) as a "prophylactic"... I had to work that word in there somehow! There's a ton of pro and con of oral EDTA all over the internet... you have to decide if it's worth the risk.

The reality is, I have been casting lead and working with lead nearly all my life, and I wonder how much lead is in my body. I just had a ton of blood work done... I should have specified a Pb test.
sparkgap
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(sorry for off-topic-ry)

@Swede:

If you're looking to reduce your body's lead-load, may I suggest that you employ the tandem of α-lipoic acid and dimercaptosuccinic acid? The thing with EDTA is that it tends to mobilize lead all around even as it is taking it away to the kidneys...

sparky (~_~)

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Rosco Bodine
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Quote:
Originally posted by Swede
 Quote: Originally posted by not_important While this makes a very neat appearing device, the work isn't needed as you can simply turn the crock's heating switch to full and connect the original power cord to the new PID controller provided your temperature sensor is a probe type. An electric deep fat fryer can be used similarly for higher temperatures, maybe up to 200 C.

HAHAHA! I thought of that AFTER I had butchered three crock pots, and I wondered how long it would take before someone figured out something so simple, yet I didn't think of it.

You are absolutely correct... turn it on high, alter or strip the plug, and attach your heater controller. The only thing you MIGHT miss out on is if the high setting does not use all of the heater(s) inside the unit. On the other hand, by hacking it and ensuring such is the case, you may be pushing the crock pot beyond its design limits.

It's still a cheap, handy source of a heat-resistant glassware, lid, heater band, etc. The 2 quart size is especially handy for prototype investigations.

Hmmm I was also wondering why you didn't wye a switched
outlet power strip output from your PID controller so you could just plug into the strip whatever you wanted to control, like if you needed an averaging control of a three crockpot array , for example .
 Quote: Rosco, I like the way you're thinking. I don't think it could be made any cheaper, short of a nearly free peristaltic pump that could flow the used electrolyte over a neutralizing bed. If the main crock is large enough, you wouldn't even have to heat the neutralizer bed if it is small enough. If it's too large, you may end up with excess cooling and crystallization, but a bit of insulation will probably be all that is needed.

I was thinking you could cut a circular plexiglass shield or if transparency isn't an issue just get a melamine serving plate
and use as an evaporation cooling reduction covering over the open crockpot, and bore a clearance hole for the siphon tube. All three crocks heating would be required, but they could be energized in parallel by one controller and controlled as an array having a summed average output
seen by the one sensor and controller operating on the center cell. Yeah the cyclical cascading siphon action "tidal flow scheme" really would be slick wouldn't it ?
Minimal plumbing, gravity flow and filtering, no wetted pump parts or thermal concerns for the pump ....because the apparatus itself is the pump.

Got Betty Crocker singing now,.... yep she's rocking

 Quote: If anyone made such a rig, a photo is an absolute requirement. Three crock pots on a balance rig! The setup would have to be quite sturdy, as even a loaded 2 quart crock isn't super light. The main crock could be 4 or 5 liters, while the ancillary crocks could be 2 liter jobs. Got to have safety backups - the thought of spilled lead nitrate is unpleasant.

Actually, It would be better to use large crocks for the sash chain or cable or braided cord suspended ones, and you would have the opening of the wye centerline axis towards the support for the beam. That would give clearance for the
siphon tube "wickets" which would connect the center cell
to the heated reservoir crockpots. The siphon tubes would be secured to the wall of the center cell and the tube would extend nearly to the bottom of the center plating cell, but
would only go the half the depth of the outboard crockpots
when everything is in the horizontal position. The vertical
motion would only be plus or minus a couple of inches from
the horizontal , maybe one cycle per minute, a very gentle
kind of motion. The center plating cell may not have to be
heated at all, but insulation may suffice, with reheating of
the circulating electrolyte to keep the plating cell temperature
high enough.
 Quote: I have become a bit paranoid about working with these lead salts, so much so that I bought two bottles of oral EDTA (chelating agent) as a "prophylactic"... I had to work that word in there somehow! There's a ton of pro and con of oral EDTA all over the internet... you have to decide if it's worth the risk. The reality is, I have been casting lead and working with lead nearly all my life, and I wonder how much lead is in my body. I just had a ton of blood work done... I should have specified a Pb test.

Working with toxic materials is survivable without injury
if you are careful and just use common sense, going overboard with protections can cause another set of hazards of its own. Lead isn't going to jump out of the bottle and bite you, but you don't want to take a bath in the stuff either.
Bikemaster
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hello,

I don't want to be out of the subject but can we use the *PID TEMPERATURE CONTROLLER* to controle the temperature of a nichrome-nickel wire heating systheme.

thx

thehemi
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On Lead & Health: Adults are pretty tolerant of lead exposure compared to children, where we keep lowering what we thought was a "safe" threshold (wish I'd known this when I took to soldering at 9-10, now I have ADHD, whee). Dermal exposure is a non-issue. Inhalation or ingestion, not so good, but primary measure for recovery is removing the source. Your body will do the rest. The internet is full of misinformation. There is no circumstance under which you should be attempting chelation of any sort, period. If you need chelation then you need to be in a hospital bed.

As for crockpots, oh what fun. Walmart carries a cooker with a digital temp controller and probe. Google "HB Probe Slow Cooker". I purchased one on sale a few months back. Here's what it looks like:

TYPE SC26 SLOW COOKER 275W

Probe left, controller center, heater right

My original plan was to wrap the heater strip around one of those 30-quart aluminum turkey frying vessels (PTFE lined) for gentle and stable cell heating.

This is not an area of expertise though, so I started to have doubts (burnout, hot spots, thermal conductivity, temp regulation, electrocuting myself). I'd like to make some movement on this - so any yes/no/stupid feedback would be useful.
Swede
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Sorry I missed these posts...

Bikemaster, the PID temp controllers would control nichrome, or a 50 foot heating tower if necessary, because they don't supply power to the heating elements... they switch a relay on and off. If your relay is big enough and can handle the current, then the PID controllers can do the job. 50 amp solid state relays are remarkably cheap.

The hemi, it looks like a right proper hardware hack to me! You could mount that digital controller and power supply separately from the fryer, say on a piece of plywood. The heating element itself needs to make good contact with whatever vessel it is wrapped around. Thermal heat transfer compound might help. The probe is probably a simple thermistor, and you may be able to replace it with one more geared towards a lab, one PTFE coated. But I'm sure the probe in the pic is stainless steel, so unless you need something better, it could probably be used as-is.

One of the problems I noted with my cheap crock pots is that the heating element has a "taper" to it because the aluminum pot it was designed for is also tapered, but that heating band looks like a good one. I'd replace the spring and perhaps try a heavier one, but at the same time, retain the original, because it may be some special heat-resistant spring, whereas the replacement spring may lose its force if it gets too hot.

Another way to tension the band is with SS aircraft safety wire. I'd say go for it.
Bikemaster
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I read some stuff on SSR and i image the setting that i need to place all thing to be right. I made a fast draw of it, so if you can check it to be sure and if something is not right, just say me how to make it right.

what tipe of SSR did you recomende me, i will use 120v AC and between 15A and 25A.

thank swede

Attachment: AC.doc (31kB)

Swede
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Your diagram is correct except you are missing one component... in the right-hand loop that has your heating element (resistance) you need to have the correct power for that element. An SSR does two things... it acts as a switch, and also pretty much isolates the two sides of the system. If you draw a dashed line vertically right through the SSR, each side has its own power supply. The temp controller has its own power, either AC or DC, and the heating element also needs a power supply, probably 115V or 230V AC.

If/when you get a SSR, you'll see the INPUT side has a rating, maybe 5 to 24 VDC to turn it ON, so you want to be sure your SSR is MATCHED with the controller. If the controller outputs a DC signal to turn the SSR on, you'll need an SSR that responds to a low-voltage DC signal. If the controller outputs 115VAC, you'll want an SSR that turns on with a 115VAC signal. Browse eBay for SSR's and you'll see there are a variety of them, each with different inputs.

Once the controller has turned the SSR ON, you can think of the two output terminals as a switch, nothing more. So on your diagram, cut the upper right horizontal wire, and connect those two wires to an AC power source compatible with your heating element. You'll want the HOT AC wire to flow through the SSR, to the heating element, and back to the return (neutral) line.

Very, very little power flows from the controller to the SSR; just enough to trip it ON, and NONE of it flows to the other side. Again, think of the output side as a switch only, not a source of power, and you'll have it nailed.

Look at this picture. The lower two terminals are the input side, and state that anything between 3 to 32VDC will turn the SSR ON. The upper terminals are your "switch" and can handle a load of up to 50 amps of AC current, anywhere between 24 and 280 volts.

I hope that's clear as mud!
Bikemaster
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I remake my diagram because i as a small error, the ssr act as a swich and the pid is just send electricity the the ssr to say on / off.
I go check on ebay and i am now able to chose the right SSR with the right PID, but i have and other problem. The thermocouple have all an operation temp of 0-400, but i want control higher temp (up to 1100 C). but in an other auction i saw -50 to 1500 C temperature measuring range and also an operation temp of 0-400. can this thermocouple be able to operate at 1100 C?

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Swede
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If the heating element AND the controller are the same AC voltage, you can use a single AC line to the system, but what you have will work fine.

For a thermocouple, consider a type K. Be sure your controller can use a type K, and do its work at those temps. Most can.

http://en.wikipedia.org/wiki/Thermocouple

The Type K TC can do > 1100C... what you are seeing as the limits for the thermocouple itself is usually the braiding, or sometimes it is a teflon coating, close to the tip. What you can do is carefully strip a few inches of the sheath off the tip, exposing the two welded wires. A better option is to locate a type K that has a stainless steel or (better) inconel sheath with the actual TC junction inside. At those temperatures, any thermocouple will have a finite lifetime... it won't go forever. But an inconel-sheathed TC, while expensive, will have the longest life.

Omega Engineering has some excellent online resources regarding thermocouples, and they'll be happy to sell to you, although the prices can be a bit high. But by browsing their thermocouple lineup, you'll learn a lot... what works, what doesn't.

In a nutshell - a bared type K tip will work, just keep the heat from the braid or sheathe, and the thicker the wires and the junction, the longer it will last, but the slower the response. At 1100 C, I'd opt for a fairly heavy thermocouple.

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