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That's a good question and the answer depends on whether you mean "two seperate blankets on two seperate molds". Generally.....no. But that doesn't mean it can't be done effectively.

If the blankets are the same size and wattage, and the surfaces to be heated have pretty much exactly the same mass, and the placement of the blankets can be said to be very similar, then there is a good chance that one controller can control two seperate forms.

These simple microprocessors can only deal with one signal at a time coming from a thermocouple and can only control one relay at a time. So as you monitor what's going on with one form, the other form could be hooked up the to relay, and recieve the same amount of switched power, but it's just going along for the ride and is being heated without really being controlled. In this scenario, the amount of material to be heated needs to be the same amount as the controlled system. If the mass of the non-controlled form is less then the controlled from, the non-controlled from will get hotter. If the non-controlled form is heavier or larger, it will remain cooler. One could rig up a thermometer or some other temperature sensor to see what's happening with the non-controlled form but one would NOT have control over the form that wasn't connected to the thermocouple. That said....I used to do this all the time with heat controlled molds but it required a mass calculation along with geomertry and airflow consideration.

So.....two or more identical forms? Sure! You could do that with one controller...but you'd want to at least monitor the non-controlled form(s).

There are microprocessors that CAN control two or more disparate loads but they require seperate relays and thermocouples for each and the cost of these controllers is high enough that two simple systems is much more economical for this application.

Assuming you might want to try this, the load capacity of the system is entirely dependent upon the amperage capacity of the relay. The silicone blankets are rated for wattage usually. Watts/volts = amps. Add up the amps and make sure the relay can handle it.

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I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

I use two blankets on the same mold while bending. On above and one below. Of course the one on top would get hotter as it doesn't have the heat sink in terms of something above and below to soak heat. I'm trying to remember Dana B's setup. Seems like he had one controller. It's been some time so I'm not sure on the details. At any rate it was more of a curiosity as I doubt I would buy one of these. I've not broke or cracked a side in 10 years just by monitoring the blankets and cycling the power on and off manually. But who knows..... I do like gadgets.

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John Mayes
http://www.mayesluthier.com

I take this to mean running two blankets on the same form. I don't know enough about the LMI controller to say. The controller can control two blankets with one thermocouple. The main issue with doing that is, can the system handle the amperage. If you run two 5 W/in2 blankets you would pull ~18-20amps(depends on size of blankets). So, first you would need to be on a 20A circuit breaker. Second, does the LMI box have adeqaute wiring, or fuse(not sure if it has one or not)

On my controller, not LMI, I run a 2.5 and 5 W/in2 blanket with a 15A fuse built into the box. So far so good.

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Matt Jacobs

"Don't tase me bro"

There are pulse type controllers that function well without PID....these really cheap controllers would probably be more than adequate for side benders.

PID controllers have the ability to calculate, on the fly, the rate at which temperature rises and falls so that the controller can adjust the duration and frequency of the pulse as the system approaches the set temperature.....in order to avoid overshooting or undershooting the set temperature. This is especially critical when trying to control chemical reactions....probably not so important when bending sides.

There are a few tricks to making a silicone blanket last.

You need to make sure you are overkilling your application with wattage rather than using a blanket without enough capacity. On a room temperature form, when initially activated, the controller is going to maintain a constant "on"...and it won't begin pulsing "off -on" until a certain temperature is reached. A lack of wattage here means the blanket will be "on" for a much longer period of time initially....one recipe for burning the blanket.


The main reason for burnout out is the heat conductivity and mass of the surface to which the blanket is applied. More metal is better. Trying to apply a blanket to a thin strip of sheet metal is more difficult than applying it to a thick mass of metal. Metal is going to act like a heat wick. The more of it behind the blanket, the cooler the actual blanket remains. When metal wicks aways heat it also homogenizes that heat. Spreading out heat evenly works to eliminate hot spots in the blanket. For this reason, aluminum, which has excellent heat conductive properties is probably best to use as a metal for this purpose. This also begs the point of making sure the blanket is very well applied to the surface with no gaps or bubbles.

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I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

Ah...in that case.....one controller can control any number of blankets all running through one relay that can take their cummulative amperage....assuming they all have the same watt per sq. in. rating and they are all the same size. If not then thermocouple placement becomes pretty dicey.

The trick with multiple and equal heat sources is placement of the single thermocouple. That can be something of an art but it doesn't sound too complex here. Let's say you had three blankets on one side bender. You want to place the thermocouple such that it falls between two of them, and then place the third on so the gap is about the same between them all.

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I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

The actual value of stuff in there is even lower than you think. You're paying a lot for the pre-programmed 'microprocessor controller'. Inside that $100 box, there's something like a $2 Atmel or PIC microcontroller and a couple super-cheap relay drivers; probably $10 or $15 of parts. On the other hand, it's half a day to make one from scratch for someone who's really good with that sort of stuff and so it's still worth buying

As for PID and PWM...it gets nerdy after this so feel free to ignore unless you're really interested:


The PWM part is the 'all the way on' or 'all the way off' part. It stands for Pulse Width Modulation and it regulates by switching a high-voltage or current source on and off quickly and varying the amount of 'on' time. So, with a 10V input it could put out 5V by staying on half the time. The switching speed, and possibly smoothing of the signal with a capacitor on the other side, means the output looks close enough to a real 5V. Most variable power supplies work this way now.

PID is a predictive algorithm, as some have mentioned, and works off of the difference between where a system is now and where you told it to go (the error). The proportional part reacts to the immediate error (magnitude) and determines the reaction speed / twitchiness of the system. The integral term watches the sum of error over time and so corrects for steady state error that might not be large enough momentarily to 'set off' the proportional term. The derivative term watches the instantaneous rate of change of the error term and counteracts the proportional terms tendency to overreact by damping the response to sudden changes. PID tuning can really suck

PWM is pretty much the standard for regulating both voltage and current from your cellphone's battery all the way up to industrial motor amplifiers. PID is the standard for most process control applications from controlling water tanks and chemical reaction rates (by controlling how open valves are) to controlling servo motors like the ones in industrial and CNC machinery.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Most of this technical talk is WAY over my head, but I've got a question, either about he LMI controller or the ones people have built themselves.

How low can you control the temperature? I use a heating blanket(actually 2, 12" X 24" blankets) between my top or back plates and radius dish when gluing braces. For some reason even with my router speed controller set as low as it will go it still gets a little too hot. I want the plates to be 100 degrees F. I've been plugging and unplugging my blanket, just like I do when bending sides, but being able to set it to 100F and forget it would be great.

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I didn't mean to say it, but I meant what I said.
http://www.brackettinstruments.com/

The essence of how these controllers work is to "plug and unplug" them. It's just a microprocessor monitoring the temperature and doing the plugging

You can have a temperature set point anywhere between 32F and well past burning yourself with the system I described earlier....which is to say that you can certainly hold 100F with one of these systems.

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I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

Great idea Woody where did you get the 12x24 blankets. If you get a very thin thermocouple that would slip between the blanket and top/back the controller will do the job, though you will need a big enough relay to do carry the extra current of 2 blankets. Search Ebay for Temperature controllers, there are lots to be had and the prices are reasonable. We went through a few threads on PID controllers in the past couple years on OLF so info is available.

Fred

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Fred Tellier
http://www.fetellierguitars.com
Facebook page http://www.facebook.com/pages/FE-Tellier-Guitars/163451547003866

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The LMI unit is currently only available in 110v. You could make your own easy enough with a 240v PID and relay and yes it will control pretty much anything as it simply turns power on an off as required to maintain a set temp, it really does not matter what that is, it could be a fan a hair dryer what ever the PID will simply read the thermocouple that is placed on the work piece and switch what ever appliance you choose on an off as necessary to maintain temp.

Cheers

Kim

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Not if the cartridge is 240v and even if it is only 110v it still depend on how many amps it pulls because if the cartridge needs more power than the LMI unit is designed to provide then an SSR (Relay) with adequate output would need to be wired into the loop. At the end of the day if you live with a 240v supply then it would be both functionally better and cheaper to simply make your own controller system with bits from ebay and the help of a sparky.

Cheers

Kim

Thanks, Zlurgh. I didn't know, and couldn't find what range they'd control.




I got 4 blankets a while back at a yard sale(for $1 apiece). I have no idea what wattage they are. I guess they're 110volt because they get hot when I hook 110 to them. They didn't have any kind of plug on them, just a pair of 18 gauge wires. I doubt they draw much current or they'd have bigger wires. It takes them ~30 seconds to reach 100F, and they keep getting hotter. With my router controller on the lowest setting they reach ~150F.

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I didn't mean to say it, but I meant what I said.
http://www.brackettinstruments.com/

Woody,
The wattage of these blankets tend to be 2.5,5,and 10 W/in2. If the blankets you have came from Omega the product code may tell you what the wattage is, but you'll have to look on their website. Don't let the small wires fool you, the wires are the resistors that heat up, so they keep them small so they heat up easier and keep the blanket flexible.

You should be able to maintain 100F with the LMI unit. However, like Zlurgh said, you have to very consistant with the placement of the thermocouple, otherwise it will be difficult to control.

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Matt Jacobs

"Don't tase me bro"

Woody, you can calculate the amperage draw for the blankets using the calculator "Calculate For Current".
http://www.the12volt.com/ohm/ohmslawcalculators.asp#current
Would need to take a resistance measurement with a VOM (Volt/Ohm Meter) first.
Nelson