Temperature limit on Soldering iron\heating element

Question

I am trying to design a soldering iron type of tool that will not heat-up past, lets just say 400 degrees F. I know I can get a PID controller and manipulate the temp, but I'd JUST like the unit to never heat above 400 F, no need to turn it up or down. The reason being is i need the modification to be as SMALL as possible...I cant have an external housing. Any ideas?

Answer 1

You could design one. Put in a temperature sensor like an RTD, a circuit that amplifies the voltage a bit if you need high accuracy, and a comparator that asserts low just below the setpoint. The comparator can shut off the heater via a relay. It won't be as clean as PID output, but should do your job. That is essentially what thermostats do, so you could shop around for one that fits the bill. In my opinion, it'd be simpler to just build your own.

I've put together a very rough schematic of what you would be looking for. Remember that this is a fairly crude control mechanism. Also, don't put much stock in the text and labelling. I was trying out the circuitlab thing and it doesn't have the parts that I needed, so I did some repurposing :P

You need something to measure temperature. I've assumed you use a K-type thermocouple. Using an RTD or a thermistor will likely prove cheaper, and will only need minor changes. It's also possible to measure this using a diode.

^{simulate this circuit – Schematic created using CircuitLab}

In this case, the thermocouple produces a small voltage (42uV/K) when there is a temperature difference between the hot and cold junctions. You would use an instrumentation amplifier, like, say, the INA114, to amplify this signal by about 50 or 100 or maybe even a 1000 times. This should let you bring the voltage up to an easy to measure level (order of 0.1 - 1 V). Note that you should use a rail to rail output opamp if you have only 0 and 5V - the INA114 typically runs on bipolar supplies (+/- 5V), and when you run it at 0-5 you won't be able to handle signals close to 0. INA333, I think, is a rail to rail single supply instrumentation amplifier. TI, for example, has plenty in its catalog.

Then, you use a comparator like the LM311 or so, whose output goes low when the temperature crosses a certain 'setpoint', set by the pot at R4. This is a very crude setting. Particularly, remember that thermocouple only measures temperature differences and not absolute temperature, so one usually also measures room temperature using some other sensor and adds that to the measured value. This is called cold junction compensation (and is why an RTD is probably a better option for something of this sort). R6, R5, and R7 set up hysteresis in the comparator - once the heater is turned off, it stays off for a little while even if the temperature falls below the threshold. This essentially moves the threshold slightly lower. The output of the comaparator is used to control the heater through a relay. The transistor amplifies the output so that the necessary current to activate the relay is available. Many comparators may be able to drive the relay directly (remove R8 and Q1 and connect the output directly to the relay.

Answer 2

No such element is available in any reasonable price point (perhaps at all). It is extremely difficult to calculate the heat generated with certain current of certain heating element, especially since once you try and solder it, you're conducting heat away from the element. Controlling the temperature using PID or other techniques is a reasonable way at low cost to ensure a consistent temperature given the amount of variables otherwise that would need to be taken into account.

Answer 3

You can use a bimetallic bar to function as a normally closed switch that opens at 400 degrees. This doesn't need to be big, but the mechanics would be tricky. Of course, even this wouldn't function real well without a controller.