Amplifying T12 tip's thermocouple, while blocking hi pwm voltage? - Page 1
Forum section: electronics / projects

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: June 11, 2020, 01:15:18 am
content: Ok so just like the subject says, i need to figure out the correct way to amplify the thermocouple in a t12 tip. Using a TC amp IC is not an option, or the CC would have to go in station instead of handle!!! So it seems like either using a comparator or using a zener clamp is the best way to keep the amplifier from seeing the high heater voltage. So I think the best way to do this is use an Instrumentation, or at least differential amp, and then just do the CC in software. This is the jist of my idea implementing the LT1167 instrumentation amp, and DS1820 digital 1 wire temp sensor embedded in t12 handle.The thing is I see a lot of schematics implementing this totally different using a single rail to rail opamp, which is strange to me because you want to the voltage across the tips + and 1 terminals, not + and ground, also isnt using a single ended design more prone to common mode noise? Obviously you only need to read from 0 to about 54mv, so a dual supply like my design isn't necessary unless the amp needs it. But whats really bugging me in a lot of schematics I see is a 10k resistor in series with the thermocouple, im guessing to protect the zener when the heater is on. But isnt throwing a 10k resistor between the thermo and amp going to cause voltage drop and Johnson noise? Here is an example of one of the setups im talking about.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 12, 2020, 06:23:55 am
content: Soldering irons are not high accuracy temperature devices, so a few degrees deviation is no problem.The 10k series resistor and zener? diode is to protect the opamp input from the high (24V?) PWM voltage. Maybe the wire usually is connected to GND, but wires break, connections get loose.The opamp has a high input impedance so the current (and therefore voltage drop) over the 10k resistor is simply assumed to be zero, or a constant (opamp bias current) and calibrated out. Noise (if it is even a problem) can be averaged over lots of samples.

author: magic
author_rank: Super Contributor
timestamp: June 12, 2020, 08:29:27 am
content: Johnson noise of 10kΩ is 1µVpp over 0-100Hz bandwidth.I have seen a station for similar irons which performed fully analog control using sample and hold implemented with NE555 and some JFET.

author: RoGeorge
author_rank: Super Contributor
timestamp: June 12, 2020, 12:57:57 pm
content: If you are trying to do it the "correct" way, you'll get cumbersome schemes, I've even seen some in which both sides of the heating element are isolated with two MOSFETs, so the thermocouple's voltage can be properly read. This is even more hilarious when we think there is no cold junction compensation for the other side of the thermocouple, which makes the whole thing about 10-30 Celsius inaccurate.However, if you search for schematics in existing products, most of them are using a LM358 and a series resistor, sometimes with diodes tied to the V+ and V- of the op-amp power supply, but not necessarily. The capacitor from the input of the op-amp to GND seems to be an essential component, too.How could that measure tens of microvolts, with 24V AC or PWM over it, with enough precision and accuracy over 1-2 meters long wires, it completly puzzles me.No need for expensive op-amps or low noise and auto offset compensated chopping op-amps.The LM358 seems to be the magic part that just works.