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Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Avalanche wrote ...
I built a TC amplifier a few months ago around an LTC1050 chop-amp which was a bit OTT in terms of performance and price, but it was a one-off project and I needed it to work!
If you're interested in why you can't just use a single thermocouple to measure temperature, I'd recommend reading the physics of the 'seebeck effect' - here's a link
It's actually impossible to only have one junction anyway, a 'thermocouple' will only give a voltage proportional to the difference between 2 junctions. The TC amp I built at work around the LTC1050 had an arrangement of thermocouple wire as shown in the diagram on that wiki page, the hot junction was attached to an IGBT baseplate, and the cold junction was on the melted surface of a block of ice. With software compensation for non-linearity, we got a ridiculous accuracy! With one junction embedded into the IGBT and the other in the heatsink, the TC amp gave a voltage proportional to heat/power flow through the baseplate
I don't need extreme acuracy for this project, I just want to know if the diodes are getting above ~100-125 C so I can shut the rectifier down until it cools, These diodes aren't cheap. I don't need reference junction compensation really, but the AD8497 has it anyway. I'm not planning on putting 600A through the rectifier, it's a bit 'overkill' for HV projects but I do plan on using it for DC welding and electro-chemistry as well (electro-cleaning, electro-etching, electro-polishing, electro-plating, etc.).
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
As an example of what you can get if you move away from thermocouples, here is the graph of the division ratio of a thermistor in series with a fixed resistor. The thermistor relation I've used is -4% per C, which is pretty typical. I've gone to the RS site, and there are more there than you can shake a stick at. This one is 68k at 25C, about £0.50 (in 5s, but then you want more than one), with a maximum operating temperature of 150C, in an 0603 package (there are packages that go up to 200C operating). I've simulated it in series with a 3200 ohm resistor, which is its nominal resistance at 100C.
As you can (just about) see, it goes through 0.5 at 100C, as you'd expect. It doesn't go up very fast at low temperatures, like you asked for. Its highest sensitivity is where it matters, around 100C. It's pretty damned linear over +/- 10C from the mid point, at +/- 20C it is starting to bend visibly. But it is still completly usable, i.e. monotonic and with some sensitivity over the whole range.
If you are lucky, you should find a value and sensitivity that match your meter resistance at your target temperature, and can tolerate the self-heating that driving your meter would entail without too much loss of accuracy. Then you can simply put the thermistor in series with the meter, and drive both from a constant voltage source. I think that wins for simplicity. Otherwise, just use a resistor as a load and scale the output voltage with an opamp and apply to your meter.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Ash Small wrote ... I'm collecting five 100k thermistors from Maplins tomorrow for £4.50
Finally! And based on Dr. Slack's approach, your chose a value that supports a very simple solution. I bet you can get the job done with, for each channel, one each of:
thermistor, 5K variable resistor, meter movement in parallel with one side of the variable R, common voltage source.
A voltage reference or regulator IC at 1.2 or 1.25 V, like LM317, would be great. Have fun!
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Destroyer of mosfets wrote ...
Won't you need a rather large heat sink for that? maby you could modify a buck converter from a cheap 12v phone charger to get 2v
I'm planning on running ALL the fans for cooling the heatsinks from 12V now. I did briefly consider using a 5V CPU fan to cool the LM317T but it didn't really make any sense.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Well, if you are worried about response speed, you have to ask yourself how quickly your big stud diodes can change temperature in the first place. You also have to ask yourself how often you will look at the temperature meters, since you never mentioned any sort of automatic cutout.
You are a serious candidate for this year's "Most Overcomplicated Rectifier" project. It's 4 diodes in a box, for goodness' sake. I'd use a bimetallic thermal switch bolted to the heatsink, tripping a contactor in the mains supply to the transformer. Or, over current protection might be all that is needed, since the temperature of the diodes depends mainly on the current flowing.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Steve Conner wrote ...
Well, if you are worried about response speed, you have to ask yourself how quickly your big stud diodes can change temperature in the first place. You also have to ask yourself how often you will look at the temperature meters, since you never mentioned any sort of automatic cutout.
This is an instrumentation system, not a control system.
Steve Conner wrote ...
You are a serious candidate for this year's "Most Overcomplicated Rectifier" project. It's 4 diodes in a box, for goodness' sake. I'd use a bimetallic thermal switch bolted to the heatsink, tripping a contactor in the mains supply to the transformer. Or, over current protection might be all that is needed, since the temperature of the diodes depends mainly on the current flowing.
The way I see it, 'four diodes in a box' will get hot without efficient cooling. Last time they burned out, hence the instrumentation. These diodes aren't cheap.
Overcurrent protection won't give any warning of cooling fan failure for example.
The most simple solution I can come up with is to use a LM317T to give 2V, then have a string of 4 resistors each supplying 0.5V to each thermistor/meter. The resistor string will also smooth out the power available to each thermistor/meter,
I have no idea how the LM317T works, other than asuming the control pin is analagous to the base or gate of a tranistor, so I was wondering how to work out the power requirements (wattage rating) for R1 and R2.
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Monitoring Heatsink Temperature...
I'd use a bimetallic thermal switch bolted to the heatsink, tripping a contactor in the mains supply to the transformer. Or, over current protection might be all that is needed, since the temperature of the diodes depends mainly on the current flowing.