Hottest of 3 thermistors? - Page 1
Forum section: electronics / beginners

author: alanambrose
author_rank: Frequent Contributor
timestamp: January 25, 2016, 12:50:57 pm
content: Hi,I would like to have a backup hardware shut-off on a heating application. (There's already a software shutoff, of course, but in one case it didn't operate as some UI loop was continuously running as something was leaning against a button).It has 3 separate thermistors (13K from memory) running into MAX6682 thermistor-to-SPI chips.I want to have a hardware shut-off which watches all 3 thermistors, and uses the highest temperature and a chosen level to activate a fail-safe. OK I can see 3 comparators and an wired-OR - but is there a better / more compact way?Alan

author: Xenoamor
author_rank: Regular Contributor
timestamp: January 25, 2016, 01:03:44 pm
content: You can get quad comparators in small SMD packages.Your fourth comparator can form the OR gate:You could also wire your thermistors in series or parallel. How far apart are they?Also you should have a watchdog timer in your program. If the program never reaches it's end. For instance, if a button is stuck down, the device will reset. On reset detect if the watchdog has triggered and then shutdown the machine

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 25, 2016, 03:08:57 pm
content: All three in parallel is the simplest possible solution.This works because thermistors are NTC and have a fairly aggressive slope. The actual "trip" temperature will be reduced when all three thermistors are nearby, but maybe that could be considered a feature rather than a bug (high average temperature being as bad as a higher local temperature?).They can possibly be connected with diodes ("wired-OR"), if the monitoring circuit is known.Tim

author: alanambrose
author_rank: Frequent Contributor
timestamp: January 25, 2016, 06:49:31 pm
content: >>> You could also wire your thermistors in series or parallel. How far apart are they?They're checking different parts and ~50cm apart (that's why there are 3) so I think it wouldn't be so great to turn them into one 'averaging thermistor'. I take the point re the slope, but nevertheless.>>> Also you should have a watchdog timer in your program. If the program never reaches it's end. For instance, if a button is stuck down, the device will reset. On reset detect if the watchdog has triggered and then shutdown the machineYes, I will check on that. The problem though wasn't that it was stuck or crashed, but that it was running the UI for such a long time that the PID loop (which only runs while the UI isn't being serviced) wasn't running.A.

author: Yansi
author_rank: Super Contributor
timestamp: January 25, 2016, 06:53:13 pm
content: First, TL08x is not a comparator. Second: "OR-ing" proper comparators is just easy, as they have usualy opendrain outputs. Please see LM339.

author: alanambrose
author_rank: Frequent Contributor
timestamp: January 26, 2016, 01:48:57 pm
content: Surely there must be an easy way of doing 'max or min of 3 voltages' in hardware? If they were low impedance sources a sort of analogue diode gate would work. Anybody?A.

author: zapta
author_rank: Super Contributor
timestamp: January 26, 2016, 02:01:17 pm
content: Quote from: alanambrose on January 26, 2016, 01:48:57 pmHow about three voltage followers op amps, outputs are diode or'ed, feedback is from the common point after the diodes?The losers will try to pull the voltage down buy the diodes will prevent it.Then you can a add voltage divider at the feedback point to have amplification.

author: Chalcogenide
author_rank: Regular Contributor
timestamp: January 26, 2016, 02:06:21 pm
content: Just a quick idea. Using a quad opamp and 3 NPN transistors, sharing a single load resistor, you should be able to get the analog minimum voltage among the 3 thermistors, assuming they are all grounded on one end.The NPN provides an inversion in the gain, that's why the feedback is provided on the non inverting input. Only the opamp with the lowest input voltage will be able to force it as the output voltage, because the BJTs can only sink current - the other 2 opamps will work open loop and saturate to their minimum output voltage. You may have to experiment with a degeneration resistors from the emitter to GND and/or a resistor from the opamp output to the base and/or whatever because the extra gain might cause the circuit to become unstable.I guarantee nothing, I haven't tried to simulate it or anything, just quickly thrown this idea on paper.The 4th opamp can work as a comparator.

author: Ian.M
author_rank: Super Contributor
timestamp: January 26, 2016, 02:08:36 pm
content: It cant be done simply sharing thermistors with MAX6682 chips as the MAX6682 powers down the thermistor when its not taking a reading.Therefore you need separate backup thermistors. Three comparators with a common setpoint can be done with one chip. Rather than wire-oring OC outputs, you may choose to diode OR the outputs so you can monitor the individual over-temperature outputs digitally so the fault location can be reported. That leaves you with one spare comparator in a quad package. Either use it to buffer/level shift/latch the wire-ORed output or for some other critical shutdown condition.

author: Yansi
author_rank: Super Contributor
timestamp: January 26, 2016, 02:27:34 pm
content: Quote from: Chalcogenide on January 26, 2016, 02:06:21 pmYour schematic will end up (most probably) as a nice oscillator. You have to linearize the transistors, by at least adding an emitter resistor so it will behave better as a kind of voltage controlled current sink.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 26, 2016, 04:12:21 pm
content: Quote from: alanambrose on January 26, 2016, 01:48:57 pmEasy is using a diode. But easy is low accuracy. It's all related to how accurate you need to be.The main thing is you haveQuote from: Ian.M on January 26, 2016, 02:08:36 pmIf you have a functional model of how it performs a reading, you can work with that and maybe come up with some methods. But it sounds like it's going to be more complicated already than just using diodes or whatever.Tim

author: Ian.M
author_rank: Super Contributor
timestamp: January 26, 2016, 04:19:32 pm
content: Probably the ideal high reliability solution is three non-resettable thermal fuses in series with double insulated connections, either directly providing the supply, or providing hold-in coil power to a relay or contactor in series with the supply, or holding the gate of a crowbar thyristor or TRIAC low against a pullup resistor.

author: alanambrose
author_rank: Frequent Contributor
timestamp: January 27, 2016, 09:07:10 pm
content: OK thanks for everybody's thoughts.Alan