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| Temperature indicator circuit with LEDs |
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| dazz:
--- Quote from: Hero999 on August 20, 2018, 07:07:00 pm --- --- Quote from: dazz on August 20, 2018, 06:16:05 pm ---Ahh, this is a massive fail. As Hero999 suggested, it's too temperature sensitive. I had the circuit breadboarded and working nicely, but as soon as I heated it up with an electric heater, one of those that blow hot air, the amps shooted up by a lot. The red LED went from 150mA to almost 2A :palm: :-DD. I wasn't measuring the current through the yellow LED, but I could tell it was about to blow up. Oh well, off to shop an Arduino I guess |O --- End quote --- Which circuit did you build? None of the circuits you've posted in this thread should fail like that. --- End quote --- The last one, the one with the darlington MPSA13's. Maybe the heat was affecting the LEDs even more than the transistors? |
| Zero999:
--- Quote from: dazz on August 20, 2018, 07:15:04 pm --- --- Quote from: Hero999 on August 20, 2018, 07:07:00 pm --- --- Quote from: dazz on August 20, 2018, 06:16:05 pm ---Ahh, this is a massive fail. As Hero999 suggested, it's too temperature sensitive. I had the circuit breadboarded and working nicely, but as soon as I heated it up with an electric heater, one of those that blow hot air, the amps shooted up by a lot. The red LED went from 150mA to almost 2A :palm: :-DD. I wasn't measuring the current through the yellow LED, but I could tell it was about to blow up. Oh well, off to shop an Arduino I guess |O --- End quote --- Which circuit did you build? None of the circuits you've posted in this thread should fail like that. --- End quote --- The last one, the one with the darlington MPSA13's. Maybe the heat was affecting the LEDs even more than the transistors? --- End quote --- The LEDs all have resistors in series with them, so the current shouldn't go that high. On the previous schematic you posted, the red LED has a 100R resistor in series with it, which would limit the current to 90mA, even if the transistor and LED failed short circuit. How hot did you heat it? If it got so hot, it started to overheat and char the resistors, then it's possible they could fail short circuit, but it would start to release smelly fumes, before then. |
| dazz:
With one of these. I'll double check everything, thanks a lot |
| dazz:
OK, I built the circuit from scratch and it seems fine now. I have no idea what I did wrong the first time but it now seems to take temp variation better. I put it in the fridge and lowered it's temperature from 30ºC to 10ºC. The current in one LED dropped from 550uA to 320uA. That's the equivalent of lowering the input from 70ºC to 62ºC. So a 20ºC drop in ambient temp shifts the measurement 8ºC. Not ideal but not terribly bad either I guess. I'm gonna try now upping the temp to 50ºC in the oven. All for the science! :-DD I also scavenged three C946 NPN transistors from an old, dead computer power supply I had forgottten about, maybe those perform better temp wise |
| dazz:
It's a lot better with those NPN's, just as Hero predicted :-+ Now a 20ºC ambient temp variation shifts the measured temp by only 4ºC. This is not a precision gauge so I'm gonna call that good. Thanks everyone! ETA: pic attached |
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