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Cute temperature controller

Status: Proposal
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November 4, 2015
TMP-36-Temp-Alarm_schem.jpg
Simple Temperature controller / alarm
Modern electronics have evolved to a very great extent today such that it over spreads on the territory of computer science on one hand while under spreads on the zone of Information technology as well. Today there is hardly any electronic project which does not use micro controller unit and or computer programming languages.

However, still there are projects which can strip down all these high tech MCU and computer programs and can still make life better and easier.

Temperature controller for many things:
In our daily life space temperature controller has umpteen numbers of uses – rice cooker, fridge, water geyser, micro wave oven, even the starter switch of a simple tube light is temperature controlled.

NiMH or Li-ion battery charger:
If you think these batteries are very fussy about charging and discharging then you are absolutely right. Therefore, if you can stop the overcharging of these batteries when the battery reaches a particular temperature (for Ni-Mh it's 45 Deg C to 50 Deg C , for Lion it's 45 Deg C) the life of these batteries will certainly extend beyond normal usage.

Tip temperature of the soldering iron:
For a normal soldering iron it's an open loop controlled temperature. However, by measuring the tip temperature or somewhere near the tip temperature we can initiate the next phase of temperature control process.

Modulate the gas burner when the water starts boiling:
While cooking once the boiling starts it's frugal to throttle the gas v/v so that less gas will be consumed. ,Just set the controller at 100 Deg C and when it reaches that temperature the relay operates the modulation switch.

TMP36 the versatile temperature sensor:
  1. TMP36 has a low voltage operation – 2.7 volt to 5.5 volt
  2. -40 Deg Cent to +150 Deg Cent operating zone.
  3. Linear operation y = mx + c
  4. Measure temperature directly into mili voltage. Uniform linearity of 20mV / Deg C
  5. Less than 50 µA Quiescent Current.
  6. Ramp up = 3 Deg C/ Second and Ramp Down = 6 Deg C / Second.

NPN-PNP junction (the Sziklai pair ) has an advantage over Darlington pair is that it's base-emitter voltage is 0.61 volt unlike 1.22 volt for the other. Principles of operation: It's a simple yet no-nonsense temp controller using very few discrete components. The main sensor TMP36 picks up temperature, converts it into millivolt. Which is then measured by the NPN, PNP junction – the Sziklai pair and then when the base voltage overcomes the bias limit the PNP transistor conducts and the 5 volt relay gets energized.

Calibration:
The 4.7K trim pot is a precise multi turn type to set the temperature set point. Heat up the probe to the desired temperature and then adjust the trim pot so that the relay just de energizes. At this point if the temperature increases beyond that point the relay will energize. The NC (or NO) contact of the relay may be used to disconnect a charger, gadget or soldering iron etc. Conversely when the temperature reduces, the TMP36 cools down and the relay gets energized again thus reconnecting the device with the power circuit once again.

Oscillation against a mean temp:
The TMP36 has a ramp up of 3 Deg C/Second and ramp down of 6 Deg C / Second. So normally it takes time to heat up and cools down on it's own thus avoiding oscillation over a mean temperature.

Advantage:
​This small ,simple yet precise temperature controller can do many amazing things out of the box. The 5Volt relay can switch on / off many common gadgets that run from power sockets. However, it's big brother always finds to do the same job through an MCU (viz: Arduino ) which is beyond the scope of this article.

The total cost of the project is <$2.0

Bye, bye

S. Bera / Vindhyanagar
 
 
 
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Images
Schematic diagram
TMP36 pin up diagram

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