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| 2N2907 PNP transistor as temperature sensor |
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| David Hess:
--- Quote from: alex.martinez on February 11, 2019, 09:07:26 am ---Thank you! I had already read this application note, together with AN137. --- End quote --- Tektronix includes a detailed circuit description in their service manuals for the DM501, DM502, and 7D13 which is also worth reading. |
| jackthomson41:
You are using 2N2907 as a temperature sensor, that's kind of a new thing for me and I still can't understand this how you are gonna implement it? Btw when I need temperature sensor I use DS18B20. I think you are trying to design a new temperature sensor may be. You can also check this DS18B20 Simulation in Proteus. |
| alex.martinez:
--- Quote from: Kleinstein on February 11, 2019, 12:16:39 pm ---250 µA is quite a lot of current and will heat up the sensor quite a bit. An advantage of the diode as a temperature sensor is that it can work with a low current and still have a relatively low impedance. So something like 1-10 µA should be sufficient. The amplifier can be non inverting - so no need for the extra buffer. Using digital decoding for the difference is a good idea, though it might need a little more resolution for the ADC: the difference is proportional to the absolute temperature. So a 8 Bit ADC would only get steps of a little over 1 K if the gain and offset are well chose. Just direct voltage reading could give higher resolution as higher gain could be used. --- End quote --- True, but according to the datasheet if the transistor is at 250 uA the temperature coefficient is the most linear. I'm still tinkering with it, so I will try 10 uA as well. What I want to do in terms of ADC is to obtain the 100°C ∆Vbe and he 0°C ∆Vbe. Then subtract that offset and amplify to the full scale of the ADC (Arduino's 10 bit). --- Quote from: David Hess on February 11, 2019, 11:54:57 pm --- --- Quote from: alex.martinez on February 11, 2019, 09:07:26 am ---Thank you! I had already read this application note, together with AN137. --- End quote --- Tektronix includes a detailed circuit description in their service manuals for the DM501, DM502, and 7D13 which is also worth reading. --- End quote --- Gonna check it this afternoon. --- Quote from: jackthomson41 on February 12, 2019, 08:18:43 am ---You are using 2N2907 as a temperature sensor, that's kind of a new thing for me and I still can't understand this how you are gonna implement it? Btw when I need temperature sensor I use DS18B20. I think you are trying to design a new temperature sensor may be. --- End quote --- It's just a bit of an experiment. Thilese parts are absurdly cheap when compariled to a dedicated temperature chip. Plus, a the end of the day, a diode connected BJT is what most silicon based chips use, but with different kinds of control systems. |
| StuartA:
Re the use of transistors as temperature sensors, am I right that they offer a more-linear response than a simple diode? I built a thermometer using a simple diode (silicon signal) many years ago, and that has linearity adequate for my purposes over 0-100'C when calibrated. The advantage it may offer over a transistor is that you can configure it so that one lead projects forward and is then folded back again, thus forming a very compact 'sensing pad'. I used it recently on a flanged resistor, together with K-type thermocouple sensor right along side. During fast heating, the diode was really fast to respond, always showing an appreciably higher reading than the thermocouple. During slow cooling, the two probes gave the same reading. S |
| Kleinstein:
Transistors are sometimes a little closer to an ideal diode than most real diodes, as the material tends to be a little cleaner. However there are plenty of diodes to choose from, so some can also be good enough. In some cases the transistor case may be more convenient (e.g. TO126). |
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