Remote Temperature Sensors — What remote BJT/Thermal diode should I use?
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There are certain criteria that should be met when choosing a transistor to use for remote temperature sensing.
1. Base-emitter voltage (VBE) is > 0.25 V at 6 μA, at the highest-sensed temperature.
2. Base-emitter voltage is < 0.95 V at 120 μA, at the lowest-sensed temperature.
3. Base resistance is < 100 Ω.
4. Tight control of VBE characteristics indicated by small variations in hFE (that is, 50 to 150)
5. Small transistor variation
6. Consistent η-factor across temperatures
Based on this criteria, the industry standard 3904 (NPN) or 3906 (PNP) are good choices for a remote temperature sensor. However, any transistor should be evaluated for η-factor and consistency.