FAQ
TL;DR: Map an RTD (141–171 Ω over 50–100°C = 0.6 Ω/°C) to 0–5 V using a constant‑current source and op‑amp; “RTDs are pretty linear in the project’s temperature range.” [Elektroda, Stephen Van Buskirk, post #21665641]
Why it matters: This helps students and makers build a simple, accurate analog front end with a clean LED in‑range indicator.
Quick Facts
- Spec: RTD spans 141–171 Ω from 50–100°C; target output 0–5 V across that range. [Elektroda, nadeen Maali, post #21665630]
- Topology: Excite the RTD with a constant‑current source, then scale/offset with an op‑amp for linear 0–5 V. [Elektroda, Stephen Van Buskirk, post #21665638]
- Indicator: Use a window comparator (e.g., LM339) to light an LED only within 50–100°C. [Elektroda, David Adams, post #21665646]
- Power: Split ± supplies simplify true 0 V output and adjustment headroom for op‑amps. [Elektroda, David Adams, post #21665646]
- Caveat: A 7805 trick lacks true 0 V output; it can require a negative rail to span 0–5 V. [Elektroda, Stephen Van Buskirk, post #21665641]
How do I turn a 141–171 Ω RTD change into a 0–5 V signal?
Drive the RTD with a constant‑current source, sense its voltage, then use an op‑amp stage to set zero and gain. Place the RTD in the feedback or input path to trim span, and add an inverting/non‑inverting stage as needed to get a positive 0–5 V. Finish by calibrating at 50°C for 0 V and 100°C for 5 V. This classic two‑op‑amp approach is simple and stable for labs and coursework. [Elektroda, David Adams, post #21665646]
Do I need linearization for 50–100°C with an RTD?
Minimal. RTDs are fairly linear across this band, especially with constant‑current excitation. “RTDs are pretty linear in the project’s temperature range.” Use the op‑amp stage to fine‑tune span and offset. Save nonlinearity fixes for wider ranges or thermistors. This keeps parts count low and calibration straightforward for students. [Elektroda, Stephen Van Buskirk, post #21665641]
What op‑amp and supply rails should I start with?
Use a general‑purpose op‑amp for gain/offset and a second for inversion or buffering. A split supply (±) simplifies achieving a true 0 V output and gives adjustment headroom. Students often use a 741 in training labs, but any suitable dual/quad works. Calibrate zero on the non‑inverting input and span via the input/feedback resistors. [Elektroda, David Adams, post #21665646]
How do I light an LED only between 50°C and 100°C?
Implement a window comparator. Feed your scaled 0–5 V into an LM339 and set two thresholds that correspond to 50°C and 100°C. The LED turns on only when the signal lies between those limits. Add hysteresis resistors if you need noise immunity near the edges. This cleanly solves the “in‑range” indicator. [Elektroda, David Adams, post #21665646]
Can I avoid op‑amps using a 7805 regulator and diodes?
It was proposed, but it fails key requirements. A 7805 cannot produce a true 0 V bottom without additional rails. You often need a negative supply to span 0–5 V at the output. Diode‑based thresholds also drift with temperature, hurting accuracy. An op‑amp approach remains the clearer, adjustable path for students. [Elektroda, Stephen Van Buskirk, post #21665641]
Should I use a microcontroller, ADC, and DAC instead?
You can, but it adds complexity for little gain here. You’d need ADC linearization, a DAC for analog output, toolchains, and debugging time. For a short project, analog signal conditioning and a window comparator are faster and teach core concepts effectively. “I use micros daily... but they make no sense at all for this project.” [Elektroda, Stephen Van Buskirk, post #21665638]
What are the calibration steps to hit exact 0 V and 5 V?
- Set the sensor at 50°C. Adjust the offset (non‑inverting input network) until the output reads 0.00 V.
- Set the sensor at 100°C. Adjust the gain resistor until the output reads 5.00 V.
- Recheck 50°C and 100°C, then fine‑trim both controls for final accuracy. [Elektroda, David Adams, post #21665646]
What’s the difference between an RTD and a thermistor here?
An RTD is a resistive temperature detector, often metal‑film, with near‑linear resistance‑to‑temperature behavior in this band. A thermistor is typically nonlinear and needs more compensation. The thread clarifies the OP’s sensor is an RTD, not a thermistor, which simplifies conditioning and calibration. [Elektroda, Stephen Van Buskirk, post #21665641]
How much resistance change per degree should I expect?
From 141 Ω at 50°C to 171 Ω at 100°C, the span is 30 Ω over 50°C, which is 0.6 Ω/°C. This simple math helps you pick excitation current and estimate signal levels before op‑amp scaling. It also sets realistic expectations for resolution. [Elektroda, nadeen Maali, post #21665630]
Can I get dual rails from common supplies for testing?
Yes. You can stack a +5 V and a +12 V supply in series to create a virtual negative rail for the op‑amps. Standard PC supplies also exposed +12 V and −5 V rails historically, which can provide the split rails you need for a true 0–5 V output. Observe grounding and current limits. [Elektroda, Stephen Van Buskirk, post #21665645]
Are diode‑only LED thresholds reliable?
They work, but thermal drift can move the threshold noticeably. A comment in the thread notes that such designs are not thermally stable. A comparator with references is more repeatable across ambient changes and parts variation. Use diode tricks only for demos, not graded accuracy. [Elektroda, Steve Lawson, post #21665643]
What if I just buy a ready‑made controller instead?
If this is not a learning build, consider off‑the‑shelf controllers from Watlow, Red Lion Controls, Oven Industries, or Dwyer/Love Controls. They support RTDs, provide programmable ranges, and include limit alarms. For one‑offs, they save time and deliver robust results with minimal wiring. [Elektroda, Kevin Parmenter, post #21665631]
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