FAQ
TL;DR: Build a 4–20 mA loop from a 24 V supply using a V‑to‑I op‑amp stage; potentiometers can vary 20–30%, and “keep the voltage across R3 the same as the voltage on its non‑inverting '+' input.” [Elektroda, Steve Lawson, post #21663691]
Why it matters: This FAQ helps PLC and instrumentation users quickly design, choose parts, and calibrate a stable 4–20 mA driver with an adjustable pot.
Quick Facts
- Headroom check: with R3 = 1 kΩ, 20 mA drops ~20 V, leaving ~4 V on a 24 V loop. [Elektroda, Steve Lawson, post #21663697]
- Potentiometers often have 20–30% tolerance; add trimmers to hit exact 4 and 20 mA points. [Elektroda, Steve Lawson, post #21663691]
- Recommended op‑amps: LM358 (single/dual supply) or LM324 (quad; inputs/outputs to ground). [Elektroda, Steve Lawson, post #21663699]
- Suggested NPN: 2N5088; PN2222A or 2N3904 also work. Choose Vceo ≥ 24 V, ideally 30 V. [Elektroda, Steve Lawson, post #21663693]
- For unstable input, use LM7818 or LM317 set ~20 V; avoid discrete zener+pass schemes. [Elektroda, Steve Lawson, post #21663703]
How does the op‑amp 4–20 mA driver actually work?
It’s a voltage‑to‑current converter. The op‑amp forces the voltage across R3 to match its non‑inverting input (Vin+). Loop current I = Vin+/R3. Select R3, then choose R4–R6 to generate Vin+ that maps your pot travel to 4–20 mA. “The op‑amp will keep the voltage across R3 the same as the voltage on its non‑inverting '+' input.” [Elektroda, Steve Lawson, post #21663691]
Which op‑amp should I pick for a 24 V loop?
Use LM358 for simple single‑supply designs. For four channels, LM324 is convenient and designed for single‑supply with inputs/outputs reaching ground. That eases low‑end compliance and calibration. [Elektroda, Steve Lawson, post #21663699]
Can I use MC33078N instead of LM358/LM324?
Yes, it can work. A datasheet review indicated it meets the needed ranges for this circuit. Verify common‑mode limits against your chosen R3 to maintain regulation. [Elektroda, Steve Lawson, post #21663695]
What transistor should I use in the loop driver?
Pick a high‑beta NPN such as 2N5088. PN2222A or 2N3904 also work. Ensure Vceo exceeds 24 V, preferably 30 V, for margin. “Use a high beta transistor.” [Elektroda, Steve Lawson, post #21663691]
How do I size R3 and what headroom do I need?
At 20 mA, R3 drops 20 mA × R3. With R3 = 1 kΩ, that’s ~20 V, leaving ~4 V from a 24 V supply for wiring, sensor, and transistor. Reduce R3 to increase loop headroom. [Elektroda, Steve Lawson, post #21663697]
What’s the lowest R3 I can use with MC33078N?
Respect its minimum common‑mode input voltage. Keep about 2 V across R3 for regulation. That implies ~500 Ω minimum at 20 mA. Going lower risks loss of control. [Elektroda, Steve Lawson, post #21663697]
How do I map a potentiometer to exactly 4–20 mA?
Add trimmers to the reference network (R4–R6). Use one trimmer to set 4 mA at one end and another to set 20 mA at the other. Pot tolerance can be 20–30%, so trim is essential. [Elektroda, Steve Lawson, post #21663691]
Are cheap ±20% pots repeatable enough?
Tolerance affects nominal resistance, not the position repeatability. A 10 kΩ ±20% pot may measure 8–12 kΩ total, yet its wiper is proportionally consistent. Calibrate endpoints with trimmers. [Elektroda, Steve Lawson, post #21663701]
How do I power multiple 4–20 mA channels from one supply?
Use one supply sized for worst case: channels × 20 mA plus the circuits’ quiescent current. For four channels, budget at least 80 mA plus overhead. A quad LM324 simplifies layout. [Elektroda, Steve Lawson, post #21663699]
Is 4–20 mA really less “jumpy” than 0–10 V?
Current loops tolerate wiring resistance and contact aging better. Keep extra headroom for noise and corrosion. Ensure your loop voltage can cover wiring drops and device compliance. [Elektroda, Steve Lawson, post #21663697]
How can I stabilize an unstable input supply for the loop?
Regulate it. Use LM7818 or an LM317 set near 20 V. They provide tighter control and fewer parts than a zener with a pass transistor like 2N3055. [Elektroda, Steve Lawson, post #21663703]
What’s the simplest LM317 parts count to set a fixed voltage?
Two resistors set the output voltage. You can swap one resistor for a trimmer to fine‑tune. Keep wiring short and add standard bypass capacitors per datasheet. [Elektroda, Steve Lawson, post #21663705]
How do I compute an example 4–20 mA range?
Example: R3 = 1 kΩ; choose R4–R6 so Vin+ spans 4–20 V. That yields 4–20 mA (I = Vin+/R3). Verify headroom: R3 drops up to ~20 V at 20 mA, leaving ~4 V from 24 V. [Elektroda, Steve Lawson, post #21663697]
How many op‑amps and transistors per loop do I need?
Use one op‑amp channel and one NPN transistor per 4–20 mA loop. For four loops, a single LM324 covers all op‑amp channels. [Elektroda, Steve Lawson, post #21663699]
Any quick steps to calibrate 4 mA and 20 mA?
- Set the 4 mA trimmer so loop current reads 4.00 mA with the pot at its low end.
- Set the 20 mA trimmer so current reads 20.00 mA at the high end.
- Repeat steps 1–2 once more to converge. [Elektroda, Steve Lawson, post #21663691]
What’s an edge case that can break regulation?
If R3 is too low with an op‑amp requiring ≥2 V common‑mode at its inputs, the loop loses control near 4 mA. Keep R3 high enough. [Elektroda, Steve Lawson, post #21663697]
