FAQ
TL;DR: Use a small current transformer (CT) set for about 10 mA secondary and a simple rectifier/comparator to flag "no-current" faults. "The secondary of a current transformer must always have a load." [Elektroda, Anonymous, post #21683780]
Why it matters: It’s a low-cost, isolated way to detect when a 240 VAC, 50 A heater opens without bringing mains onto your PCB. This FAQ is for appliance engineers asking how to sense open-circuit heater failures safely and cheaply.
Quick Facts
- Heater spec in thread: 240 VAC, 50 A elements that fail open; goal is go/no‑go current detect near 0 A vs ~50 A. [Elektroda, Anonymous, post #21683774]
- CT burden rule of thumb given: max ~1 Ω per turn (100 turns → ≤100 Ω); start at 47 Ω for robust signaling. [Elektroda, Anonymous, post #21683781]
- No RMS IC needed for on/off detection; rectify CT output and feed a logic input or 555 missing‑pulse detector. [Elektroda, Anonymous, post #21683777]
- CT provides inherent galvanic isolation; never run with open secondary—use a load or indicator LEDs. [Elektroda, Anonymous, post #21683780]
- Alternatives: Hall/AMR transducers (Allegro, LEM, Phoenix Contact, Weidmuller, Emphatec limit detector ≥1.5 A). [Elektroda, Anonymous, post #21683782]
What’s the cheapest safe way to detect a 240 VAC, 50 A heater going open?
Use a pass‑through current transformer on one line conductor, add a burden resistor, then rectify and compare the voltage. If the MCU drives the SSR/contactor, it already knows heater “on” commands; a near‑zero CT signal during “on” indicates an open element. This avoids routing mains to the PCB and keeps parts count low. Start with an off‑the‑shelf CT and a diode bridge, RC filter, and a comparator or MCU ADC input set to a simple threshold. [Elektroda, Anonymous, post #21683777]
Why pick a current transformer (CT) over other methods?
A CT gives cheap, strong isolation and scales large primary currents to small, safe secondary signals. You only pass the heater’s 6 AWG lead through the core; no series drop is added. Designers note the secondary must always have a load. You can even put two antiparallel LEDs across it for a visual “current present” indicator. Cost and complexity stay low compared with full metering solutions. [Elektroda, Anonymous, post #21683780]
How do I choose the CT and burden resistor?
Select a CT rated above your line current (e.g., CST206‑1A 110 A). Follow the burden guidance: maximum about 1 Ω per turn. With 100 turns, keep burden ≤100 Ω; many start at 47 Ω to get a healthy voltage. Measure with heater on; adjust burden for your comparator or MCU input range. Add a clamp (e.g., zener) if needed for transients. [Elektroda, Anonymous, post #21683781]
Do I need RMS conversion to detect failures?
No. For go/no‑go current presence, rectify the CT output and feed a logic input or use a missing‑pulse detector. RMS ICs add cost without improving the binary decision. A simple bridge, filter capacitor, and threshold works well when the controller already knows heater on/off commands. [Elektroda, Anonymous, post #21683777]
Can I keep mains off my PCB while sensing current?
Yes. Use a PCB‑mount CT with a pass‑through window so the 6 AWG heater lead threads the core, while the low‑voltage secondary stays on your PCB. Pair it with your MCU input for a compact design. This approach avoids bringing 240 VAC onto control electronics while enabling reliable open‑circuit detection. [Elektroda, Anonymous, post #21683778]
What’s a safe rule about CT secondaries?
Never leave a CT secondary open. Always install a burden resistor or a protective load module. Open‑circuit CTs can generate high voltages that stress insulation and damage parts. As one expert put it, "The secondary of a current transformer must always have a load." Products exist that integrate the burden automatically. [Elektroda, Anonymous, post #21683783]
How do I wire a simple CT‑to‑MCU detector?
- Pass one heater conductor through the CT; fit a 47 Ω burden across the secondary.
- Full‑wave rectify the secondary; add RC filter; clamp if needed.
- Feed a comparator or MCU ADC; flag fault when filtered voltage falls below threshold during commanded “on.” [Elektroda, Anonymous, post #21683781]
What’s a good secondary current target for indication?
Design for about 10 mA secondary current when the heater is healthy. That level easily drives antiparallel LEDs for a visible “current present” check, or creates a clear voltage across the burden for digital detection. This keeps dissipation modest while ensuring strong signal margin. [Elektroda, Anonymous, post #21683780]
What if my heater is PWM’d by an SSR—will detection still work?
Yes. Use the controller’s knowledge of SSR on‑times and a missing‑pulse detector or filtered rectified voltage. During commanded “on” windows, expect pulses; a persistent absence signals an open element. A 555‑based missing‑pulse circuit or MCU timing works well without expensive RMS parts. [Elektroda, Anonymous, post #21683777]
Are there off‑the‑shelf modules with setpoint outputs?
Yes. Limit detectors accept CT inputs and close a contact when current exceeds a threshold, such as about 1.5 A. Industrial transducer makers offer DIN‑rail options with isolation and scaling. These reduce certification burden and speed compliance for appliance designs that need a relay output. [Elektroda, Anonymous, post #21683782]
What are Hall‑effect and AMR current sensors, and when use them?
Hall and AMR sensors measure magnetic fields from current and provide isolated analog outputs. They integrate the conductor and signal conditioning, simplifying PCB design. Brands mentioned include Allegro, LEM, Phoenix Contact, Weidmuller, AsahiKasei, and Aceinna. Choose them when you want DC capability, compactness, or standardized modules over raw CTs. [Elektroda, Anonymous, post #21683784]
What is a current transformer (CT) in plain terms?
A CT is a small transformer that senses current in a primary conductor and produces a proportional, isolated secondary current. You route one heater lead through the core (one turn). The secondary’s many turns reduce current and raise voltage across a burden for easy measurement. [Elektroda, Anonymous, post #21683776]
What is an SSR or contactor in this context?
An SSR (solid‑state relay) or a contactor switches the 240 VAC power to the heater under MCU control. Your controller can correlate its on/off commands with the CT signal. If the CT shows no current during an on command, the element is likely open. [Elektroda, Anonymous, post #21683778]
Can I prototype with maker boards?
Yes. You can feed the rectified CT signal into a Raspberry Pi or an Arduino (e.g., Nano) for thresholding and logging. Use proper burdening and isolation practices, then read the voltage with an ADC or GPIO via a comparator. This speeds proof‑of‑concept before hardening the design. [Elektroda, Anonymous, post #21683785]
What edge cases should I watch for?
Two key pitfalls: leaving the CT secondary open and mis‑interpreting zero current when the heater is intentionally off. Always burden the CT, and cross‑check with the MCU’s knowledge of SSR or contactor state to avoid false failure flags during controlled off cycles. [Elektroda, Anonymous, post #21683780]
Is there a simple visual indicator without an MCU?
Yes. Put two LEDs in opposite directions across the CT secondary. With proper burdening, they light when AC current flows and stay dark on open circuits. It’s a quick, isolated, and low‑cost way to show heater current presence during commissioning or service. [Elektroda, Anonymous, post #21683780]
