FAQ
TL;DR: Use a biased input and the right burden strategy; “The ADS1115 has four inputs,” so dedicate channels instead of risky auto‑detect. “Make sure the burden resistor is ALWAYS connected across the CT.” [Elektroda, Anonymous, post #21682530]
Why it matters: This FAQ helps engineers building three‑phase current monitors avoid blown ADCs, bad scaling, and field wiring mistakes.
Quick Facts
- Bias the CT signal at approx. +2.5 V mid‑supply before the ADS1115 to read AC on a unipolar ADC. [Elektroda, Anonymous, post #21682529]
- Open‑circuiting a CT can create very high voltages; use shorting bars or input clamps for safety. [Elektroda, Anonymous, post #21682542]
- ADS1115 offers 4 inputs and a PGA, letting you map different CT types per channel. [Elektroda, Anonymous, post #21682530]
- Some standards require tying one CT secondary lead to ground; plan grounding and noise mitigation. [Elektroda, Anonymous, post #21682534]
- Field‑proofing tip: use a 4‑pin CT connector with a detection jumper or put the burden inside the plug. [Elektroda, Anonymous, post #21682539]
How do I connect 100A:50 mA and 100A:1 V CTs to an ADS1115?
Give both signals a mid‑supply offset, then apply the right burden strategy. The 100A:50 mA CT requires an external burden across the secondary plus the offset to center the waveform. The 100A:1 V CT already includes a burden, so route it to the same offset stage without adding another burden. Keep inputs within ADS1115 limits and reference the offset node cleanly. [Elektroda, Anonymous, post #21682529]
Is automatic analog CT‑type detection a good idea?
It’s unreliable. Any analog detection needs the CT to be connected and producing signal, which creates a chicken‑and‑egg problem for switching. Prefer a defined connector/keying method or dedicate ADC channels and select scaling in firmware. [Elektroda, Anonymous, post #21682534]
What happens if I add a burden to a CT that already has one?
You place two burdens in parallel. The effective resistance drops, so the output voltage decreases relative to current. Your calibration must account for the new total burden value to recover accurate scaling. [Elektroda, Anonymous, post #21682542]
What happens if I connect a CT with no burden?
The secondary can generate very high voltage and damage electronics. Use current‑limiting resistors and clamps, and consider a shorting bar so the CT is never open. This is a critical field failure risk. [Elektroda, Anonymous, post #21682542]
Do I always need the burden across the CT?
Yes. “Make sure that the Burden resistor is ALWAYS connected across the CT.” You can AC‑couple into the offset network and still keep the burden across the secondary. This preserves a defined current path and safe operation. [Elektroda, Anonymous, post #21682531]
How can I make the system field‑proof without auto‑detect?
Use a 4‑pin CT connector: two pins for the CT, two for a presence/jumper code. Or place the burden inside the CT plug. The PCB senses the jumper and applies the right scaling or path. [Elektroda, Anonymous, post #21682539]
Should I ground one side of the CT secondary?
Some jurisdictions require grounding one CT lead. Grounding also reduces noise and undefined float. Confirm local code and design your offset and protection network to reference that grounded side appropriately. [Elektroda, Anonymous, post #21682534]
How fast should I sample 50/60 Hz currents?
Target about 1 ms per channel to capture peaks or zero crossings. For three phases, budget three times that rate. Ensure your MCU and ADS1115 settings meet throughput and resolution goals without aliasing. [Elektroda, Anonymous, post #21682536]
Can ADS1115 cover three phases and a voltage sense?
Yes. It has four inputs and a PGA, so assign A0–A2 for three CTs and reserve A3 for voltage sensing or future use. Calibrate each channel’s gain to its CT type. [Elektroda, Anonymous, post #21682530]
How do I add offset without corrupting the burden divider?
Keep the burden across the CT, then AC‑couple the CT signal into a buffered mid‑supply reference. An op‑amp buffer for the divider provides a low‑impedance bias. Add surge protection at the input. [Elektroda, Anonymous, post #21682531]
How should I protect the ADS1115 from spikes?
Add series resistors, clamp diodes or zeners to the ADC rails, and ensure the CT is never open‑circuit. Protection components prevent transients from exceeding input limits during connect/disconnect. [Elektroda, Anonymous, post #21682542]
How do I simulate a CT path in LTspice for validation?
Model the source as a current source with series resistance and a burden. A variac‑driven 6.3 V transformer example shows current limiting via series resistors and indicates secondary current ratings (e.g., 8 A max). [Elektroda, Anonymous, post #21682547]
What do 100A:1 V and 100A:0.33 V CT specs mean for scaling?
They specify the secondary voltage at rated primary current with the intended burden. If you change total burden, voltage per amp changes. Recalculate the transfer function and update ADC gain and firmware scaling. [Elektroda, Anonymous, post #21682542]
Is it acceptable to always include a PCB burden and correct in firmware?
Yes. Fix the hardware burden on the board, then apply software multipliers for CTs with internal burdens to match expected scaling. This avoids risky auto‑switching. [Elektroda, Anonymous, post #21682546]
What’s a safe way to support installers who may swap CT models?
Ship CTs with connectorized burdens or keyed 4‑pin plugs. The matching PCB detects the jumper and selects the right analog path and math automatically. It reduces installation errors and support calls. [Elektroda, Anonymous, post #21682539]
Quick how‑to: map two CT types to one ADS1115 cleanly
- Add a buffered 2.5 V reference and AC‑couple both CTs into it.
- Place the external burden across the 50 mA CT; route the 1 V CT directly.
- Use ADS1115 PGA per channel and calibrate each path in firmware. [Elektroda, Anonymous, post #21682530]
