Burden resistor on CT.   - Page 1
Forum section: electronics / projects

author: viper
author_rank: Regular Contributor
timestamp: September 11, 2025, 12:33:19 am
content: I'm designing a circuit to monitor AC current on a wire. That current is expected to be 8-15A, with sometimes 100A of starting inrush. I need to monitor that with an ESP32. I am curious of the best practice for placement of a burden resistor, as it is said there could be some potential higher voltage risk from the CT in an open circuit condition? Ideally, I would rather put this resistor on the PCB, but I am not sure if that is wise? I don't really expect to come into contact with those CT leads while operating, but..... in the name of safety and best approach.I'm also unsure how I would even estimate the potential voltage at open circuit to know if that is really a risk or not.helpsies?

author: Hiemal
author_rank: Regular Contributor
timestamp: September 11, 2025, 01:13:24 am
content: Well, one never runs a CT open circuit. That's how they burn up, and isn't something you should really ever have happen in normal circumstances.You can put a TVS diode in parallel with the CT, which can help protect against overvoltage. Putting the resistor on the PCB is fine, just make sure it's rated to handle the power dissipation and is pulse rated to what you're expecting.

author: uer166
author_rank: Super Contributor
timestamp: September 11, 2025, 04:22:10 am
content: Do not run it open circuit ever. The challenge is estimating the required burden resistance; generally it's a trade-off between core size and signal level: the larger the burden resistance, the earlier the core saturates and vice-versa. Ideally you want a burden resistor of 0 ohms and an amplifier of infinite gain to capture all frequencies.Obviously that's impossible so choose something reasonable for your application.

author: viper
author_rank: Regular Contributor
timestamp: September 11, 2025, 06:03:43 am
content: Appreciate guys! Reality is this is a semi-commercial device so having to think about protecting people from themselves. I like a TVS in parallel but sort of think I need to add redundancy. There should be no reason for someone to touch this while running, but I also have a small pigtail from the CT going into an enclosure. Enough for me to think carefully.I didn't do a deep dive on saturation of these but just considering basic math, a 100:1 CT would be 100mA on CT from 10A of line current. At the same time, if at 120V on that leg, that is 120*100=12kv. ? I'm not sure, but enough to carefully consider idiot behavior.I'd prefer any components just land on the PCB, but that does leave those wild CT wires so I need to think on that. Probably feel better if there was at least a TVS right at the CT, but I don't know what is actually common in industry.

author: Jeroen3
author_rank: Super Contributor
timestamp: September 11, 2025, 06:11:23 am
content: Contrary to that uer166 says, you do not want a burden resistor of 0 Ohms as that would make the CT inaccurate, but too high makes in inaccurate as well.You want to match the VA rating of the CT.For a small unit like yours, possibly PCB level CT, the manufacturer typically recommends one in the datasheet.Accompanied by how much ratio error it gives.Eg. this one is good with 50-200 Ohm:https://datasheet.octopart.com/AC1020-Nuvotem-Talema-datasheet-140257281.pdfWhat you can do to avoid the open circuit scenario is using parallel resistors. In which case if one of them isn't placed or soldered properly, or breaks, it will not go open circuit.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: September 11, 2025, 08:31:24 am
content: Quote from: viper on September 11, 2025, 12:33:19 amwhy do younotwant to place the burden resistor at the CT? i can't see having it directly at the CT presenting any disadvantage.cheers,rob :-)

author: viper
author_rank: Regular Contributor
timestamp: September 11, 2025, 03:40:10 pm
content: Quote from: robert.rozee on September 11, 2025, 08:31:24 amThe CT will be installed in a hostile environment certainly not on a PCB. If I install anything across the leads right at the coil, I would have to figure out how to protect it, unless I find something with the built in protections.If it weren't for some risk of injury, this would be a moot subject. I'm just trying to assess if this is 'really' much of a risk

author: Terry Bites
author_rank: Super Contributor
timestamp: September 12, 2025, 05:23:52 pm
content: OK, so you read the data sheet and chose the right resistor.I'd go for a good quality 1W type here.You connect the CT to the PCB over some shielded twisted pair and ground the shield via a 10-100 ohm resistor near the PCB connector.Do not ground CT end of the shield!Add a common mode choke or wind a few turns of the STP on a ferrite torroid. Apply low pass filtering in the amplifierThe voltage across Rburden is likely to be less than a volt at the rated current so add a pair of back to back diodes or a low voltage bidirectional TVS across it.That should deal with high current surges.Use an inst amp, better still for single supply systems use a "fully differential amplifier". So much easier to interface to an ADC.

author: floobydust
author_rank: Super Contributor
timestamp: September 12, 2025, 06:28:54 pm
content: Popular low-dollar chinese CT's with a connector... like the YHDCSCT-013have either a built-in zener clamp or burden resistor or both, to prevent drama.Note the ESP32 has a lousy ADC so accuracy is not possible here.

author: viper
author_rank: Regular Contributor
timestamp: September 12, 2025, 10:26:54 pm
content: Quote from: Terry Bites on September 12, 2025, 05:23:52 pmI appreciate you taking the time for a drawing. I'm trying not to over complicate the setup. distance from CT to PCB will be max 4ft, usually less than 1ft. Curious if EMI is typically an issue with these? Never setup a circuit for a CT. Not sure what to expect.

author: viper
author_rank: Regular Contributor
timestamp: September 12, 2025, 10:31:55 pm
content: Quote from: floobydust on September 12, 2025, 06:28:54 pmAppreciate that! I was trying to stick with a solid core as I feel it will remain more reliable and consistent in the environment so sort of surprised those split types seem to be cheaper than solids? maybe not shopping the right sources?As for accuracy and connection options, I am quite open on that but between probably 5-20A, which is where I need best resolve, if I am within 1A, that is probably fine but .1A is really preferred, just in that range. For high range, I probably don't care if it saturates because that is only inrush. I just need to know it went high and how long the event was.If ADC is that bad, are there options? I'm going to burn up the two I2S legs so does that leave analog?

author: floobydust
author_rank: Super Contributor
timestamp: September 13, 2025, 02:11:39 am
content: A split-core is much easier to use and no danger requiring a user to disconnect a HV mains wire and poke through a CT.Looking at current only for a load with poor power factor, like a motor you need decent resolution I think. Some motors read high current with no load on them. Seehttps://www.loadcontrols.comas an example with pump motors. They sell power meters for that.I think you need the CT feeding a burden resistor and then op-amp active rectifier and then to the ADC. Average current is much easier to look at.There are a zillion Arduino + CT circuits out there.ESP32ADCis affected by RF TX on WiFi or BT. Nominally its scale is 0-1.1VDC for 12-bits. You have to calibrate it too.

author: ME
author_rank: Frequent Contributor
timestamp: September 13, 2025, 09:46:16 am
content: sorry for being thick but what is a "CT"?.

author: pqass
author_rank: Super Contributor
timestamp: September 13, 2025, 12:21:40 pm
content: Quote from: ME on September 13, 2025, 09:46:16 am"CT" =current transformerA special transformer consisting of a spool of many turns of fine wire (secondary) on a constant load (burden resistor) formed with a thick wire that's threaded through the spool (single turn primary). The AC volts across the burden resistor is proportional to the current through the wire threaded through the spool. This is a non-contact way to sense the AC current of the wire threaded through the spool.