Is the ZMPT101B mains voltage sense circuit safe? - Page 1
Forum section: electronics / beginners

author: e100
author_rank: Frequent Contributor
timestamp: July 03, 2022, 07:49:46 am
content: Or to put it another way, why is it designed with a 1:1 transformer instead of a step down transformer?Is there something specific about the transformer that makes it particularly suitable for this application?Transformer datasheethttps://innovatorsguru.com/wp-content/uploads/2019/02/ZMPT101B.pdf

author: eblc1388
author_rank: Frequent Contributor
timestamp: July 03, 2022, 08:48:58 am
content: Quote from: e100 on July 03, 2022, 07:49:46 amIt is not a voltage transformer. It works as a current transformer and the input current needs to be user limited to 2mA or less. That's the purpose of the series resistor R12(820K).Quote from: e100 on July 03, 2022, 07:49:46 amWhile the ZMPT101B is safe if you adhere to its current rating, the PCB with ZMPT101B on it is not. According to the ZMPT datasheet, a resistor of 200K is used for sensing 220V. If R12=820K on the PCB, then perhaps the designer would want to use the PCB for up to 1000V sensing. While the resistor value is good, the form factor of a single SMD resistor is a definite no no because of the high voltage involved. Its bad even for 110V or 220V sensing and may result in a flashover.You would need proper HV rated resistor(or usually several of them in series) for such voltage level.

author: e100
author_rank: Frequent Contributor
timestamp: July 03, 2022, 09:35:13 am
content: Quote from: eblc1388 on July 03, 2022, 08:48:58 amWhy use a current transformer for measuring voltage?Is this standard practice?

author: Gyro
author_rank: Super Contributor
timestamp: July 03, 2022, 10:59:27 am
content: Using a current transformer means that the transformer doesn't have mains voltage applied directly across its primary winding (the voltage is dropped across the series resistor) so it doesn't have to be as big [Edit: Smaller core]. This makes it smaller and eases primary winding insulation - although primary-secondary insulation is still just as critical of course.P.S. As eblc1388 says, that smd footprint is far too small to drop mains across. You should bypass it (or fit a smaller value) and dorp most of the voltage across an external resistor, suitably insulated in heatshrink. Most 0.5W leaded resistors have around 350V element limiting voltage, but check and preferably use two in series (for mains).

author: Ian.M
author_rank: Super Contributor
timestamp: July 03, 2022, 12:58:13 pm
content: I'd call that irredeemably unsafe.See attached photo which I measured to find there is only 1.75mm clearance between the Neutral trace and the output side ground plane, which isn't even fit for 120V AC use. Also %DEITY% only knows what the trace separations are topside under the current transformer!

author: langwadt
author_rank: Super Contributor
timestamp: July 03, 2022, 01:13:06 pm
content: why the ... did they not turn the transformer 90 degrees?

author: Gyro
author_rank: Super Contributor
timestamp: July 03, 2022, 01:17:48 pm
content: How to take a perfectly sound idea and...How do you screw something up that badly without deliberately trying to?

author: zaphod_b
author_rank: Newbie
timestamp: May 13, 2025, 08:32:05 am
content: I came across these modules a while ago when researching options for voltage measurement. They are everywhere on AliExpress as well as on Amazon.Obviously I was shocked about the low clearance distances on the board and the fact (as mentioned above) that turning the transformer 90 degrees would have made it a lot safer.But probably the designer was able to shave off some millimetres from the board size that way, saving a lot of money and risking so many lives.I even contacted Amazon about product safety (including links to relevant standards in the EU) and was brushed off twice with a notice that further nagging could result in the termination of my account.TL;DR Do not use this board. It is has a inherently dangerous design and you are risking not only all devices attached to it downstream but also the lives of everyone in proximity.

author: Terry Bites
author_rank: Super Contributor
timestamp: May 15, 2025, 05:04:36 pm
content: Its a CTR with 4kV of withstand. So fine on mains.Never, ever scrimp on AC power components. Dont buy cheap crap, get it from a traceble source.Fire, death and litigation will come your way soon enough if you dont.