...The radios we'll always remember


 Posted By: Robert Nickels (W9RAN)
Posted: 05/05/2022

What's New? 0 Comments 05/05/2022 

UNI-T UT210E Mini Digital Clamp Meter Handheld RMS AC/DC Resistance Capacitance Tester

Clamp-on DC current measurement!


Digital meters aren't new nor are they quite "dime a dozen" but when you can guy one for under five bucks right off the rack, why get excited about one for $50?    One reason:  "non contact DC current measurement"!

The UNI-T UT210E does all the usual things you'd expect from a handheld meter in this class:

  • AC DC Current measurement
  • Resistance measurement
  • LCD backlight
  • Continuity buzzer
  • VFC measurement
  • NCV ,non contact detection
  • Auto Range
  • True RMS Measurement

But what sets it apart is the ability to do NON-CONTACT DC Current measurement.    Other meters with this capability are easily twice the price, so this makes the UNI-T worth a careful look.    After owning one for a year or two I've used it for many radio-related projects where nothing else would do the job, such as measuring plate current in a transmitter by simply clamping it onto one of the power supply leads,   watching the charge current through unknown electrolytic capacitors and reforming them in-place without unsoldering by merely clamping onto the rectifier output and running the voltage up with a variac, and measuring battery current in class E transmitters.     This is in addition to all the usual reasons you'd use a DMM, but whenever I wonder what the current through a wire is, the UNI-T is the tool I reach for. 

To measure AC current without breaking the wire is simple and it's been done for decades - the name "Amprobe" is nearly synonymous with clamp-on meters and every electrician has one on his belt.   But AC is simple because, well because it's "Alternating" thus the meter clamp just acts like the wire is a one-turn transformer primary and measures the voltage in the secondary which is proportional to current.     But everyone knows transformers don't work for DC - so how is non-contact measurement possible?

The answer is "magnetoresistance" or "the tendency of a material (often ferromagnetic) to change the value of its electrical resistance in an externally-applied magnetic field."   The first magnetoresistive effect was discovered in 1856 by William Thompson,  better known as Lord Kelvin, but he was unable to lower the electrical resistance of anything by more than 5%.  Modern materials produced using semiconductor processes can create changes of orders of magnitude and have found wide application in sensors for biomedical applications, automobile engine timing, and hard drive pickup heads to name a few.   By embedding an MR sensor in the ferromagnetic pole pieces that make up the UNI-T meter's clamp jaws, it becomes possible to measure very small magnetic fields corresponding to just milliamps of current.   That's what makes this such a useful tool for hams and experimenters!     The lowest scale (which I use most often) is 2A full scale and easily shows currents in the 10 to 100mA range with good accuracy.    Higher scales are provided for bigger currents but they all work the same.   You simply select the current range desired,  toggle to "DC" and before current is applied, press the "Zero" button.    This is necessary because the MR sensor is so sensitive it will try to measure residual magetism in nearby objects - and even that of the Earth's magnetic field!   Once the tool is in position, pressing Zero will remove all these undesired effects and then when current is applied, an accurate measurement is possible.

The meter also can measure capacitance and detects voltage without making contact - a feature that could be a lifesaver.   For people like me who forget to turn it off, it does so automatically so battery life is many months of normal usage.   

Similar meters may be found under different names and I have no affiliation with Amazon, but here is an AMAZON LINK for those who'd like to get more information.


   

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