Voltage and Current Protection Upgrades for UNI-T UT61E
Introduction: Voltage and Current Protection Upgrades for UNI-T UT61E
The UNI-T UT61E has become one of the most popular multimeters in the hobby electronics market. Not only because of its surprising precision but also for its rigidity and sturdiness. All of this within a contained price tag of 60€.
However, behind all its magnificent features, the meter hides one clear drawback: input protection. Due to the competitive pricing of the meter, both overvoltage and overcurrent protection have been left behind in the Chinese version.
In this Instructable, with very little spending, we are going to explore the upgrade possibilities of the meter, which will make it an overall better tool.
Step 1: Why It Is Important to Have Overvoltage and Overcurrent Protection in a Multimeter
A multimeter is very often used to test unknown voltages or currents, I don't think I'd need to explain the possible consequences of testing a high energy line with a meter not rated for the voltages involved. But, just to be sure that everybody knows the dangers of electric and electronic appliances I linked a video about this topic from EEVBlog showing a fair bit of examples.
In the video, a huge variety of meters were tested against a high voltage and high energy source. Needless to say, the cheaper meters performed pretty badly in these tests. However, we can observe that the more expensive units, although some of them did suffer huge internal damage, did not explode nor tear themselves into pieces.
This is enough to not harm the user or expose him to the high voltages of the source. In addition, some of the higher-end meters may be back to life with very little service, since only some well-designed and selected components were damage.
Step 2: UT61E Improvement Capabilities
In order to address some of these issues, there is a UT61E version, called "GS", made exclusively for the German market which is slightly different than its Chinese counterpart. 71
It comes with a lesser voltage rating of CATIII 300V, CATII 600V, rather than CATIV 600V and CATIII 1000V in the regular version.
However, as you may have guessed, these ratings, as in every Chinese multimeter, are completely made up. If you look at the first picture of this step, you will see the internals of the German version on the left and the Chinese version on the right. The former features varistors, PTCs, and bigger HRC fuses, which makes the CAT ratings far more believable.
If you look close enough at the first and second pictures, you will notice that both PCBs are exactly the same, giving us the opportunity to install such features ourselves, considerably improving the meter input protection capabilities.
The third and fourth pictures show the complete schematic of the meter with the fourth being a snippet of the input protection design of the German GS version.
Step 3: How Input Protection Works in the UT61E GS
Multimeters are meant to measure multiple magnitudes from their "V" and "COM" terminals, such as resistance, continuity, capacitance, etc- For this reason, there are a lot of protection mechanisms for both the user and the meter in this section.
The UNI-T UT61E GS offers a very classical approach to this regard. A pair of PTCs are connected in series with both terminals in order to limit the maximum current in the event of a short, as well as a set of varistors for high voltage transient events. As for the millivolt range, a pair of transistors are used as Zener diodes to prevent overvoltages in the input of the microcontroller.
The micro and milliamp range is also protected against overcurrents that may damage the shunt resistors. This will usually happen by user mistakes, such as connecting this terminal in parallel to mains voltage.
The second picture represents how it is easily implemented in the UT61E with a 5-diode bridge. It consists of a regular 4-diode bridge rectifier, for positive and negative transients, and then an extra diode shorting the bridge. This is an added protection mechanism since fuses do not blow sufficiently fast enough to protect the shunt from destruction.
In the Chinese version, none of the varistors are installed leaving the meter without anything to do against transients. In addition, low-quality 250V fuses are installed, which are not ideal for a multimeter.
These are the upgrades that we are going to perform in this Instructable
Step 4: Extensive Explanations on Typical Multimeter Input Protection
If you want to know more about the topic, I linked a 40-minute EEVBlog video about it. 71