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.

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.

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. 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.

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

If you want to know more about the topic, I linked a 40-minute EEVBlog video about it.

Also, if you prefer a readable version, it is all explained in detail on this webpage:

https://lygte-info.dk/info/DMMDesignProtection%20U...

Disassembling the meter is pretty straightforward, there are only three screws holding the case together as you can see in the pictures.

There are also three screws holding the PCB to the top cover. Remove these screws and push outwards the gaps on the sides between the case and the PCB, with a flat head screwdriver. The board will eventually come off the case.

In the schematic shown in a previous step, the varistors used in the GS version are BOURNS MOV-07D751K with a rating of 825V and 70 pF of stray capacitance. I have installed a set of EPCOS S10K385 B72210S0381K101 instead since I already had them lying around in my drawers. They are rated for 505 VDC and have a stray capacitance of 150 pF.

In order to install them, simply cut the legs to a fairly short length allowing them to sit in the PCB but not to protrude too much, so the case will still close easily.

Then, solder the legs directly at the top side of the PCB pads.

High-end multimeters include a special type of fuses, called "HRC" or High Rupturing Capacity fuses, which are able to carry a rated short circuit current for a given period of time without blowing off.

They are made out of heat-resistant ceramic materials and sturdier metal-end caps than typical glass fuses. Also, the internal part of the fuse is filled with sand-like powder to prevent air exposure-related problems. Because of this filling material, they can dissipate heat better than regular fuses, which allows them to carry the rated current without overheating.

Also, when they eventually blow off, the Silver compound of the fuse wire and the filling, start a chemical reaction that results in a high resistance path that quenches any electrical arc.

This is why it is of good advice to have quality HRC fuses in a multimeter.

Replacing the milliamp range fuse is a fairly easy task. First, remove the old 6x25 mm 250V 1A fuse and observe that there is an extended footprint for a bigger fuse. This alternative footprint is already compatible with the fuse holders present in the meter, so, in a second step, simply desolder the one close to D1A and D3A.

Remember to apply new solder to the pads to prevent overheating or damaging the PCB. Production lead-free solder has a higher melting point, which makes it difficult to rework manually, so be careful.

To finish, resolder this same fuse holder to the alternative footprint. Now, this new mounting setup is compatible with 6x32 mm fuses. I selected a SIBA FF 500 mA 1000V fuse.

This is the last and the trickiest part of this Instructable, although it is still very easy to perform.

As with the milliamp range fuse, there are alternative footprints to install a bigger fuse on the GS version, so, first of all, we should remove the old 250V 10A fuse and desolder both fuse holders.

I decided to buy better holders for the 10A range than the ones that come with the meter. So, I went for Littelfuse 01020071Z 6.3 mm fuse holders, however, any similar device with the same dimensions will work too.

The new fuse has a 10 mm diameter, but the footprints in the meter are clearly for 6.3 mm fuse holders (believe me, I tested this out, a 10 mm holder will not fit the footprint nor will allow the case to close properly. You know, Chinese engineering...), so we have to widen them up a bit, pushing them outwards with a pair of pliers. Once you are done, solder them to the wider footprint.

Last but not least, cut the edge mounting bars of one of the holders (or both) with a pair of cutting pliers. Finally, install the new fuse pressing gently against his new holders.

After applying all the changes, your meter should look something like the picture in this step.

Now it is time to reassemble the meter screwing the PCB again and checking if the box still closes correctly with the HRC fuse not blocking the way.

Congratulations! You are done!

The last step is to plug in the battery and check if the meter still works.

In this Instructable we have explored the possibilities of improving the input protection of the UNI-T UT61E making it less prone to failures in higher voltages environments and safer for the user.

Total costs of all the mods:

- 6x32 mm SIBA FF 500 mA. Qty: 1(3€)

https://es.aliexpress.com/item/32808289000.html?sp...

- 10x38 mm HRC Fuse. Qty: 1 (3€)

https://es.aliexpress.com/item/32909758127.html?sp...

- Littelfuse 01020071Z 6.3 mm fuse holders. Qty: 2 (0.5€)

https://lcsc.com/product-detail/Fuse-Holders_Litte...

- EPCOS B72210S0381K101 Varistors. Qty: 4 (2€)

https://es.aliexpress.com/item/32452420265.html?sp...

https://www.mouser.es/ProductDetail/EPCOS-TDK/B722...

- Or BOURNS MOV-07D751K. Qty: 4 (2€)

https://www.mouser.es/ProductDetail/Bourns/MOV-07D...

Grand total.............8.5€

Time: 15 - 30 minutes

There is one little inconvenience about performing this mod, which is that varistors have capacitance and the Chinese version is not calibrated to zero this effect. So, a static measure of 350 - 400 pF will be shown in the capacitance reading, but, it is not something that bothers me giving the major improvements performed to the meter.

I hope you liked this project! I encourage you to comment and share this article, so I could write better ones in the future. I would be glad to answer all of your questions.
Enjoy! You can also follow me on Instagram!