Tenma Handheld DC Power Supply - Review

Table of contents

RoadTest: Tenma Handheld DC Power Supply

Author: vplacint

Creation date:

Evaluation Type: Power Supplies

Did you receive all parts the manufacturer stated would be included in the package?: True

What other parts do you consider comparable to this product?:

What were the biggest problems encountered?: After a specific time, when is heated due to the load power consumption, either on the main banana plugs or the USB connector, the display is not readable due to a high contrast issue. This issue was only recovered after a specific time, when the power supply has been cooled down. Hence, I think is a thermal issue combined with bad choose of the capacitance on its power rail, see the testing results.

Detailed Review:

1. Introduction

Many thanks to element14 for selecting me to evaluate this product, Tenma Handheld DC Power Supply. It is really a compact, handheld and light weight DC power supply covering a broad range of the voltage levels, from 0.3 to 30 V, and allowing a maximum power of 50 W. It has been designed to be fanless in a AC-DC followed by an synonymous DC-DC buck converter configuration. In figure 1 can be seen its specifications extracted from the datasheet.

    Figure 1. Tenma Handheld DC Power Supply => specifications

 

I was very excited to be chosen to evaluate this product, as I wanted from a long time ago to have a power supply on my home working bench, but I didn’t had time to design it. The product arrived as it is in figure 2.

 

    Figure 2. Tenma Handheld DC Power Supply => package

 

Before powering up, I had to see how it was designed, so I have opened the case to see the hardware. The design is split in 2 pieces one with the power supplies, figure 3 and 4, and one with the digital control and readout, see figure 5.

  Figure 3. Tenma Handheld DC Power Supply => power supply board top view

Figure 4. Tenma Handheld DC Power Supply => power supply board bottom view

  Figure 5. Tenma Handheld DC Power Supply => digital control and readout board bottom view

 

As you can may see from photos, in my opinion here is the first drawback of this product, as the design in some portions seems to be really low-cost, e.g. low filtering capacitance on the output (only one big capacitor, from my side at least 2-3 capacitors in parallel should be used, to have a lower ESR value), there are some traces of hand soldering and the PCB is not cleaned, multiple components like capacitors or resistor are placed on the same footprint etc. However, I can live for now with that.

 

2. Testing and Evaluation

To test this product, I have used two experiments. First experiment was done with a professional electronic load where I measured parameters like: transient recovery time, power-up ramp, output ripple, and thermal analysis. The second experiment was carried out with a custom electronic load which was interfaced with a LabVIEW-based graphical user interface. From the GUI, the user can trigger the tests and can set the current pattern to be sink from the power supply. Please note that the voltage drop on cables is not covered, as there is not yet a possibility to allow remote feedback to compensate them on this power supply. It will be added in the future on a to-do list.

 

2.1 Professional Electronic Load Experiment

In this experiment, I have tested the Tenma Handheld DC Power Supply to study its behavior when delivering close to its maximum power: 12 V @ 3.7 A. A first test to study its thermal distribution was done while delivering for ~ 30 minutes 12 V @ 3.7 A. The power supply behaved well, to deliver and stabilize the required amount power sink by the professional electronic load. However, I have seen some thermal issues, results which are presented in figure 6a and 6b.

Figure 6a. Tenma Handheld DC Power Supply => thermal scan of its top view case

 

Figure 6b. Tenma Handheld DC Power Supply => thermal scan of its bottom view case

 

Issues with its LCD have been seen, after such tests when it was hot, see picture 7a and 7b. These issues have not been recovered with a simple power cycle of the power supply, however after a few hours when it was cooled off, the issue seem to be recovered.

Figure 7a. Tenma Handheld DC Power Supply => LCD issue 1

 

Figure 7b. Tenma Handheld DC Power Supply => LCD issue 2

 

I have repeated the tests either with the USB ports or with the main power supply via banana plugs, and the issue seem to appear when is heating to a certain point. It is still not clear for me why, but I think it may be due to a bad capacitor somewhere, which is affected by temperature. However, even with this issue, its outputs were OK, hence it think its affecting only the LCD.

Moving on from the thermal issue, I have placed the oscilloscope on its outputs to measure the noise and other power supply related parameters: power up ramp time and transient recovery time. In figure 8 the output ripple has been measured to be close to 130 mVpk-pk as stated by its datasheet while the power supply was delivering close to its maximum power, 12 V @ 3.7 A. Given the fact that this was not an accurate measurement, due to its construction is pretty hard to measure accurate without intervening in its hardware. However, this value may be lower if the measurement is done properly. Another way to reduce the noise, is to add some capacitance on its output, with the caveat that it may affect its transient response, so careful attention has to be given here.

Figure 8. Tenma Handheld DC Power Supply => output noise

 

The power-on ramp time has been measured, with the power supply turned-on in full load. The results is presented in figure 9, and besides the small oscillation at around 2 V the behavior is as expected.

Figure 9. Tenma Handheld DC Power Supply => power-on ramp time while turned-on in full load

 

With the same output parameters, 12 V @ 3.7 A, transient recovery times have been measured when the load was switched from 0.1 A to 3.5 A, and vice-versa. The testing results, shown in figures 10a and 10b, highlight that the power supply needs ~ 1 ms to recover, in either scenario. This recovery time is acceptable for a medium-performance power supply. However, just for comparation, high-performance power supplies like Keysight N7952A have 100 us to recover within the 100 mV (0.25 % full scale output).

Figure 10a. Tenma Handheld DC Power Supply => power supply transient recovery time when the load was switched from 0.1 A to 3.5 A

 

Figure 10b. Tenma Handheld DC Power Supply => power supply transient recovery time when the load was switched from 3.5 A to 0.1 A

 

2.2 Custom Electronic Load Experiment

For the second test I have designed a custom electronic load which was controlled via a LabVIEW-based graphical user interface (GUI), and used to sink a specific programmed current, figure 11. Please note that due to the lack of a remote feedback function to compensate the voltage drops across the cables and connectors, the voltage drops are presented in the following measurements.

Figure 11. Tenma Handheld DC Power Supply => connections with a custom electronic load

 

Tests with different programmed currents at different voltages have been carrier out to check its stability with different loads at different voltages. Figure 12 shows a test of the power supply tested with 25 V at about 1 A. The current has been increased in steps of ~0.16 A at each 100 s.

Figure 12. Tenma Handheld DC Power Supply => 25 V @ 1 A test

 

In figure 13, a test with 16 V and 2 A is shown with current being set in steps of ~0.32 A at each 100 s. When the programmed current to be sink reached 2 A the power supply enter in over-current protection by cutting off its output. This is another drawback, which sometimes can be very annoying.

 

Figure 13. Tenma Handheld DC Power Supply => 16 V @ 2 A test

 

Another tests with 10 V up to 2.5 A and 5 V up to 2.5 A are presented in figure 14 and 15. As same as previous tests, the currents are set in steps at each 100 s.

Figure 14. Temna Handheld DC Power Supply => 10 V @ 2.5 A test


Figure 15. Tenma Handheld DC Power Supply => 5 V @ 2.5 A test

 

A last test has been done at even lower voltage,1 V and 2.5 A. The results, given in figure 16, shows a good stability of the power supply at low voltages. During these tests the voltage drops are more persistent.

Figure 16. Tenma Handheld DC Power Supply => 1 V @ 2.5 A test

 

3. Conclusions

 

Overall, I am happy with this handheld power supply, despite its issues find during testing it behaved well during tests. It is small, pretty powerfully, and covers a broad range of voltage and current which make is excellent for low voltage and low power applications. However, the following list highlights some personal issues which need to be fixed, maybe by the vendor, if another version is intended to be released:

-when using both the USB charger ports and the main power supply, and if a short is done on either one, then the power supply is cutting off both. This can be annoying, maybe individual cut-off can be implemented;

-the output filtering can be improved;

-the banana connector are of low quality. I noticed that they were heating a lot when sinking high currents with respect to the cables;

-the LCD issue need to be fixed. I will search for a solution; [Maybe another user or roadtester of this power supply can confirm this issue?];

-maybe some thermal dissipation improvements;

-it can be a long shot but maybe remote feedback could be implemented, to remove the voltage drop on cables;

-being a lab power supply, maybe an earth connection has be implemented, to avoid unwanted/dangerous episodes.

 

 

PS: Let me know if I missed something, or something is not clear.

 

Cheers,

Vlad-Mihai Placinta

Anonymous
  • Hi Ralph,

     

    Thanks for your input. Which means I got a faulty unit. But I am still happy with this device, hence I will live with this issue, till I will find a way to mitigate it.

     

    Cheers,

    Vlad

  • I haven't seen the LCD issue but the case does get pretty warm in operation as you can see from Vlad's thermal pictures.  I somewhat expected it because of the fanless design and there aren't any vents in the case.  Not an issue for me as I don't intend to run continuously fully loaded.  The heat is a function of the load but I haven't quantified that.  Wish I had a thermal camera. 

     

    That reminds me that I need to finish my review.

     

    Ralph

  • Good road test report.

     

    Good detail and spotting the LCD defects.

     

    DAB

  • Hi Gough,

     

    Thanks for your input. I have corrected the brand name, sorry about that. I will have to tear it down someday so see exactly were the heat comes from. I have used it in 3 different rooms, where I felt comfortable, hence to temperature was about 25 degr C.

     

     

    Cheers,

    Vlad

  • Looks like one of the output filtering chokes may have been getting quite warm (along with the bulk of the transistors) ... maybe that was conducting into the banana plugs rather than the plugs themselves being low-quality and making poor contact. Having heat affecting the LCD would not be unusual in the sense that LCDs tend to get sluggish in the cold as well, thus are definitely temperature sensitive. The design is a little strange if the LCD does not display correctly even loaded within the design parameters. I wonder if your room is quite hot where you are testing the unit?

     

    As for earthing - perhaps not having an earth is a benefit as it is a set of floating outputs with no direct reference to earth, thus can be more flexibly used. The casing appears to be plastic and the unit probably complies to double-insulation, thus earthing is not a necessity. Four-wire kelvin connection (remote sense) is a fairly rare feature to find in lower-end non-precision supplies - I wouldn't really expect it on such units.

     

    By the way, the brand is Tenma not Temna (as you've captioned all your images).

     

    - Gough

  • Hi David,

     

    Thank you, Yes I am curious too to see what other users have seen.

     

    Vlad

  • Hi @shabaz

     

    Thanks!

    Thats my plan, to dismantle the power supply and see what it is underneath the LCD, and maybe to find the issue. but till then I am waiting to see if other user have seen this issue, just to be sure if its an isolated issue or a general one.

    I agree with you, it is very handy with its dimensions and features.

     

    Vlad

  • Hi Frank,

     

    Well, it is not really a matter of load, as it is a matter of heat. I have seen this issue, after charging my phone from the USB ports, which cause the power supply to heat to a certain point. I will dig into it.

     

    Vlad

  • A great review. Nice to see the thermal analysis. It should be easy for the other reviewers to confirm whether the LCD issue is common or if you have a slightly dodgy one.

  • Hi Vlad-Mihai,

     

    Nice review, and very detailed information. The photos were interesting, it's good to see there is filtering on the PCB on the output, so they have not cost-cut there. The noise looks acceptable too, considering this is a very compact switch-mode supply. A lot of the remainder could be removed by using a ferrite core on the output leads (and a capacitor at the end of it, but this could be an existing capacitor in the equipment to be powered up). I have a switch-mode bench supply, if I use a ferrite on the output wires and even a small capacitor (e.g. 10uF) the difference is huge.

     

    Regarding the LCD, I'm wondering if there was some flex of the LCD holder or the outer enclosure, if maybe the LCD has a zebra-strip for example not making contact until it cools down. It would be surprising though, since it doesn't seem that hot near the LCD (if I'm reading the image correctly). I guess any flexing of the outer case could be a reason too, although that shouldn't happen either.

    Or it could be a one-off faulty unit, even high-end gear can be faulty from factory sometimes (I've had that twice, one I worked around, the other needed the manufacturer to repair it).

     

    It's very cool that USB is built-in, it makes testing these frequently-encountered USB-powered devices much easier. Also, it reduces the need to have a couple of multimeters and so many cables, since the current measurement is integrated. I think for uni/home use, it would also greatly reduce the risk of error and blowing up multimeters with probes left plugged on current by mistake!

     

    I have not looked at buying a power supply too often, dreading that it could be expensive, but after seeing the reviews, and by coincidence having tried a bench Tenma supply suggested by last week, it's clear they are good value for money too.