Tenma Handheld DC Power Supply - Review

Table of contents

RoadTest: Tenma Handheld DC Power Supply

Author: ralphjy

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?: The specification had multiple errors and the ripple at low voltage was out of tolerance.

Detailed Review:

The Tenma 72-2660 is a compact portable DC power supply.  It is apparently manufactured by a Chinese OEM and resold by quite a number of vendors including Tenma.  The design was introduced back in 2015 so the good news is that there is lots of user experience.

 

I have done much of my detailed review as a series of blogs, so this is a wrap up with some application testing and a summary of my overall impressions of the unit.

 

Here are the links to the blog posts:

Tenma Handheld DC Power Supply - Unboxing

Tenma Handheld DC Power Supply - Output Current Range

Tenma Handheld DC Power Supply - USB Output Functionality

Tenma Handheld DC Power Supply - Product Features

Tenma Handheld DC Power Supply - Performance Tests

 

USB Cable resistance testing

I was inspired by Doug's roadtest and I also must admit to being somewhat cavalier using USB cables to supply power to my various SBC and microcontroller boards although  I do run into occasional issues (more frequent now with current requirements of 2+ amps).  So I put together a little test jig so that I could attach USB to microUSB cables to the power supply output.  Then I went to my cable box and randomly selected 16 cables of various lengths (I probably have 40-50 micro USB cables in total with about half of them in use).

 

Here is my setup testing a short cable

:

 

Here is my pile of cables with the bad ones marked with a red flag:

 

Of the 16 cables, two of them actually had open ground connections (I suspect this is a connector issue).  Of the rest, I found 2 that I marked as bad because of too much overall voltage drop.

Here is a plot of resistance per inch for the different lengths tested (measured at 1A):

Based on the data I would say a "good" cable would have less than 30 mOhms per inch resistance.  It is interesting that the longest cable has the lowest value.  It is using a larger wire size to mitigate the total resistance.  I wish vendors would just indicate the AWG of the wire in the cable.  Over a small sample there is no correlation with voltage drop vs length or O.D of the cable.  So, my mantra of shorter is better doesn't always apply.  I probably should start measuring the delivered voltage in my different setups.

 

On a related note, since I had the load assembled, I thought I'd try checking a couple of power adapters that have the power wiring attached.  I recently bought some Rhino brand supplies that have been giving me issues:

Rhino 5V, 2.5A with 36" cable

Unloaded: 5.24V

1A load: 4.95V

 

Raspi branded supply (Stontronics) 5.1V , 2.5A with 58" cable

Unloaded: 5.33V

1A load: 5.19V

 

It is interesting that the Rhino brand has "Raspberry Pi 3B+" prominently labeled on the box.

 

USB Power Bank Input Current

I have a number of different USB power banks that I would like to charge from small solar panels and I want to characterize the input current vs input voltage requirement.  I have a number of power banks from different vendors at capacities from 2Ah to 20Ah.  I'll need to check the characteristic over various charge states, so for this roadtest I've just verifying that the procedure will work.  I wish the supply had automated control capability since it is using a STM microcontroller but that would be asking for a lot at this low price point (I think it mainly a programming effort, plus adding an interface).

 

I am using the same USB test jig for this setup and manually sweeping the voltage while monitoring the current.

 

I tested 3 of the 2Ah units and here are the results:

 

Reasonably well matched but I'll need to check vs charge state.  So, I should be able to get the info that I want, just need to invest the time.

An interesting side note: I decided to test these power banks under load and one of the three would not maintain a constant output voltage, even after I fully charged it. It would vary randomly between 4-5V.  I assume there is a problem in the regulator circuit.  I guess I am learning that I need to test everything

 

Summary and Overall Impression

I really like the portability and ease of use of this power supply.  It will be a good utility supply for my workbench.  That being said, there are a number of issues that the user needs to be aware of.

  1. Excessive ripple at voltages under 1V - not an issue for me.  Maybe might affect measurements with diodes.
  2. Negative offset voltage of -600mV when output is disconnected - probably only an issue testing parts with multiple power rails.
  3. Possible thermal issues with continuous operation with full load due to lack of case ventilation.
  4. No power switch - just use it with a switched power strip.
  5. No true constant current mode - supply sometimes shuts down going into current limit.
  6. Cannot operate USB output independent of main output - just need to be aware that the USB voltage will show up at the main output.
  7. Cannot adjust USB voltage, only current - requires that you use a fixture on the main output to do any USB voltage testing.
  8. I saved the funny one for last - you can accidentally set the current limit above 3.75A - not sure what it will do and I decided not to try it but here is the proof:

 

And finally, I need to pay more attention to what I'm doing.  I was playing with the voltage with the current limit set out of the way and while I was looking at the oscilloscope there was an odor of burning plastic.  Nice impression of a 25W sandstone power resistor running at 40W.

Anonymous
  • Negative offset voltage of -600mV when output is disconnected

    I also found the negative voltage issue when I was testing one of these supplies. I found that leaving different kinds of loads would change the offset value. I could get the offset down to -400 mV. Such a limited range made me suspect it might be a diode junction of a BJT. As I understand it, there is a drain circuit that is used to pull the supply back down to "0 volts" when it is turned "off." That's the source of this extra offset.

     

    It turns out if you remove a transistor and replace a couple of capacitors with resistors you can eliminate it. The trade-off is that the supply has a significantly sharper turn-off.

     

    I have a video coming out soon that shows which parts to change on the Tenma supply. (There are multiple versions of the PCB, the Tenma PCB is slightly different from the pictures I found.) When nothing connected (or a 10 Mohm DMM) I still get a small negative offset. But with the 1 Mohm load of my scope, it is 0.

  • Hi Ralph,

     

    Great review and blog posts!

    It's neat that it can act like a USB power supply, while providing current consumption measurements. I tried building a passthrough type device to measure current from existing USB supplies, but didn't get beyond a protoboard:

  • Hi Ralph,

     

    A great review, thank you. It is always nice to see a real-world test being undertaken (hopefully you are throwing the broken leads out? ).

     

    That is an interesting pattern from the resistor - it seems to show it is hotter in the middle as the melt-back is further, which would make sense. Please keep us informed of Mat's recovery

     

    Rod