RoadTest: Tenma Handheld DC Power Supply
Author: dougw
Creation date:
Evaluation Type: Independent Products
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?: The only other products comparable to this one seem to be private branding of the same basic power supply.
What were the biggest problems encountered?: I found it difficult to measure power supply noise, but this is not an issue with the product. it is an issue with my measurement technique.
Detailed Review:
The Tenma Handheld DC Power Supply is a really exciting product which is a pretty unusual statement for me, especially about a power supply. Based on the large numbers of road test applicants I'm not the only one who feels this way. There have been many instances over the course of my career where this instrument would have been extremely useful.
I have done a lot of field work where we were delivering or installing one-of-a-kind custom designed products. These system installation scenarios and troubleshooting scenarios often needed a "known good" power supply to at least eliminate one of the major culprits of system problems. However it was always extremely difficult to carry all the equipment needed and lugging around a normal bench power supply was often out of the question or at least very difficult and expensive. Many of those situations ended up being logistical and physical nightmares because I did not have a solution like this available. This product form factor immediately resonates as being indispensable for some applications and very handy in a much wider range of applications. I have blogged about the importance of having the right tools when operating in remote environments and without even having tried this product, it is obvious to me it would qualify as a magic tool as described in that blog. If I had this power supply in some of those situations where it was especially needed, I would have some great stories about how it saved the day. One obvious reason I wanted to participate in this road test is it has lots of great features that make it a very useful device every day, but the big reason is that I know situations will occasionally occur where this instrument will save the day.
In this road test I will make some videos of the unboxing, product features, user interface and performance tests. But I want to spend most of my effort on showing some real applications of this intriguing device.
Here is the unboxing video:
I had to split of the initial features run through video:
Quick Performance tests of response times and noise
The first test was to look at the output voltage during turn-on to see how well behaved it is:
Turn was a smooth ramp that took about 7 ms to reach 5 volts with a 10 ohm resistor load. There was no overshoot.
Next the turn off decay was recorded:
This is with a 2.5 Ohm load: - it takes about 1 ms.
This is with a 10 Ohm load: it takes over 3 ms.
This is with no load: it takes over 70 ms.
This performance is fine with me.
Next I looked at what happens when the load is suddenly doubled:
I'm not sure if some of this is due to the switch bouncing, but the step in voltage is due to increased voltage drop in the cable as the load gets doubled using a slide switch - the power supply did not sag.
Next I put a 10 Ohm resistor in series with the load to measure the current as the device goes into current limit:
It looks like there is a short spike above the current limit before it gets it fully under control.
And this is what happens to the voltage at the load as current limit kicks in:
Note that the load is doubling via a slide switch to cause this response.
Since this is a switching power supply I wanted to look at the noise it generates. However it is tough to measure noise properly and I am sure it could be done better than what I did. My tests at least showed the test method greatly affects the results. I tried to replace the scope probe ground lead with a very short wire clipped to the probe barrel. This made a large difference in the reading, but I am sure it could be done better.
Here is what it looked like with the standard probe ground:
The probe ground lead is definitely causing significant spurious noise.
And here is what the noise looked like with the little stub ground:
I don't think either one of these images accurately depicts the real noise, but it is what I captured.
Next I will start looking at some applications - I'm much more interested in using this power supply in real applications than simply regurgitating datasheets.
Application #1 - Testing Voltage versus Current Relationships
I have a Sharp GP2Y0A21YK0F optical analog proximity sensor which is very nice, but it is highly non-linear and it seems to consume a lot of current. I built a linearizing circuit which improves its performance at longer distances, but I also want to see if it is possible to reduce the current by reducing the power supply voltage. The Tenma power supply can easily be employed to adjust the voltage and measure the current while I determine the performance of the linearizing circuit and the sensor. It is also very useful for applications like this where you may need to test somewhere where there is more room than at your electronics workbench and you have to take your power supply to the test site.
I was pretty neat that it got to use my Tema digital multimeter with the Tenma power supply.
Application #2 - LED Testing
I have a bunch of LEDs which are all clear plastic and of course there are no markings on the LEDs at all. I have no idea what colour they are, what their forward voltage is or how bright they are. This power supply will make it easy to set a 20 mA current limit and a higher than needed voltage and just plug any LED in to discover everything I want to know. This testing makes use of the Multifunction Socket that came with my Tenma digital multimeter.
Application #3 - Testing Cable Losses
The objective of a powers supply is to get voltage and current to a circuit, but both of these parameters are dependent on the wires that make the connection. This power supply makes it easy to test the leads before hooking up a circuit. I assumed it was as simple as setting the current limit and shorting the leads together to see the amount of voltage drop in the cable at that current level. However if the resistance is so low that the current generates less than the supply's minimum voltage setting of 0.3 volts, the supply will turn off. The following video shows some of the things it can and cannot do.
Application #4 - Battery Substitute
I have a Dymo label maker that requires 9V at 2 Amps. It has a battery compartment which takes 6 AA batteries, but they only last a few minutes, because as soon as their voltage drops at little bit the machine does not work. Alkaline AA batteries struggle to supply over an amp and it doesn't take long for the voltage to sag at these current levels. I hate putting in fresh batteries every few minutes or every time I use it.
I made up a little cable to the Tenma power supply and it is a whole lot more portable than lugging around a big bench power supply.
This application gave me a chance to feature my Tenma soldering station and Tenma fume extractor when building the cable.
I also designed and printed a tandem holder/stand so I can snap the pair in when going on a labeling mission and the pair can be hand-held, or at least not flopping around.
Application #5 - Raspberry Pi power
A very common issue with Raspberry Pi computers is that they push the envelope of what the USB standard allows and many USB power supplies and USB cables aren't quite good enough to supply what Raspberry Pi's need.
I wanted to experiment with the voltage and different cables with various Raspberry Pi's to get a feel for what does and doesn't work.
(video pending)
Conclusions
Publication of this road test was delayed because it took a whole week to design the 3D printed parts and production of all the videos always seem to take many more hours than the results would suggest. However this road test was a good opportunity to spend some time learning about the Tenma Handheld Power Supply and what it might be good for. I found it easy to figure out how to operate the device, and the buttons had a nice tactile feel. I especially like the 5 programmable preset configurations - they can save a lot of button pushing when changing configurations.
The rectangular shape made it far easier than the Dymo to design a case that fit properly.
There are a few quirks to get used to such as the minimum voltage of 0.3 volts and the all push button interface, which was likely implemented to minimize the size of the device. The fact that a USB port can be set to deliver 3.75 amps could be very useful when trying to sort out USB power issues.
One application not mentioned so far is that this supply will be very useful when creating blogs, it's small size will allow it to fit in the camera frame better and the large digital display makes the display easy to read in a video or still shot. Those digits may be the largest digits I've ever seen on a power supply readout.
Variable power supplies provide enormous flexibility and their adjustability is a key advantage, but 95% of the time power supplies are set up to supply standard voltages like 3.3 V or 5 V or 12 V. This Tenma embodies the best of both worlds, by providing a dedicated button to select one of 5 pre-programmed configurations. And having 2 high-current USB power ports really addresses an enormous segment of modern power supply activity. Not only that, but when you turn it on, it starts with the same configuration it had when turned off, whether it was per-programmed or not. These features are worth the price of admission. This makes it a super flexible power supply while addressing the needs for convenience and high productivity. Some big, expensive supplies could learn a few things from this design.
This little power supply packs a lot of features and functionality into a much smaller form factor than traditional bench power supplies.
I am exceedingly happy to have this power supply, it will definitely see a lot of use.
Relevant links:
Tenma Handheld DC Power Supply - Road Test info
Tenma 72-2660 Bench Power Supply - purchasing page page
Tenma 72-2660 - Handheld Power Supply - datasheet
Top Comments
Excellent Review. I am surprised at the capabilities of the device.
Good job.
A nice road test. I'm tempted to get one, but can't seem to find it on the Farnell UK site any more. A quick search shows it does seem to go under a few different brands in other places..
Nice adapter Doug!