Thanks to Project14 and the Electronic Toys contest I have a new bench power supply! Thanks tariq.ahmad. In this blog I will outline why I selected it and my first impressions. I also do a quick out of the box check to make sure things seem to work as they should.
I have needed to upgrade my power supply for quite some time and have been on the lookout for something with the following specifications:
- Reasonably priced
- 0-30 V
- 0-3 A
- mA setting resolution and low mA load accuracy
- Dual channel
- USB / LAN interface
Dual channel would be nice as I occasionally need a second rail for either op amps or 3V3 and 5V but it was of lower priority. In a similar manner I would like to be able to control the unit from a LAN but it is for the time lower priority also. The mA setting resolution and accuracy are difficult to meet in a low cost unit and twiddling with the potentiometers on the one I have (fine and coarse) make it all but impossible. Recently I noted that the Tenma 72-2685 Tenma 72-2685 was on sale for $68.02 at Newark (although it appears to be selling for $119.36 as I write this). Here are the specifications - note the Current Load Regulation, Setup Resolution, and Setup Accuracy along with encoder control:
Unfortunately it is not dual channel and it can't be controlled by the computer but those are wants rather than needs. So I snagged one :-).
Unboxing
It came nicely packaged and undamaged in a box which I shall keep in case I need to transport it. The instructions are terse but sufficient. My unit has a North American 110 V mains power cord (it is switchable to 220V). It does not come with leads.
Quick Look Around
This is a basic power supply so our tour won't take too long.
Front:
- The digit display is bright and easily read from all angles.
- There are bright LEDs for Constant Voltage, Constant Current, Lock, and Over Current Protection
- The encoder controls for current and voltage are great! They allow precise setting and are reasonably quick to set once familiarity is gained
- There is a power on / off button (blue) which completely turns the unit on and off
- There are ground, positive and negative terminals that accept banana jacks - there are no binding posts
Sides:
- Ventilation
- Four Phillips screws on each side to hold case together
Back:
- Fan exhaust (runs only when needed)
- Power socket and switch for 110 or 220 V
- Fuse accessible from outside with a spare fuse
- Approx. 1.1 meter power cord supplied
Top:
- Handle
Bottom:
- Four rubber feet
Overall it looks good and well built from the outside.
A Peek Inside
I am not qualified to assess a mains powered unit but popped the cover and had a quick look around. I make no judgement other than the following:
- Inside is clean, and things generally neat and tidy
- Cable and wire is held securely and out of the way so as to not get pinched
- Mains were grounded to the case all the way around as was ground terminal
- The power MOSFETs are on a PCB that back to a large heat sink attached to the fan
- Solder joints and connections look OK with no flux residue
- Large Jicon capacitor has silicon to hold it in place, rated at 105 C and placed away from MOSFETs
- Silicon used to hold some connectors and wire in place
- The front panel PCB was clean and solder joints looked good.
- What I assumed to be vias and cooling traces was interesting.
- One largish capacitor on its side with no silicon to hold it down
- The terminals did not look to be of high quality
- No bodges or repairs seen anywhere
Trying it Out
I don't have my electronic load working yet and I don't plan to look at ripple or reaction to load changes so this is going to be quick and dirty. I may come back for a deeper investigation in future but for now here is a quick look at the things that most interested me.
The transformer taps are at 7V, 14V, and 21V. The specifications give the voltage range to be 0-30 V but it can be adjusted up to 31 V. I hooked up a 10K resistor (measured to be 10.01 k) in series just to see how accurate the voltage setting and readout was at low currents. The voltage was checked with a Tenma 72-102072-1020 bench digital multimeter (DMM1) and the current with an Aneng AN8008 (DMM2). I have an assortment of inexpensive multimeter with the 72-102072-1020 being the best. There are no calibration certificates but they are all in close agreement with better resolution than the Power Supply Unit (PSU). Voltage was measured at the terminals so as to avoid influence of resistance in the wire leads.
The results are in the following table.
| PSU Setting (V) | PSU Reading (V) | DMM1 Reading (V) | PSU Reading (A) | DMM2 Reading (mA) |
|---|---|---|---|---|
| 1.00 | 1.00 | 0.997 | 0.000 | 0.986 |
| 2.00 | 2.00 | 1.992 | 0.000 | 1.972 |
| 3.00 | 3.00 | 2.995 | 0.000 | 2.966 |
| 4.00 | 4.00 | 4.000 | 0.000 | 3.962 |
| 5.00 | 5.00 | 5.002 | 0.001 | 4.956 |
| 6.00 | 6.00 | 6.000 | 0.002 | 5.946 |
| 10.0 | 10.0 | 9.994 | 0.006 | 9.907 |
| 15.0 | 15.0 | 15.00 | 0.011 | 14.876 |
| 20.0 | 20.00 | 20.00 | 0.015 | 19.84 |
| 25.0 | 25.00 | 25.00 | 0.020 | 24.821 |
| 30.0 | 30.00 | 30.00 | 0.025 | 29.801 |
I found the user interface fairly easy to use once I got used to it. Setting a value is not as quick as rotating the potentiometer on my other bench power supply but it is WAY MORE PRECISE with the encoder on this unit. Voltage is easily set to within 0.01 volts and readout is quite accurate as can be seen from the table. A somewhat minor complaint since this is an entry level supply is that settings and readout are not displayed at the same time. To get a setting it is necessary to push the adjustment knob which then displays the set point.
Current adjustment can be done down to 1 mA, but readout and setting accuracy is not as accurate as voltage. As can be seen from the table (note that PSU readings are in A while DMM2 is in mA), readout does not occur until the current approaches 5 mA measured with the DMM. Readout accuracy is not given in the specifications, and this is much better performance than my other bench PSU but it was still a bit of a disappointment. I decided to test with a smaller resistor in order to see how it performed at higher currents. In the following table a 10 ohm resistor (measured to be 10.05 ohms) was set up as described above.
| PSU Setting (V) | PSU Reading (V) | DMM1 Reading (V) | PSU Reading (A) | DMM2 Reading (A) |
|---|---|---|---|---|
| 0.50 | 0.50 | 0.499 | 0.045 | 0.045 |
| 1.00 | 1.00 | 0.996 | 0.095 | 0.093 |
| 2.00 | 2.00 | 1.990 | 0.193 | 0.192 |
| 3.00 | 3.00 | 2.993 | 0.292 | 0.290 |
| 4.00 | 4.00 | 3.997 | 0.389 | 0.390 |
| 5.00 | 5.00 | 4.998 | 0.488 | 0.487 |
| 10.0 | 10.0 | 9.986 | 0.974 | 0.970 |
| 20.0 *** Fan turned on | 20.0 | 19.98 | 1.858 | 1.853 |
The fan turned on at 20 V and roughly 2 amps. The resistor I was using was not rated for this load and I halted the test after a few seconds. However, as can be seen voltage setting and readout accuracy are good throughout the range, as is the PSU current reading.
My main interest is performance at lower voltages and low current so I hooked up the following circuit:
Basically I want to turn up the voltage slowly with the LED in series at various current control settings to see how accurate the current control setup and current readout accuracy are. For these readings the DMM2 probes were not exactly as shown in the sketch above but were across the LED leads and not the terminals on the PSU. Accordingly the voltage readings between the PSU and DMM1 are off a bit. The item of interest here is the current setting and reading.
| PSU CC Setting | PSU Voltage Reading | DMM1 Voltage Reading | PSU Current Reading | DMM2 Current Reading |
|---|---|---|---|---|
| 1 mA | 1.62 V | 1.62 V | 0 mA | 0.002 mA |
| 1 mA | 1.65 V | 1.646 V | 0 mA | 0.006 mA |
| 1 mA | 1.85 V | 1.841 V | 0 mA | 0.981 mA |
| 1 mA | 1.89 V | 1.875 V | 0 mA | 2.241 mA |
| 1 mA | 1.91 V | 1.889 V | 0 mA *** Constant Current | 3.016 mA |
| 2 mA | 1.95 V | 1.910 V | 0 mA *** Constant Current | 3.109 mA |
| 3 mA | 1.91 V | 1.889 V | 0 mA *** Constant Current | 4.825 mA |
| 4 mA | 1.96 V | 1.922 V | 2 mA | 5.994 mA |
| 4 mA | 1.98 V | 1.927 | 3 mA *** Constant Current | 6.612 mA |
| 5 mA | 1.97 V | 1.936 V | 4 mA *** Constant Current | 7.638 mA |
| 6 mA | 1.99 V | 1.944 V | 5 mA *** Constant Current | 8.530 mA |
| 7 mA | 2.00 V | 1.951 V | 6 mA *** Constant Current | 9.455 mA |
| 8 mA | 2.01 V | 1.956 V | 7 mA *** Constant Current | 10.254 mA |
| 9 mA | 2.04 V | 1.963 V | 8 mA *** Constant Current | 11.270 mA |
| 10 mA | 2.05 V | 1.968 V | 9 mA *** Constant Current | 12.126 mA |
| 15 mA | 2.10 V | 1.998 V | 14 mA *** Constant Current | 17.465 mA |
Observe the first column which has the current control setting (mA) and the last column which has the current observed (mA) on a DMM. Ideally, the current control setting will always be at or below the DMM current reading. In actuality it exceeds it by up to 2.5 mA or so. This is within the specification sheet value of 0.5% + 5mA. Now consider the next to the last column which shows readout of current on the PSU and also whether it was in constant current mode. The PSU does not start reading current until it reaches almost 5 mA (an error of approximately 5 mA). The largish error exists throughout the low range. As noted above the specifications do not specify readout accuracy for current.
Controlling the Unit
Control using the encoders is a bit different than the standard coarse / fine potentiometers and is probably best shown with a video.
Conclusions
I am quite pleased with the Tenma 72-268572-2685 and recommend it for those looking for an inexpensive entry level bench DC power supply. Setup is easy and resolution is very good for an inexpensive power supply. The unit met or exceeded the specifications in the areas I tested. The 72-268572-2685 current control does not perform as well as I would like at very low currents but it meets the specifications and the specifications are superior to the other low cost units I looked at and match some higher priced units in key areas. The power supply seems well built and was an absolute bargain at the discounted price of $68.02 at which it was purchased. This is only about $10 more than I paid for my "eBay special" and there is no comparison in terms of performance or quality.
There are higher priced models in the series for those needing higher current output, higher voltage output, and USB / RS2332 interfaces.
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