Tenma Handheld DC Power Supply - Performance Tests

21 Apr 2019

Now to get on with some performance verification.

Noise Measurements

I'm not really set up to do precision noise measurements but since switching frequencies are 10's of kHz, I should be able to get a qualitative look against a 100mVp-p spec. You can see from the following pictures that the power supply probably doesn't meet spec at lower voltages and has a fair amount of low frequency noise below 1V. In general purpose use this probably won't cause a problem. All pictures have the same vertical and horizontal resolution (100mV/div and 50uS/div). The low frequency noise is essentially gone at 3V and noise is reasonably similar at higher voltages. The noise doesn't appear to vary much with load as long as the supply is in CV mode. It will get noisier in current limit (CC mode) probably due to the limiting circuit. I did not test with reactive loads.

PS Noise @ 300mV PS Noise @ 400mV

PS Noise @ 500mV PS Noise @ 1V

PS Noise @ 2V PS Noise @ 3V

PS Noise @ 4V PS Noise @ 5V

Output Voltage offset when off

This power supply is from a Chinese OEM and is rebranded by many vendors (Tenma, TekPower, Velleman, etc). I read in a review on Amazon that the output has a negative offset voltage when the output is off, so I thought that I should check that out. I believe that the OEM is Ningbo Jiuyuan but I'm not sure.

I found the following reference to a dump circuit at the TekPower website but unfortunately no schematic.

Question： Why the output shows not at Zero when the power is set to off?

Answer: In order to make a quick discharging when the power is set to zero, we put a transistor at the output end, so it will discharge the current faster, but this generates a very small voltage at the output, which they think it as harmful.

Yes, you can do as follows to make it zero:

1. Remove the transistor form the image, and

2. then add two 50 ohms resistor.

Measurements:

Output off, no load: -635mV
Output off, 1K ohm load: -339mV
Output off, 100 ohm load: -36mV

So it looks like the transistor junction with less than 500uA available current. The fix looks nasty enough that it's probably not worth the bother. And I haven't seen any data about how fast it discharges without that transistor.

The USB output also has some offset but it is much smaller [USB on/off, Output off] (-263mV). And there is some bleed thru [USB off, Output on] (58mV - independent of output voltage).

Transient performance

Here's a picture of the output turn on. Ground is at the center of the screen. It starts from the -600mV negative offset and drops almost 200mV before switching to ground and then the programmed voltage (300mV in this case). You can see all the noise at 300mV.

It does better at higher voltages. Here is 10V with a 10 ohm load. It stays at 0V for about 5ms and then takes about 15ms to get to 10V. Very little overshoot.