TENMA 72-10505 Current limit problem

[brumbarchris]

Hi all,
Just started to actually used my recently purchased TENMA 7210505 power supply and it turns out it is having a current limit problem on channel 1.

If I set the current limit to anything lower than 87mA, the channel goes into Constant Current mode as soon as I enable it (without external load), although it displays "0" for the drawn current. Once I gradually increase the current limit, the channel goes out of that state and manages to eventually ramp up the voltage to whatever value it is being set to.

For the moment, I am not sure if this is a bug or "feature"; I plan to open it up over the weekend, but until then I would be interested in knowing if anybody else had a similar problem, if it is a fairly common thing for these "cheapies" and how you solved it if you faced that.

Regards,
Cristian

[wraper]

If you set, say, 100mA, does it limit current correctly? Measure actual current.

[brumbarchris]

If I set current limit at 100mA and turn the output ON without a load, the current consumption indicator shows 0. As I attach a VERY high load and start decreasing its resistance, the indicator correctly indicates the current drawn by the load, but starts to get in CC mode at about 23mA (which is exactly the difference between the 100mA setting and the 87mA I've mentioned in my previous post.

Cristian

[Zucca]

Meay be there is an offset of 87mA? Can you calibrate the current limiting circuit? Any pots there?

[wraper]

Meay be there is an offset of 87mA? Can you calibrate the current limiting circuit? Any pots there?

Yep, I was thinking the same. There could be an offset in either current sensing or voltage generated by DAC, calibration done incorrectly? I don't think there are any pots, in any case should look for KORAD KA3003D-3S when doing research as Tenma is just a rebadge.

[wraper]

These PSUs have DACs made of discrete resistors, something wrong might happened there as well.

[Armadillo]

The LM1117, 3.3V power supply is taken as a reference. Firstly make sure the reference is correct then go through the power supply calibration procedure.

Edit: Power off, press and hold M4 button then at the same time press the power on button to enter calibration mode. Read the instruction manual....