East Tester ET5411A+ (and ET5410A+) Electronic Load: Schematic reversed...

[morris6]

For your entertainment the reversed schematics of these instruments are added here. There are three boards and an SMPS power supply. The SMPS power supply that is at the backside of the instrument looks rather "generic" so I didn't include it. The instrument itself consists of a large "Main" board mounted to the bottom side of the main heat sink, a "Slave" board on the top side and a "Front" board with the display and the control buttons. The schematic of the main board is spread over 5 pages. So we have a total of 7 .PDF plots of the schematic pages in the .ZIP file.

The schematics were done in KiCad 7. I used the part references as they are silk screened on the boards. Because the part references are not unique for the whole system, they sometimes appear on more than one board, I had to make separate KiCad projects for main, slave and front. For your convenience the schematics are published as .PDF's. And some pictures of the main board. The bottom side picture is marked with the different ground planes and the interconnecting zero ohm resistors.

Comparing the ET5411A+ main board with pictures I found here of the ET5410A+ the only hardware differences are the type of power Mosfets and the value of the resistors that determine the high measuring range for voltage and current. And the firmware is adapted of course to reflect these differences in measuring ranges.

Funny thing is the main board contains in the isolation section all the parts for the double channel ET5420A+ variant, I wonder why that is..?

The input sense section has some nice provisions to cope with the advertised maximum 500V input (ET5411A+). It has isolation slots in the PCB and two ladders of 8 resistors for the voltage input circuit. However, the heads of the board mounting screws have only about 2 mm spacing from the traces. The screws attach the board to the main heatsink, The main heatsink is connected to the V+ terminal while circuit ground is connected to the V- terminal! Even for the ET5410A+ variant that is restricted to 150V on its input it makes me feel a little uneasy. Use with care, keep the boards dry and clean.

I am working on some small mods. I took the encoder out of the front board and soldered it back with A and B pins free of their pads; connected the pins with short wires to the opposite pads. Now rotating the encoder to the right increases numbers, it feels more natural IMHO.

Next is taming the noisy fan.

Another thread here discusses the addition of an external voltage sense connection.
https://www.eevblog.com/forum/testgear/et5410-electronic-load-mod-add-external-sense/msg4837247/#msg4837247
But be aware of the problems that can arise when these external sense wires loose their connection.

Your questions or remarks are welcome. I connected some wires to specific points of the circuits trying to get an idea of the dynamic behaviour.

[andyB2022]

Hi, first I want to thank you for all of your work. Do you won ET5410 (first model)? If so, could you please extract the firmware from the stm32? I've managed to screw my unit...

[morris6]

Hi Andy,

About your request, I don't have the ET5410, the older model.
The newer model, the ET5411A+ I have here, has a different hardware design and also the processor is different. So I am almost sure the firmware is NOT compatible. Also, it has to be seen howfar it is possible to read the content of the processor flash...

[LinuxHata]

Maybe someone is familiar with ET5408/09 models?
They appear to have bigger screen, more buttons and features also appear to be similar to 5010/11 models, but they cost about 20% less. So what is the catch?

[CloverGit]

Hi, do you have the parameters about the second order low pass filter circuit (U7) in the DAC output section with its capacitors C10, C8 (or C5, C3). Also, the values of capacitors C6, C9 and C2 are unknown. I am building my first electronic load and would like to determine if these parts are necessary. Your work has inspired me, thank you for your work!

[morris6]

I did try to measure these values in circuit but gave up due to the inconsistent results. Seems I can't help you.
 
The low pass filter circuits on U7 are for cancelling noise from the DAC in the MCU. These signals are not pwm but dc levels. The other caps you mention are part of the integrator circuits. Maybe try a simulation to determine a value for these?

Have fun, Maurits

[CloverGit]

I spent a few days reversing part of the circuit of the Maynuo M9812B and I found a similar design. the capacitors at C6, C9, and C2 are usually large (several to several tens of nF, even up to 10uF in one case) and use a complex circuit, which I guess may also be related to the op-amp self-oscillation problem.

[cnkz]

Thank you for this interesting re-engineering work.

[modoran]

I am working on some small mods. I took the encoder out of the front board and soldered it back with A and B pins free of their pads; connected the pins with short wires to the opposite pads. Now rotating the encoder to the right increases numbers, it feels more natural IMHO.

Why do you do that ?   Rotating encoder to the right or left can be changed from menu, in which way you want, reverse or forward.
menu -> system set -> knob set.


Perhaps older firmware does not have this setting ?

[morris6]

Checked the menu of my ET5411A , yes, indeed, you can change direction forward <> reverse for the encoder knob. Thanks for pointing this out. It's not mentioned in the "manual".

However, after my modification the knob is working the "right" way and accordance with the menu setting. So, not a totally wasted effort after all.

[KubaSO]

In the output stage, a TL072 drives 3nF of NMOS gate capacitance through a \$100 \Omega\$ resistor. TL072 has a fairly anemic output - about 25mA max. The series resistor doesn't help drive strength either. For a gate driver, that's very anemic. A buffer stage would make things better.

[andyB2022]

Anyone that tried to download the firmware from GD32F303VET6?

[temperance]

In the output stage, a TL072 drives 3nF of NMOS gate capacitance through a 100Ω resistor. TL072 has a fairly anemic output - about 25mA max. The series resistor doesn't help drive strength either. For a gate driver, that's very anemic. A buffer stage would make things better.

Not only the drive current is a problem. The open loop output impedance of the TL072 is 125 R and responsible for a phase shift in the feedback. Good luck trying to compensate that. Or you buffer the op amp or you have to implement a different compensation network.