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Opensource Electronic Dummy Load – Overload Review

We have sold out about 80 units OverLoad in this month and get some feedback, here is the most detailed review by Bogdan Kecman:

1. I tried it on the low side, so charging some nimh batteries and it is very precise, 0-1A it was 1:1 with my desktop amper meter, but after it went over 1A it started to drift a bit, 1.5A was displayed as 1530mA, 2A was 2011mA … so it has decent precision over low end too, I am satisfied.

– It means 2% and 0.5% error, and we use the 1R 1% sampling resistor, it will cost this error.

2. the current is set via pot and not trough software. I have not checked the schematic yet but preliminary board examination show that pot goes directly to lm335 so it’s not controlling the mcu so mcu is only measuring stuff, ain’t controlling anything. This pot is well, single turn so 0-3A on a single turn pot means that you touch it and it jumps 5mA to one or another direction. Using some multiturn pot here would be a must imho.

– Yes, so the adjusted value is continuous. We can not find a multiunit pot that suit for this board – size, footprint, any suggestions is appreciated : )

3. terminals – I like them

– It took a lot of time to install, but we are very pleased that you like it

4. I missed to see the manual, I downloaded schematic and firmware but no manual…. will probably be available soon

– The manual will release soon , now we are making the OverLoad and DDS manual.

5. ampermeter function – when I short terminals – it show 300mA, when I push 100mA trough it it show 430mA so .. it is not precise on the low end .. donno how it behaves on the high end nor what the limit is (waiting for manual)

– You can calibrate the ammetter in the firmware, we didn’t do this step for this OverLoad but now we do the calibration before all boards be shipped out.  How to calibrate the current value: in the firmware you can see the calculate function in about line 251: DispilayValue = (int)((CalValue-1480)*9.2857) ,You can change the 1480 value to calibrate the initial value.

6. voltmeter function – 0-5V very precise, no idea if it can measure over 5V (waiting for manual)

– It can measure 0-30V

8. encoder – nice

– The same encoder will be used in our DDS signal generator and LC meter

9. display – contrast issues – might be related to firmware, if you go to about the contrast is to low, some positions have also too low contrast and i think backlight might be too bright from time to time

– we will try to modify the display library to fix it

10. firmware ..
– contrast issue on display is most noticeable problem.
– There is no constant voltage load only constant current. Question is if it is possible at all taken that you set load current with a pot directly on the op amp, not sure if there is dac on board at all
– There’s no nf filter for display (in any mode) so the numbers change too fast
11. board/design
– build quality – excellent
– the radiator on board is oriented in such a way that air goes trough connectors, if one rotated the radiator by 90 degrees one would push air trough power resistor and cool it too. It is important as the cooler that resistor is the more accurate measurements are

–when we get the radiator, we found this problem, so we add some slot in it to cool down the resistor. Yes, you are completely correct, if we can rotated it by 90 degrees, it will bring the better cooling effect for the sampling resistor, then maybe more slight error : )

All in all, interesting device. The only major flaw I see attn is that instead of using DAC and drive the current value trough mcu they are doing that analogue … I’ll check schematic to see if there is DAC on board at all, but if not I see that there is i2c on the connector, maybe it would be an easy hack to add DAC there and link instead of the pot .. I haven’t used i2c dac’s mostly spi but I assume there are some nice i2c versions too smiley

– Let us know have you done this hack : ) We are also sourcing some IIC DAC, and we will use them in some other project.