Programmable DC Electronic Load - First Look

RE: Programmable DC Electronic Load - First Look

fmilburn — Sat, 31 Aug 2019 17:18:25 GMT

There is an interesting discussion on Hackaday about how Arduino code for reading ADC is incorrect for the SAMD21 which is the microcontroller used in the MKR1000. Basically it attempts to read the ADC before it is finished and ready. This may be part of the problem encountered above but I have not investigated it yet. The Feather M4 uses the SAMD51 and I haven't looked into whether it suffers from the same problem.

RE: Programmable DC Electronic Load - First Look

three-phase — Tue, 30 Jul 2019 18:40:29 GMT

Some good progress being made, thanks for keeping us updated.

Kind regards.

RE: Programmable DC Electronic Load - First Look

fmilburn — Tue, 30 Jul 2019 03:59:28 GMT

I have made a little progress...

Received these parts which more or less match the 2nd schematic drawn above:

Bourns PWR220T-35-1R00FBourns PWR220T-35-1R00F TO-220 style 1% 1 ohm resistors
IRLZ34NPBFIRLZ34NPBF Infineon N-Ch MOSFET
MCP4725A0T-E/CHMCP4725A0T-E/CH Microchip 12 Bit DAC
OPA192IDBVROPA192IDBVR Texas Instruments Op Amp
10C035410C0354 Bourns 2 Ch Encoder

I also recently purchased an Adafruit Feather M4 Express which has the ATSAMD51J19 microcontroller with 12 bit ADCs and 12 BIT DACs so I did some experimentation with that. The DAC was programmed in the Arduino IDE to output various voltages and then measured with via ADC (2 pins away) and a digital multimeter. The first ADC reading was thrown out and then the next 5 averaged. The following plot shows the deviation of the programmed DAC output and ADC readings from the meter.[View:/resized-image/__size/620x368/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-faa45e32-5181-4034-ae38-500f2f17752c/8836.contentimage_5F00_204157.jpg:620:368]

The DAC output in blue is very linear and could be easily corrected to a large extent in software. There is a lot of scatter in the orange ADC readings. One ADC reading at around 1800 mV is in error by approximately 35 mV which is outside what the datasheet gives as maximum error - not sure what caused that. I seem to remember the MSP432 giving much better results and had a look at the TI datasheet. It reports error differently but does seem to be both more accurate and precise. It might be interesting to do a real world comparison to the Atmel chip. It is a shame there isn't a MSP432 variant with a DAC.

The following plot shows the percent error for the ADC and DAC on the Feather M4 Express. The title is a bit misleading - the X axis is the meter reading in mV and the Y axis is the percent error for the DAC in blue and the ADC in orange.

[View:/resized-image/__size/620x367/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-faa45e32-5181-4034-ae38-500f2f17752c/7331.contentimage_5F00_204158.jpg:620:367]

Percent error in the DAC would improve if the software correction described above was implemented.

In summary, DAC output seems good on the Atmel SAMD boards so far. The ADC is probably only good for 20 mV or so. This may be good enough for reading the input voltage of the DUT but not good enough for reading current. For that, it would be better to just report the DAC setting.

The Bourns 1 ohm resistors were measured with the milliohm meter and are within 0.1% of 1 ohm.

RE: Programmable DC Electronic Load - First Look

fmilburn — Fri, 14 Jun 2019 20:38:28 GMT

Thanks Donald

RE: Programmable DC Electronic Load - First Look

three-phase — Fri, 14 Jun 2019 19:29:51 GMT

An excellent start to your project Frank. Look forward to the updates as they come.

Kind regards.

RE: Programmable DC Electronic Load - First Look

Andrew J — Fri, 14 Jun 2019 08:48:17 GMT

I shall follow this with interest. A DC Load is going to be my next project.

RE: Programmable DC Electronic Load - First Look

michaelkellett — Fri, 14 Jun 2019 08:04:13 GMT

I expect that with some settings and loads your current sink will oscillate.

You'll be able to mitigate this with the following changes:

about 47R between op amp output and MOSFET gate

4k7 MOSFET source to op amp -ve

100pF op amp output to op amp -ve

These are starting values - if you have a spice model for the MOSFET you can tune by simulation.

Sometimes a resistor in series with the feedback cap helps..

Otherwise you can tune with a scope while applying pulses to the op amp +ve input, either with a

signal generator or software to the DAC.