Recalibrating Array DC Electronic Load 3715A??

[marmelade]

Around two years ago I bought a DC Electronic Load from Array. Its the "basic" 3715A model, which I beleive is also branded and sold by Gossen Metrawatt.

Haven't used it very much until now. To be honest, I am quite disapointet with the accuracy/performance of the instrument. In CC-Mode, set at 0A, it sinks roughly 60mA. The voltage-readout seems reasonable accurate, but the current readout is absolutely off (i.e. showing 1.2A whilst sinking 0.5A).

Is anybody aware of a calibration/adjustment procedure for these instruments?? What can I do?

[bicycleguy]

Is that in the 3A or 30A Current range?  30A would be in spec, 3A out.

[marmelade]

I am currently not able to set a particular range. The manual is stating the existance of a low and high range but the UI does not show anything alike.

Just this evening I tried establishing a remote connection via RS-232 in order to set the range remotely  (yet unsuccessful). I am a bit confused, the manual mentiones DSR/DTR flow control but on the very same page only TX+RX+GND are connected....? Does the number of bits include or exclude the stop bits? Has anybody figured out the proper parameters yet?

A quick test with a DMM hooked up to the lab supply: (PSU-Voltage = 10V)

Current(DMM) [A] |  Setpoint(load) [A] | Readback(load) [A] | Readback(load) [V]
 0.060                   | 0.0                         | 0.357                       | 10.04
 0.160                   | 0.1                         | 0.457                       | 10.04
 0.362                   | 0.3                         | 0.658                       | 10.03
 0.563                   | 0.5                         | 0.860                       | 10.03
 1.065                   | 1.0                         | 1.363                       | 09.99
 2.070                   | 2.0                         | 2.370                       | 09.93
 2.570                   | 2.5                         | 2.873                       | 09.90
 2.977                   | 2.9                         | 2.363                       | 09.88
 3.077                   | 3.0                         | 3.060                       | 09.88
 3.580                   | 3.5                         | 3.560                       | 09.85
 3.980                   | 3.9                         | 3.960                       | 09.82

Above 3A, the initial offset remains, but the readback values seem ok. Below the deviation is obvious. Is there a possibility to manually adjust the current-sensing?

[marmelade]

Having built a small multi-output SMPS just recently, the topic pops up again.....

I totally dis-assembled the unit and started debugging. The unit is pretty well-made with state of the art an genuine components (IRFP460, STM32F105, OP07C), good quality FR4 pcbs and solid housing constructed around it. Only the Heatsinks look a bit cheapish to me - but they seem to be fit for the purpose. There is a full JTAG connector and a 24LC04B EEPROM. THe RS-232 interface is built araound a MAX202 and two 6N137 optos. And, unlike stated in the manual, there is a third opto connected to Pin9 of the SUB-D connector for remote ON/OFF (its actually mentioned in the chineese version of the manual).

Apart from the 371x Series loads, where you can find the calibration procedure on the website, the 3715A can not be calibrated. The calibration seems to be fully digital in the MCU.

I had a brief look at the analog circuitry.
*There is one diff-amp sensing the voltage across the main shunt with a transfer ratio of 1V/10A (0-3V, 0-30A current range). Its output is monitored by the STM32 and also fed into the analog current control loop which compares it to the setpoint voltage generated by the STM32 DAC.
* The output of the diffamp is amplified by a second OPA with gain of 10. This signal is only monitored by the STM32 (0-3V, 0-3A current range) but is obviously not used for set-point derivation nor the analog control loop.
* The current read-out and display ist particular bad in the 0-3A range because of a high dc-offset at the output of the second OPA.
* The 60mA constant offset from the setpoint is due to a non-zero setpoint-voltage for Iout=0. Since the load current is in their given datasheet limits above 3A it seems this offset is needed to achieve enough high-level accuracy. This way the low-level accuracy was sacrificed.

Initially this thing was bought for automated efficiency tests for SMPS circuits, but I was never able to establish a connection via the serial interface. No response with any of the settings given in the menu. Today I gave it another shot an wrote a python script the iterates through all possible combinations of serial interface settings, requesting an '*IDN?'. But no response at all! I have verified, that the UART message is present at the RX pin of the MCU (Rx=PA10, TX=PA11). Maybe this SCPI compatability is just a datasheet-joke? Has anybody ever used it?

If anybody is interested, I have an EEPROM dump.

I am finished with this unit. I'll keep it for manual testing and try to find something better for the automated application.

[marmelade]

A more complete trace out of the current sense schematic revealed no surprises. Actually the dc-offset of roughly 300mV at the output of the second OP-AMP  is intended operation of the design.

Most probably my unit somehow "lost" its calibration values, wich is the root cause for the absolute deviation of current-readout. The displayed values in mA match quite accurately with the output Voltage in mV of the second OPA (when probed with DMM).

I wish I could get a FW-update from ARRAY.

Attachments: LTSpice screenshot and EEPROM dump.

[robosilo]

I have a similar issue with my unit. I purchased it used and it was very off on the low end. I emailed their tech support and they sent me a document on how to calibrate. below is the procedure.

Array 3715A Electronic Load Calibration Methods
1.   Calibration Equipments
PC             1PC;
3311           1PC;
Power Cable     1PC;
Large Current Power Source NL08-30;
High-voltage Voltage Source ARRAY 3665A;
DC Electronic Load ARRAY 3721A;
Flat Head Screw Driver   1PC;
Fastening Adhesive Glue    1Bottle;
Standard Current Sampling Resistance    1mΩ;
Digital Multi-meter;

2.   Calibration Methods
A.   Current Regulation
Connect the Large Current Power Source NL08-30, the 3721A Electronic Load, the Standard Current Sampling Resistance and the 3715A in series.
Hold down the decimal point “.” on the keyboard, press the switch of the power supply and select the calibration current option.
a.   Input 0.5A in CC mode of the 3721A, read the current value on the voltage meter, input it into 3715A and confirm;
Input 2.5A in CC mode of the 3721A, read the current value on the voltage meter, input it into 3715A and confirm;
Input 25A in CC mode of the 3721A, read the current value on the voltage meter, input it into 3715A and confirm;
b.   Read the current value on the voltage meter directly, input it into 3715A and confirm; (This is the low point of the calibration current DA.)
Read the current value on the voltage meter directly again, input it into 3715A and confirm; (This is the high point of the calibration current DA.)

B.   Voltage Regulation
Connect the High-voltage Voltage Source ARRAY 3665A, the voltage meter and the 3715A in parallel.
c.   Regulate the voltage value of the voltage source to 0.3V, read the current value on the voltage meter, input it into 3715A and confirm;
Regulate the voltage value of the voltage source to 3.6V, read the current value on the voltage meter, input it into 3715A and confirm;
Regulate the voltage value of the voltage source to 4.5V, read the current value on the voltage meter, input it into 3715A and confirm;
Regulate the voltage value of the voltage source to 38V, read the current value on the voltage meter, input it into 3715A and confirm;
Regulate the voltage value of the voltage source to 42V, read the current value on the voltage meter, input it into 3715A and confirm;
Regulate the voltage value of the voltage source to 200V, read the current value on the voltage meter, input it into 3715A and confirm;
d.   Read the voltage value on the voltage meter directly, input it into 3715A and confirm; (This is the low point of the calibration voltage DA.)
Read the voltage value on the voltage meter directly again, input it into 3715A and confirm; (This is the high point of the calibration voltage DA.)

3.   Current Accuracy
0~2.999A should meet the requirements ±(0.2%±5mA); and 3~30.00A should meet the requirements ±(0.2%±30mA). Otherwise it is unqualified.

4.   Voltage Accuracy
0~3.999V should meet the requirements ±(0.2%±3mV); and 4~35.99V should meet the requirements ±(0.2%±30mV); 36.0~360.0V should meet the requirements ±(0.2%±300mV). Otherwise it is unqualified.