A question about a commercial 15W electronic load - Page 1
Forum section: electronics / beginners

author: John_doe
author_rank: Regular Contributor
timestamp: September 21, 2022, 03:37:03 pm
content: Hello everyone.In the following link you can find schematics I did a couple of years back for a 15W USB electronic load designed by Ruideng:https://oshwlab.com/eeda5ux455/ruideng-15w-electronic-loadI hope it is accurate and there are no errors.Also, I just found out that there was another user of this forum that made a different schematic for the same product a couple of months after me:https://www.eevblog.com/forum/reviews/circuit-diagram-for-amazonebayaliex-15w-usb-load/Here is his schematic (ignore the blue block, it is a modification):https://www.eevblog.com/forum/reviews/circuit-diagram-for-amazonebayaliex-15w-usb-load/?action=dlattach;attach=1164510;image;PHPSESSID=d2jspm85a12m1mf3bqc4rfilu2Anyway, my question:This load has a thermistor that turns on a fan when the temperature reaches a certain value. The thing is, that the total current draw of the entire circuit remains the same even with the fan on, meaning that if the fan draws, lets say150mA, then now the TIP122 will draw 150mA less.How do they achieve this? What part of the circuit causes this compensation?

author: strawberry
author_rank: Super Contributor
timestamp: September 21, 2022, 04:16:56 pm
content: thin wires or current shunt PCB layout error

author: magic
author_rank: Super Contributor
timestamp: September 21, 2022, 04:31:58 pm
content: I assume that the unit runs from USB and has no other power supplies.One simple way to achieve correct current regulation is to send the fan's ground current through the shunt resistor, together with the darlington's current. On your schematic, Q0 source would be connected to RSHUNT rather than directly to GND.

author: John_doe
author_rank: Regular Contributor
timestamp: September 21, 2022, 07:41:44 pm
content: Quote from: magic on September 21, 2022, 04:31:58 pmYes!! You are absolutely correct! Such a simple solution but I couldn't think about it myself. Both me and the author of the other schematic made the same mistake... This is no coincidence, even now I can't find the darn trace that connects the source of the A2SHB to the darlington's emitter, it is well hidden.Before, I checked for continuity but my DMM can't measure the shunt's minuscule resistance, so now, armed with your suggestion I turned the load on, put a big current on it and measured the potential differences and sure enough, confirmed it easily.Many thanks.

author: magic
author_rank: Super Contributor
timestamp: September 21, 2022, 08:19:43 pm
content: Such details are easy to miss when tracing a circuit with DMM continuity test.It's possible that more things are also connected there. I looked at the photo, there appears to be a via near the "5" on the R025 resistor. It's possible that it goes to GND pins of the LDOs on the top side, and those are connected through another via to Q0.

author: John_doe
author_rank: Regular Contributor
timestamp: September 22, 2022, 08:33:23 pm
content: Thanks again, I actually tested every component on the board after I wrote the previous reply, and although it's not through the via you described, both regulators' ground pins are in fact connected to the shunt and not to the USB ground. other than that I couldn't find any other errors in the schematic.