Yet another dummy load thread :D - Page 1
Forum section: electronics / projects

author: metalphreak
author_rank: Frequent Contributor
timestamp: March 02, 2013, 01:50:42 pm
content: So I've decided to build myself a dummy load, but I want to incorporate a bunch of different things I'm playing with at the moment. Basically I want to learn a bit more about using ADC/DACs, opamps, these OLED displays I've got, and so on. I wouldn't mind a bit of feedback on my proposed designI've got a box of Dell M915 copper heatsinks I plan on making use of:http://imgur.com/a/qlNlwThe idea being to have quite a compact footprint case, with a couple of temp controlled 40mm fans blowing air through as required. These heatsinks are meant to cool 130W+ multicore CPUs so should be up to the task. They will mount to the PCB with screws from the underside of the board (the thumbscrews shown actually have a threaded female on them rather than male).My dilemma comes with choosing which MOSFET to use and how to mount them to the heatsink. I was initially thinking DPAK/D2PAK style but the heat would have to be conducted through the package body rather than the tab. This is where International Rectifiers DirectFET products come inThe IRF8304 looks suitable, is less than $3 in a reasonable volume, and the entire body is metal to conduct heat to both a heatsink, and through the pcb. Datasheet:http://www.farnell.com/datasheets/1641133.pdfIt can dissipate 100W! if the case is kept at 25deg C. This obviously isn't going to happen, but the Junction to Case is only 1.2 C/W max. Assuming the heatsink is 0.3 C/W (no idea what this actually is), I can comfortably dissipate 50W without the junction going over 100C @ 25C room temp? Judging by the rest of the specs (inc Vgs(on)) it should be good. 3.5V at the gate allows over 100A.The rest of the circuit is just based around controlling the current. Plan is to have it controlled by some kind of PIC18-series - it's what I'm comfortable with using. I have some Microchip rail-rail opamps in dual package and quad package. Each mosfet will have its own current sense resistor and its own opamp feedback - no dodgy current sharing stuff here.Case will probably be a nice cheap multicomp one from Element-14 unless I find something nicer elsewhere. Display is a little 1" 128x64 OLED screen I have a few of:I was thinking of using a cheap PIC18F usb series chip to do USB/RS232 comms. Same price as an FTDI chip, and you can do a whole lot more custom stuff later. Basic use would be for a simple USB to serial bridge optoisolated for comms to the main controller. 5V from USB can also be used to power everything through an isolated DC/DC supply like Dave has in his little USB supply thing. Otherwise, separate battery - possibly powered from device under test (system power consumption would have to be accounted for then). Switch or auto-detection circuit to toggle between options (USB > Batt > DUT).My main concern though, is picking the right MOSFET to start withEither 2 or 4 of those IRF ones depending on what kind of power dissipation I can support with my heatsink. Design should be good up to 10A, 30V, and ~120-150W (TBD). Input voltage and current will be measured by the microcontroller. I2C/SPI eeprom for storing data - discharge logging and so on. Also monitoring heatsink temp with a sensor.

author: Spikee
author_rank: Frequent Contributor
timestamp: March 02, 2013, 03:02:24 pm
content: Use two IRFP064N each one their own 0.1 ohm resistor resistor and opamp to drive them.My schematic has been testend on 20V 10A (200W) for a couple of hours and it was stable at 38 deg C.In one of my video's you can see the 63A peak test and it survives it perfectly. If you want to improve it a bit youcould replace the LM324. To controll it i am designing a user interface using a 2.8 inch touch screen and STM32 micro.it has a 12Bit adc and Dac to measure the currents etc.My design:https://dl.dropbox.com/u/100819959/constant%20current%20load/Constant%20current%20EEVblog_Spikee.pdf