Linear MOSFET Electronic DC Load — Part 2

Dual Electronic Load  assembly view from top
Dual Electronic Load assem­bly view from top

Finally assem­bling the elec­tron­ic load mod­ules for some basic test­ing of min­i­mum volt­age at dif­fer­ent cur­rent lev­els, and heatsink ther­mal capacity.

Before design­ing the micro­proces­sor con­trolled ana­log board, I want­ed to test the capa­bil­i­ties with the mod­ules mount­ed in an enclo­sure, under semi-real world con­di­tions.
I used a vari­ant of my orig­i­nal ana­log test board that set the cur­rent lev­el using a poten­tiome­ter for each chan­nel. It is capa­ble of dri­ving the L2 Linear MOSFET’s at up to 10 volts

Dual Channel Analog DC Load control board top view
Dual Channel Analog DC Load con­trol board V 1.4 top view

The 60mm fan has it’s own ded­i­cat­ed 12 volt pow­er sup­ply and can pull up to 1.4 amps. The fan is a Delta Electronics PFC0612DE-F00 and is capa­ble of 67 CFM (1.2 M3/Min) of air­flow at zero sta­t­ic pres­sure. It’s speed can be con­trolled by a PWM sig­nal from a start­ing speed of 30% to 100%. It is a real scream­er at 100%.
The con­trol board has its own sep­a­rate 12 volt pow­er supply.

Dual Electronic Load modules and fan
Dual Electronic Load mod­ules and fan

I am using two 15 AWG wires per chan­nel to reduce the volt­age drop from the input ter­mi­nals to the board. Ribbon cables con­nect the mod­ules to the con­trol board.

Dual Electronic Load front panel components
Dual Electronic Load front pan­el components

So far every­thing is work­ing as expect­ed. I may end up increas­ing the cur­rent sen­sor gain from 50 to 100, to get some addi­tion­al cur­rent capa­bil­i­ty from the mod­ules at low­er volt­ages and con­trol the max­i­mum wattage in software.

DC Load enclosure with drilled sides
Dual Electronic Load enclo­sure with drilled sides 125W testing

By strate­gi­cal­ly plac­ing the inlet air holes I was able to achieve some addi­tion­al cool­ing of the mod­ule heat-sinks and MOSFET hold down clamps. The Wakefield-Vette heat-sinks have addi­tion­al fins on the com­po­nent side, and by direct­ing incom­ing air direct­ly at these fins there is a sig­nif­i­cant increase in mod­ule cool­ing vs using the fan alone in my ini­tial test­ing. The rear MOSFET in each mod­ule still runs hot­ter by around 5 degrees F under heavy loads though.

As it sits, the Dual-chan­nel L2 MOSFET DC-load works well for gen­er­al load test­ing. I still would like to add a graph­i­cal LCD dis­play and micro­proces­sor con­trol, but cur­rent­ly that is not a pri­or­i­ty.

Eagle CAD 7.7 board and schemat­ic files for L2 MOSFET board

6 Replies to “Linear MOSFET Electronic DC Load — Part 2”

  1. Hi there any chance you can sup­ply the schemat­ic of the MOSFET board and dri­ver board ? I’ve been look­ing around to build an E Load for a while and your design looks good .….

    thank you


    1. Hi Graham,
      I have added the Eagle CAD board and schemat­ic files for the Dual L2 MOSFET board to the bot­tom of the DC Load — Part 2 post.
      The dri­ver board was devel­oped for my test­ing pur­pos­es, and will be incor­po­rat­ed into a fin­ished project, so it is not avail­able for release yet.

      Greg (Barbouri)

  2. Hi Greg,

    I am look­ing at your board and schemat­ic. I can­not find any R5, R6 and R7 on the board as well as C3, C4 and C5. Do you have a new­er board or .…?


    1. Hi Alex,
      Those 6 com­po­nents are clus­tered most­ly around the MAX44284 IC and just above the cur­rent sense resistor.

      Greg (Barbouri)

      1. OK, I see. I do not have these com­po­nents on my brd file. I have just down­loaded a fresh copy of your file and now I can see them. Thank you for point­ing it out. 

        You are dri­ving it with 0–10V con­trol board based on MAX6225 (2.5V) volt­age ref­er­ence (2ppm/C tem­p­co)? How do you find it drift­ing so far/influencing your DC load ? My per­son­al expe­ri­ence so far with mul­ti­ple Vrefs with­out going into LTZ1000/LM399 ultra high pre­ci­sion ter­ri­to­ry was that the low­est low long term drift was LTC6655 in LS8 pack­age (her­mit­i­cal­ly sealed). 


        1. Hi Alex,
          I did­n’t see any appre­cia­ble drift, and only used the MAX6225 vref, because I had some excess stock.
          I was only shoot­ing for a sta­bil­i­ty of 1 mA on the ana­log con­trol board for test­ing pur­pos­es, before mov­ing the design to full dig­i­tal control.

          Greg (Barbouri)

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