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Milliohm Meter

MilliOhm Meter Display 0.025ohmThe Milliohm Meter Project

I built this mil­liohm meter sev­er­al months ago which is based on a project by Scullcom Hobby Electronics and pre­sent­ed on YouTube.

Louis at Scullcom Hobby Electronics has a new video updat­ing the mil­liohm meter, and fea­tures the board I pro­duced in this post.
NEW Link to updat­ed video on YouTube
NEW Version 1.5 board avail­able at OSH Park requires addi­tion­al 0.01 uF ceram­ic capac­i­tor, 2 pin head­er, and 250 mA PTC fuse.
NEW Updated project post with Milliohm Meter ver­sion 1.5 board.

It uses a 4 wire kelvin mea­sure­ment sys­tem that sup­plies a pre­cise 100 mA cur­rent to the device under test using one pair of leads, and then mea­sures the volt­age drop across the device under test using a sec­ond pair of leads.
As designed it can be used to mea­sure low resis­tances of under 2 ohms (Ω).   Some of the design changes I made to fit my needs were:

  • Change the num­ber of resis­tors need­ed in the cur­rent source cir­cuit
  • Use high­er pre­ci­sion resis­tors with a low tem­per­a­ture coef­fi­cient
  • Add sep­a­ra­tion and shield­ing between input, out­put, and sup­ply cir­cuits
  • Use low­er ESR and high­er val­ue capac­i­tors for the MAX680 pow­er sup­ply
  • Add addi­tion­al decou­pling capac­i­tors on the INA106, and MAX680 IC’s
  • Change the volt­age reg­u­la­tor from a TO220 pack­age to a SOT-223 pack­age

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