Kerry Wong did a teardown of an old HP 6181C DC current source:
HP 6181C is a lab grade 0 to 250 mA current source manufactured back in the late 70′s and early 80′s. There does not seem to be any pictures on the Internet for this particular model so I thought I would do a teardown here.
Before doing the teardown, let’s take a look at some of its key features. HP 6181C has three output current ranges: 0-2.5 mA, 0-25 mA and 0-250 mA. The output voltage limit can be set from around 0.5 V to all the way to 100 V. The output current can be adjusted via a 10-turn potentiometer and has a set resolution of 0.03% (under 1µA in the 0-0.25 mA range). The output current stability is rated at around 100 ppm, while not top-notch by today’s industry standard but remains quite impressive for hobby use.