Computing stuff tied to the physical world

Reverse diode current

In Hardware on May 27, 2012 at 00:01

After measuring the forward voltage drop over a diode, I should also have measured the reverse leakage current, i.e. how much current the diode lets through when it’s supposed to be blocking. I never did until now, because I couldn’t detect any current in a quick check I did a while back. Time to build a better setup – here’s what I used:

JC s Grid page 17

The voltmeter’s own 10 MΩ or so internal resistance will skew the readings by 10%, but that’s no big deal.

It turns out that the reverse leakage current is pretty small when applying 5V:

  • 1N4004 – a high power diode: 1.3 mV = 1.3 nA
  • 1N4148 – a low power diode: 3.4 mV = 3.4 nA
  • BAT34 – a Schottky diode: 50 mV = 50 nA

That’s nanoamps, i.e. milli-milli-milli-amps. The Schottky diode does indeed leak a tad more than the others. Here are the specs of that BAT34 diode – note that the reverse current could even be used as temperature sensor!

Screen Shot 2012 05 14 at 17 15 55

FWIW, I found a minuscule “RB751S” SMD Schottky diode, about 1 mm long, which does a bit better at 7.0 nA:

DSC 3221

It was quite a challenge to get some wires soldered onto it. I used the core of 30 AWG Kynar “wirewrap” wire:

DSC 3222

Anyway – the BAT34 is good enough: 50 nA leakage is acceptable while dealing with circuits which consume µA’s.

  1. What kind of camera or setup do you use to so AMAZINGLY illustrate this FANTASTIC journal?

  2. Heh – very simple: a Nikon D40 plus an illuminated magnifying glass, shown at the end of this weblog post. Just held the object under the magnifier with one hand and took the shots with the camera in the other.

    Pictures are adjusted and cropped in iPhoto.

    The schematic was drawn on an iPad, using a free app called Bamboo Paper.

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