I have a metal cased device powered by 2x 1.5v AA batteries with a standby current draw of 19.7 microAmps. If I then cause a build up of static by shuffling across a synthetic floor and touch the metal casing, the current draw at least doubles to +50microAmps. I am unsure of the principles behind this effect.

What external factors (to the device) could cause increased current draw? Could an appliance in close proximity that charges capacitors to provide a burst of 10's of 1000's of volts which then discharges cause increased current draw?

Edit - 14:30 29/10/14 The board has a conformal coating and has passed EMC testing covering immunity for residential, commercial and light industrial environments.

65 identical devices were measured for current draw and all were consistently within a current draw range of 0.81uA.

Discharging a static charge by touching the metal casing cause a measurable increase in current draw. The larger the charge, the larger the current draw.

Measurement has been using a digital multimeter wired in series with the positive power feed.

Edit - 15:17 29/10/14 Some further information that has come to light - the device has a battery pack that is located within a metal cabinet (the device itself is on the outside of the cabinet). Also within the cabinet is a piece of electrical equipment that performs automatic self-tests. The battery pack has two wires running inside of the cabinet before going through a small hole to the external mounted device. From my research and suggestions below, it would appear that perhaps ESD or EOS is somehow in play. I do not pretend to understand this field and would appreciate any further answers regarding possible factors.

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    \$\begingroup\$ Downvote not particularly mature or helpful without a reason... if the question is not well worded, then a little assistance would be of more value. \$\endgroup\$
    – Emjx
    Oct 28, 2014 at 16:45
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    \$\begingroup\$ Can you provide more information about the circuit? Is everything inside the case? If so, how can you measure the current? If the circuit is single resistor I would be very surprised by the behavior you mention. If it contains a lot of high-impedance points (or worse: unconnected CMOS inputs) I would not be surprised at all. \$\endgroup\$ Oct 28, 2014 at 17:03
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    \$\begingroup\$ Are you sure your measurement equipment and setup is capable of measuring such small currents accurately? \$\endgroup\$
    – Phil Frost
    Oct 28, 2014 at 17:14
  • \$\begingroup\$ Also, is this 50 microamps only while you are shuffling? When does it return to the nominal 19.7 uA? \$\endgroup\$
    – Phil Frost
    Oct 28, 2014 at 17:15
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    \$\begingroup\$ 50 uA is pretty small, maybe it's just getting rid of the excess charge you put on the case. \$\endgroup\$ Oct 28, 2014 at 17:43

1 Answer 1


Static discharge could be causing one of the inputs to the processor (or even an input to another chip) to change state. Look for unterminated inputs, add strong pullups or pulldowns as appropriate, or just add capacitors to the unterminated inputs (something like 0.1uF). I have seen problems like this many times. I have seen units which would power on as a result of static discharge, and I have seen units where reset could be asserted due to static discharge.


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