[hackspacebristol] Measuring low current

Tom Gardner tggzzz+hs at gmail.com
Thu Feb 22 01:10:14 UTC 2018


Other things we could try are measuring a short current pulse with a scope,
and reducing that 500nA.


On 21 February 2018 at 23:24, Jolyon Jenkins <jolyonjenkins at googlemail.com>
wrote:

> Thanks. Yes, you characterise the problem accurately. To elaborate: if I
> have the Fluke (25) set to the mA range, all is fine in active mode, but
> when the device then goes into sleep mode, the resolution isn't good enough
> to know the current being drawn. If I then switch to the uA range, I'm not
> confident that the (mechanical) switch isn't bouncing, or momentarily
> disconnecting, and that I therefore have a true uA reading. It looks low
> (0.5uA) but I would like to be sure.
>
> If, on the other other hand I start with the Fluke set to the uA range,
> then two different things happen while the device is in non-sleep mode.
> Either the  Fluke shows a high but incorrect uA reading, and the device
> doesn't work at all; or it partially works and the Fluke shows 0uA. In
> either case it is as if the meter is being overwhelmed by the mA current
> (which I don't actually know but I think is around 1mA - I only "think"
> because it is too brief to measure.)
>
> I kind-of understand your answer but I'm sorry to say I don't know what
> UUT and FSD stand for. Assuming the UUT to be the device I'm powering, then
> yes it can tolerate a 0.9V drop.
>
> A little while ago I was in the avionics lab at Bristol university for
> other purposes and we were measuring current with a hall effect device that
> went around the wire rather having to be in the circuit itself. Is such a
> method likely to be sensitive enough for these low currents?
>
> On 21 February 2018 at 22:48, Tom Gardner <tggzzz+hs at gmail.com> wrote:
>
>> On 21 February 2018 at 20:44, Jolyon Jenkins <
>> jolyonjenkins at googlemail.com> wrote:
>>
>>> ​Is there any equipment in the Hackspace that can reliably measure very
>>> low current (<1uA but probably >100nA)? I don't trust my Fluke to do it
>>> reliably or not to alter the behaviour of the device being measured.
>>> I'm trying to measure the current draw of my device while in sleep mode,
>>> but when the fluke is switched to the uA range it can't cope with the
>>> higher currents that precede sleep mode.
>>>
>>
>>
>> In what way "can't it cope"?
>>
>> My *guess* is that you are asking for the impossible - but there is a
>> workaround.
>>
>> I'll assume you want to measure 1uA, but at other times the UUT will draw
>> 100mA. You want to measure the 1uA but not the 100mA.
>>
>> Current measurements are done by inserting a resistor in series with the
>> UUT, and measuring the voltage drop across it - the so-called "burden
>> voltage". The FSD voltage is usually fixed, and hence a lower current
>> requires a higher resistance. If the UUT flips to a much higher current
>> then there will be the concomitant much higher voltage drop across the
>> resistor - and the *UUT* might not be able to cope with that extra drop.
>> The meter will be fine, except that the display shows "OL".
>>
>> The Fluke 25 has a burden voltage of 0.5mV/uA up to 320uA, which
>> translates to 500ohms and a FSD voltage drop of 160mV at 320uA. But at
>> 100mA the voltage drop would be 50V, which the UUT probably can't tolerate.
>>
>> One dodge is to make the meter have a non-linear resistance by strapping
>> a conducting diode in parallel with the meter terminals. The objective is
>> to get the diode to be almost non-conducting when making the measurements,
>> but to conduct when the UUT is taking a high current.
>>
>> So, supposing you are trying to measure 1uA through a 500ohm resistor.
>> How much current would go through a parallel diode? According to Horowitz
>> and Hill p294 (available in the 'space) a jellybean 1N914/1N4148 at low
>> voltages behaves like a 10Mohm resistor. Since that is >>500ohms,
>> negligible current should go through the diode. Alternatively at 160mV it
>> indicates around <100nA would go through the diode, which would also be
>> negligible.
>>
>> When the UUT takes 100mA, the data sheet indicates the voltage drop
>> across the diode will be about 0.9V. Can the UUT tolerate that voltage
>> drop? If not the you will have to use a beefier diode, which will also be
>> more "leaky" at low voltages.
>>
>> Alternatively, I have a Datron 1061 with a 1nA resolution but a 1kohm
>> resistor. You could borrow that for a few days.
>>
>> I also have a Solartron 7081 which doesn't measure current, but does
>> measure volts with a 0.1uV resolution. You could supply your own resistor,
>> and make measurements while at the 'space.
>>
>>
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>>
>
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