[hackspacebristol] Measuring low current

Jolyon Jenkins jolyonjenkins at googlemail.com
Thu Feb 22 09:35:00 UTC 2018


Thanks - any idea what sort of time, roughly? (I have to negotiate some
domestic arrangements.)

On 22 February 2018 at 08:52, Tom Gardner <tggzzz+hs at gmail.com> wrote:

> Yes, barring events (, dear boy, events).
>
>
> On 22 February 2018 at 08:22, Jolyon Jenkins <jolyonjenkins at googlemail.com
> > wrote:
>
>> ​Thanks - will you be in this evening? ​
>>
>> On 22 February 2018 at 01:01, Tom Gardner <tggzzz+hs at gmail.com> wrote:
>>
>>> Sorry; standard electronic TLAs :) FSD is full scale deflection, and UUT
>>> is unit under test,sometime DUT for device under test.
>>>
>>> I don't know enough about hall effect devices to measure current, but I
>>> have my doubts about sensitivity. But before I could be sure, I'd have to
>>> RTFM.
>>>
>>> Try putting a diode in parallel with the meter, select the appropriate
>>> meter range, hold the range, and measure a low current. Bring it along on
>>> Thursday evening, if you like, and we'll play with it.
>>>
>>>
>>> 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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>>>>> Hackspacebristol at lists.aktivix.org
>>>>> https://lists.aktivix.org/mailman/listinfo/hackspacebristol
>>>>>
>>>>>
>>>>
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