[hackspacebristol] Measuring low current

Jolyon Jenkins jolyonjenkins at googlemail.com
Thu Feb 22 08:22:46 UTC 2018

​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
> 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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