Author Topic: EEV μCurrent for Christmas  (Read 523 times)

Offline Joules

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EEV μCurrent for Christmas
« on: January 26, 2018, 12:37:29 PM »
I got a belated Christmas pressie from a friend last week.  He bought me a Dave Jones (EEV Blog) μCurrent kit.   Dave designed and produces these to allow normal multimeters and oscilloscopes to be able to measure ridiculously small currents accurately.

https://www.eevblog.com/projects/ucurrent/

This allows you to really see what is happening in very low power systems by monitoring the current without killing the circuit with a burden load from the measuring equipment.  I needed something to have a play with, so pulled an old battery watch out of the draw that still worked.

The setup is a bit errrr rough, as the bits of wire and clips do introduce some resistance, however I was wanting to see the current pulse taken by the watch each tick than get an accurate measurement of current.

I got what I wanted in the One Tick image below.  The pulse was too fast for the multimeter to catch so used a USB scope to catch the signal.  The pulse is just under 1mA for about 2ms, I haven't done that maths as to what that works out over the life of a battery, 3yrs ?

As the pulse was so fast I needed to measure its width, and this was when I got the surprise, the pulse I first saw was actually 10 pulses with varying height and width.  At first I thought is it a 10bit code or some sort of PWM, after capturing a few more they are all the same.  It seems to follow a 3/4 sine wave pattern.   Having done a little research it seems watches use stepper motors, but I would have expected a steady stream of pulses and smooth sweep of the second hand.   To me the shape of the pulses got my attention, why do they have this particular shape and its consistent.   The only thing I haven't tried is adding resistance to the battery to lower its voltage and see if the PW changes.

The μCurrent is an interesting addition to my electronics test equipment, sort of like a microscope for current allowing you to see things you hadn't been able to before.

I would be very interested in learning more about this pulse stream if anyone has further knowledge.

Just get doing and make swarf, you can decide what its going to be later.   :thumbup:

Offline Joules

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Re: EEV μCurrent for Christmas
« Reply #1 on: January 26, 2018, 01:10:44 PM »
An interesting thought occured.  I see the current pulses as all being positive, that doesn't mean the coil driving the gear does.  The little stepper inside only has a NS pole magnet, so as it spins around, unless inertia pulls it past the stator position it would need a negative kick to pull it back round.   I see no mechanism to limit the direction of movement, so my thinking...  Do the ten pulses actually equate to five rotations of the tiny pinion, I am still puzzled by the ten pulses though.
Just get doing and make swarf, you can decide what its going to be later.   :thumbup:

eskoilola

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Re: EEV μCurrent for Christmas
« Reply #2 on: January 27, 2018, 02:44:06 AM »
If the shape of the pulses is reproduced accurately then that shape is exactly like charging an inductor. Might there be a booster system, charging a capacitor and then discharging the capacitor, now at larger voltage than the battey,  into the actual stepper motor ?

Offline Joules

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Re: EEV μCurrent for Christmas
« Reply #3 on: January 27, 2018, 05:08:13 AM »
Thanks eskoilola, hadnít even considered that, though nothing obviously capacitor inside the watch, so it must be tiny.  Likewise the back emf diode must be all on the ic.
Just get doing and make swarf, you can decide what its going to be later.   :thumbup: