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NP25 Current Consumption / ucurrent
10-11-2015, 03:14 PM (This post was last modified: 10-11-2015 03:38 PM by TomC.)
Post: #1
NP25 Current Consumption / ucurrent
To those interested:

I've been 'using' one of the fine NP25 units from Jim Johnson.
[Thanks Jim Johnson / Chris Chung]

I've done some preliminary measurements using a 3V power supply and the uCurrent from Dave Jones.
[Thanks Dave Jones]

Current consumption is very close to 1.2mA - regardless of P/R mode or slow/fast mode.

Here are some of the modes:

1. 0.00 in display (wakeup).
2. Program running.
3. Full display (-8.88888888-88).

The ucurrent module makes these measurements most easy - including being able to view the transient nature of the current on an oscilloscope. I am using a ucurrent Gold which has bandwidth of 300kHz; it sufficiently shows much of the transient nature of the current draw. I will have to study the display/keyboard strobing to better understand the timing. I may work on a higher bandwidth current measurement too.

All for now,
TomC
From lovely/autumn Detroit, Michigan, USA, 3rd Stone.
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10-11-2015, 04:14 PM
Post: #2
RE: NP25 Current Consumption / ucurrent
(10-11-2015 03:14 PM)TomC Wrote:  To those interested:

I've been 'using' one of the fine NP25 units from Jim Johnson.
[Thanks Jim Johnson / Chris Chung]

I've done some preliminary measurements using a 3V power supply and the uCurrent from Dave Jones.
[Thanks Dave Jones]

Current consumption is very close to 1.2mA - regardless of P/R mode or slow/fast mode.

Here are some of the modes:

1. 0.00 in display (wakeup).
2. Program running.
3. Full display (-8.88888888-88).

The ucurrent module makes these measurements most easy - including being able to view the transient nature of the current on an oscilloscope. I am using a ucurrent Gold which has bandwidth of 300kHz; it sufficiently shows much of the transient nature of the current draw. I will have to study the display/keyboard strobing to better understand the timing. I may work on a higher bandwidth current measurement too.

All for now,
TomC
From lovely/autumn Detroit, Michigan, USA, 3rd Stone.


1.2mA seems far to low. The problem with the hardware design is that there is no propper current limiting for the LEDs. The limiting factor is usually the CR2032. But that results in significant battery drain.
With the power supply it is probably the shift register that limits current.
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10-11-2015, 05:25 PM (This post was last modified: 10-11-2015 05:29 PM by TomC.)
Post: #3
RE: NP25 Current Consumption / ucurrent
Hello Harald:

Thanks for the prompt reply.

Agreed, this number seems very small -- I have verified this value.

I will try to post full test schematics soon; I have a uCurrent Gold current sensor in series with the 3 volt power supply. The output of the uCurrent device is connected to a HP3468 voltmeter and a Tek465 oscilloscope. The ucurrent may limit the response to 300kHz, but I do not believe this will be an issue with this design. The scope has 100MHz bandwidth, so I believe these measurements to be 'sound'. On the scope, one can observe the max current draw of about 1.2 mA with much (depending on what is displayed) of the time dropping to about 1/2 that amount (i.e. 600uA).

The design shows some interesting characteristics in that a full display [-8.8888888-88] actually draws LESS average current than with a single '1' in the display! I assume this is due to how the display is strobed. One would have to study the code to understand what is really happening here.

Also, the HP 'bubble' displays luminosity is specified at 1.0mA.

TomC
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10-12-2015, 08:08 AM (This post was last modified: 10-12-2015 08:16 AM by Harald.)
Post: #4
RE: NP25 Current Consumption / ucurrent
(10-11-2015 05:25 PM)TomC Wrote:  Hello Harald:

Thanks for the prompt reply.

Agreed, this number seems very small -- I have verified this value.

I will try to post full test schematics soon; I have a uCurrent Gold current sensor in series with the 3 volt power supply. The output of the uCurrent device is connected to a HP3468 voltmeter and a Tek465 oscilloscope. The ucurrent may limit the response to 300kHz, but I do not believe this will be an issue with this design. The scope has 100MHz bandwidth, so I believe these measurements to be 'sound'. On the scope, one can observe the max current draw of about 1.2 mA with much (depending on what is displayed) of the time dropping to about 1/2 that amount (i.e. 600uA).

The design shows some interesting characteristics in that a full display [-8.8888888-88] actually draws LESS average current than with a single '1' in the display! I assume this is due to how the display is strobed. One would have to study the code to understand what is really happening here.

Also, the HP 'bubble' displays luminosity is specified at 1.0mA.

TomC

Hi Tom,

hmm, I don't quite understand why you would get those low measurements. Maybe the Hardware has changed?
I mesured on my NP25 just now and here is the screenshot:

[Image: lecroy14dvpjz.jpg]

As you can see at a constant 3V (measured on chanel 2) the peaks are just below 40mA and the average is 23mA. By the way, as the battery resistance isn't present, I lost one segment of the 8 leftmost digits. Probably due to the excessive current (not beeing limited by the battery).

I always wanted to do a redesign of the hardware - but as always, there are more projects than time to work on them Smile
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10-12-2015, 10:44 AM
Post: #5
RE: NP25 Current Consumption / ucurrent
Harald:

Thank you for posting.
Very curious. Even the character of your waveforms look very different than my measurements. I will retest later this week.

TomC
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10-12-2015, 07:46 PM (This post was last modified: 10-12-2015 07:47 PM by BobVA.)
Post: #6
RE: NP25 Current Consumption / ucurrent
(10-11-2015 03:14 PM)TomC Wrote:  Current consumption is very close to 1.2mA

That does seem odd.

Using the 2032 coin cell as a power source and a DVM I measured about 7.5mA with "0.00" on the display, up to a little over 12mA with all the digits lit. Since it's certainly missing the peaks your setup is catching, you'd think it would be lower, not higher than your measurments.

Maybe the scope, uCurrent, NP-25 and perhaps your power supply are having some odd interaction that's causing a false reading?
Would it be worth substituting a simple, resistive load on a battery for the NP-25 and see if that produces an expected result? Then you could start adding in your power supply and then NP-25 and see what happens?

Interesting!

Bob
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10-15-2015, 01:07 PM
Post: #7
RE: NP25 Current Consumption / ucurrent
(10-12-2015 10:44 AM)TomC Wrote:  Harald:

Thank you for posting.
Very curious. Even the character of your waveforms look very different than my measurements. I will retest later this week.

TomC

I thought about your test setup again. From what I gather the ĀµCurrent is basically a shunt with a precision amplifier. No galvanic isolation.
Could it be that it doesn't like driving the capacity of your entire setup against scope ground? Or even worse, is your power supply grounded or has a bypass capacitor?
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10-15-2015, 03:17 PM
Post: #8
RE: NP25 Current Consumption / ucurrent
Thank you all for your comments.

I did verify ground loops and the setup does not have this issue.

--- However ---

I have not yet collected new data, but have found the problem. I seem to have been mis-using the uCurrent in that I have been saturating it's output. As it is powered by a single CR2032, one has to be aware of its output swing limitations. (Oops - problems 'in the cockpit'!)

I hope to have more data within a couple weeks.

TomC
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