Accurate and Stable low amps measurement

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Re: Accurate and Stable low amps measurement

Postby hexibot43 » Tue Dec 18, 2012 4:16 pm

Can I ask - How are you guys incorporating this into the use of a picoscope? Are you using buffer triggers, and saving the buffer? How are you managing a recording over the course of a couple hours to a couple days? I have just started learning all the methods of triggering this scope, which there are many. Is it possible to make a simple time based trigger that takes a snapshot once every say 5 seconds. Not a complete waveform, but just a point. Add the data to a file the would become a waveform / data log.

How are you guys doing this?

Thanks,

Mab
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Re: Accurate and Stable low amps measurement

Postby Alan » Tue Dec 18, 2012 4:42 pm

I would not bother with a trigger, a timebase of 1000s/div gives about 3 hours across the screen. Set the memory to 1MS which will give you 100 readings per second.

If you need to measure over longer time periods then it will just split it across multiple waveforms in the buffer. If you are paranoid about the PC crashing / a power cut etc and loosing the data then use the alarm option from the tools menu - select event as capture (ie a screen full of data) and the action as save to disk. A timebase of 500s/div will then give you one file saved to disk every 90 mins or so. The next day you will have a dozen or so files to review - just load the first one then press Pg Dn and it will load each file in the directory in sequence.

We have been asked to add some longer timebases so that you can capture a 24 hour test on one screen - thats on its way in the next release, but I would still use the above if collecting for 24 hours to avoid any risk of data loss.
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Re: Accurate and Stable low amps measurement

Postby hexibot43 » Tue Dec 18, 2012 8:31 pm

Alan,

If I'm calculating this right with that time base you would take a sample every 16 2/3 minutes and as you've recording over 3 hours. I was thinking of more like sampling at least once every minute to give enough resolution to get a good picture of the length of drains. I've seen a lot of these where they come on for 2 or 3 minutes draw 3 amps and then shut right back down. It will do this repeatedly all night long. Computer turns on, and then realizes nothing is going on so shuts back down. So I'd probably have 30 times as many points, and only 1/10th of an hour on the screen.
So with each filled buffer you'd save it, and then be able to quickly move through all of it as if it were basically one file? One buffer at a time. That will work. I've got a car in the shop right now that has a problem that seems to take 2 days to show up. I'll give it a try and see how much data I create.
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Re: Accurate and Stable low amps measurement

Postby Alan » Tue Dec 18, 2012 8:39 pm

Let me check my maths again.

1000s/div * 10 divisions = 10,000 seconds (2.77 hours)

1MS (1 million samples) across 10,000 seconds = 100 samples per second

That should be more than enough for battery draw.

If your vehicle takes 2 days to play up then thats going to be 17 or 18 screen-fulls of data with the above settings.
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Re: Accurate and Stable low amps measurement

Postby hexibot43 » Tue Dec 18, 2012 10:20 pm

Alan,
Thank you for correcting me. I am still not use to how this works. Can you please explain it a bit further for me.

1000s / div means 1000 seconds per division ( I would think this to mean one sample every 1000s - where I went wrong)

10 divisions I am guessing this is how many divisions on the screen?

1MS Is what we set the buffer too....hence one million samples over the period of one screen.

I'm still fighting the very reason I bought a PC based DSO. My head is still thinking like I'm working with my very old Tektronix scope.

I'm going to put this to the test over the next couple days.
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Re: Accurate and Stable low amps measurement

Postby FioranoCars » Wed Dec 19, 2012 12:23 am

I guess the resistor thing is what VW did, calculating the resistance of the fuses (i guess there must be some manufacturing tolerances and variance?)
from the tables using a 7.5amp mini fuse (for easy of maths):
A 1mv drop across the fuse is quoted as equivalent to 100ma drain, so that must mean the fuse has a resistance of 100ohms? (or is it 0.01 ohms?)

I will go and have a play!

thanks
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Re: Accurate and Stable low amps measurement

Postby Alan » Wed Dec 19, 2012 6:35 am

hexibot43 wrote:Alan,
Thank you for correcting me. I am still not use to how this works. Can you please explain it a bit further for me.

1000s / div means 1000 seconds per division ( I would think this to mean one sample every 1000s - where I went wrong)

10 divisions I am guessing this is how many divisions on the screen?

1MS Is what we set the buffer too....hence one million samples over the period of one screen.

I'm still fighting the very reason I bought a PC based DSO. My head is still thinking like I'm working with my very old Tektronix scope.

I'm going to put this to the test over the next couple days.


Yes, you have pretty much got it there.
1000s/div is 1000 seconds per division of the screen. There are 10 divisions so 10,000 seconds.

Setting the memory to 1MS means the scope will try to collect 1 million samples across the 10,000 seconds.

1,000,000 samples across 10,000 seconds is 100 readings per second. If you upped the memory to say 10MS then the scope will take 1000 readings per second. No real downside to doing this other than when you save the file to disk it will be bigger than required.

If you right click in the graph and select view properties you can see the actual no of samples per second, memory used and other "under the hood" stuff.
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Re: Accurate and Stable low amps measurement

Postby KimAndersen » Wed Dec 19, 2012 11:35 am

I did some intensive search on the internet regarding finding the specifications on standard/Mini fuses. What i was interested in - were the resistance through the fuse and i found a document from LittelFuse which is a manufactor of automotive fuses.

Based on this document and in conjunction with the document from Volkswagen about voltage drop across a fuse - can i clearly see, that there is a standard.

Here is a example from Littelfuse.

Mini Fuse 7.5 AMP = Nominal cold resistance Ohms ( 0.011 ohm ) and here the same specification from the document from Volkswagen - where a Mini Fuse 7.5 AMP with a voltage drop on 0.1 millivolt across the fuse equals 10 milliamp.

Then some math calculations: 0.1 millivolt divided by 10 milliamp equals 0.01 ohm.

I would say - it the same result i get - but judge for yourself.

For further study - read this document.

Standard_Mini_Fuse_Resistance.pdf
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Last edited by KimAndersen on Thu Dec 20, 2012 9:46 am, edited 2 times in total.
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Re: Accurate and Stable low amps measurement

Postby FioranoCars » Wed Dec 19, 2012 5:09 pm

I've enclosed a sample file using the amp clamp method, if doing longer readings we would use a 30minute screen (200sec/div) and sample of 1k/sec and let it fill as many screens as it runs for. Then we have the detail to zoom in on when needed. Our basic 8minute screen uses a trigger so that locking the doors triggers the recording, and this allows overlay analysis of other examples of the same model to see the shut down sequence, some cars can take 5+minutes to fully shut down and have up to 8 distinct steps of reduced current.

Over a weekend this would leave us with 50 screens of data, but that's minutes to examine. Nothing worse than getting a result but not having the detail to work out what it was.

We often use the other channels to monitor suspect circuits and/or battery voltage.

I'll try and run some fuse resistance tests in parallel with the clamp and see what we get, it will be interesting to see!

I'm hoping this will eliminate the need to an expensive clamp!

Parasitic drain 8mins.psdata
Sample parasitic drain recording
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battery discharge.jpg
Battery discharge


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Re: Accurate and Stable low amps measurement

Postby Avdr » Wed Dec 19, 2012 8:50 pm

dealing with the option of using the voltage drop across a fuse, aren't the numbers all getting a bit small?
I am asking here, not telling
But for example:

A fuse resistance of 0.011 ohm, surely that would be easily altered, by temperature, the coating used on the fuse blades, even grease deposited from the fingers? You would literally have to measure the fuse in question first, to be sure it was true to spec.
But then you come down to a voltage drop of 100micro volts, would that not get lost in noise in the signal?

I've recently been having a fault with the vectra, which I had been living with for some time already. If I didn't use it for say 10 days, the battery would be virtually flat (70Ah Varta - bought new - less than 12 months old). Even after say 5 days it was quite affected, locks would be slow when unlocking, engine would turn over slower than usual, and a fault with the HID lights would come up on dash - I have found them to be very voltage sensitive.

Anyway, I managed to resolve the drain down to a permanent drain of approx 180mA, of which 25mA was attributable to the various systems requiring memory - ECU, radio, instruments & clock etc. - and to the alarm.
All acheived with the 60amp clamp on 20A setting.
The remainder turned out to be the power windows, amazingly they require all that current just to remember the glass stop positions.
The point being anyway, that in this case the 60A clamp was a success. By making sure to zero it in the same plane, as that in which the measurment was to be taken (an invaluable piece of info I have read on this forum) I had good results.
With relays added into the window power feeds (from fusebox) the car can now be left for weeks on end, and no longer do I have problems with HID warnings.

EDIT: Forgot to mention there, that while watching the waveforms it was easy to spot the increased drain everytime the alarm LED flicked on, as it does every second. ie. On for a second, off for a second. It's a tiny LED. Will have to post the waveform up.
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