Accurate and Stable low amps measurement

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Re: Accurate and Stable low amps measurement

Postby Alan » Thu Dec 20, 2012 5:45 am

Richard - I hope you dont mind but I added a screengrab of your waveform. You can also see the current spikes every 2 sec or so which I assume is an alarm LED flashing.

AVDR - 100uV is pretty small to measure with the scope but you might be OK if you use the 50mV range and use the resolution enhancement to 16 bits (in the channel drop down menu). On the other hand if you are probing around the fuse box to find the offending circuit then a meter will do as well as a scope?
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Re: Accurate and Stable low amps measurement

Postby Avdr » Thu Dec 20, 2012 7:53 am

I couldn't open Richard's .psdata file, I was getting an error stating I needed a newer version of picoscope software. This was true, but even after I upgraded to the current release it still wouldn't open.
Thanks for the screenshot Alan, I can now see the data.

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Re: Accurate and Stable low amps measurement

Postby FioranoCars » Thu Dec 20, 2012 3:45 pm

Sorry the data is saved using the latest beta, should not affect opening it normally, but sometimes it can ... you can install the beta as well as the release software and run them side by side, they do not conflict with each other, so you can always download the beta if you want to take a close look at the data.

The 2 second spikes are the LED for the alarm. This was a sample of a good waveform, with circa 40ma drain which is what we would expect for this car. We usually only do a 8minute run unless there is a known issue that does not show within the 8 minutes, then we go for 30minutes, then hours ... until we find something!

The use of the scope rather than a meter is to establish that a drain exists, then if it is really hard to find to trace separate circuits to narrow the search (ie remove the fuses and relays from half the components to eliminate them) although 80% of the time the drains come from components not using the fuse boards, ie the tracker or the alternator!

But when it is a permanent drain, visible straight away (ie the current never drops and stays at a sensible level - it remain too high), then a multimeter is quickest, it's just that 20%+ that we find are not so simple.

The fuse method of testing these simple faults, as per the VW article seems a much better approach than the pulling of fuses, for all the reasons they mention in the article, we would probably still have the pico recording total battery drain when using the multimeter, so we know the drain is still present at the point of the DMM test, would be easy to get lost during testing if the drain disappeared!

We will try some tests, simulated with and without a "forced fault", to see how the fuse method goes in parallel with a clamp ... will post results when done!

Richard
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Re: Accurate and Stable low amps measurement

Postby hexibot43 » Thu Dec 20, 2012 4:54 pm

I'm really enjoying this thread. I have been continually looking for a better solution to dealing with Parasitic draw. Thought you all might be interested in this diy project. And now that I have a better understanding of the way the PicoScope works - I would like to see hardware to go with it. I used it last night and it worked great to performed the recording. It was very easy to go through the screens recorded, and zoom in as necessary. And as stated you have three other channels available as needed.

http://hackaday.com/2009/12/16/automotive-current-monitor/

http://allegromicro.com/en/Products/Current-Sensor-ICs/Zero-To-Fifty-Amp-Integrated-Conductor-Sensor-ICs.aspx/

They have quite a few choices. Looking into it now. It will be easy to create a very output from - just need a input in the range we are looking for that with enough resolution.

I've been looking at the chips made by Allegro to see if there is something that fits this need. I will try and make something that will supply a varying voltage in relation to current draw like our Inductive clamps but be based on a shunt (series ) type Ammeter. I'm thinking something that could read up to 20 amps with enough resolution to be useful for parasitic draw.

Something like the "lightening bug" ammeter with a range better suited to our needs. Or like the other mentioned that would in the +/- 80 ma range. But have a range of 20 amps.
Last edited by hexibot43 on Thu Dec 20, 2012 6:12 pm, edited 2 times in total.
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Re: Accurate and Stable low amps measurement

Postby FioranoCars » Thu Dec 20, 2012 5:44 pm

20 amps is fine for this sort of work, although if as we do, using the door locking cycle as a trigger to ensure all readings are based on the same time line, then beware as some of these can exceed 20amp during the locking phase!

Most drains rarely exceed 1-2amps, only twice has it been more (both about 6amps cycling on and off several hours after the locking phase!), and most have been in the 100-400ma range (hence the need for good resolution).

Let us know how you get on, we'd certainly be interested in something (even if we had to DIY it), we just don't have the time or electronics expertise to start designing things right now! Our "project" list is pages long already, so time is always at a premium :-(

Richard
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Re: Accurate and Stable low amps measurement

Postby Alan » Sat Dec 22, 2012 7:50 am

Hi Hexibot,

The current sensors from Allegro look interesting but as you have to break into the circuit why not try measuring the voltage drop across a small resistor. If the battery voltage is say 12.5V and we drop 0.1V across a 1ohm resistor (discussed above) then thats not going to effect the running of any circuits on the vehicle.

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Re: Accurate and Stable low amps measurement

Postby FioranoCars » Sat Dec 22, 2012 11:24 am

In theory I would agree, if the circuit can be maintained while achieving accessible, accurate, reliable, detailed resolution without wandering results then great, what more do you need...

I have not yet tested the fuse method, but have had a look at a few cars, and especially on Mini fuses we have a high proportion with no access point in the plastic to be able to probe for a reading, so the circuit will need to get broken in any event. ;-(

We will do some tests using sample fuses from real cars of various amperage using a known current load (probably 50ma, 500ma and 5amp) to see the consistency of the readings of the fuses and how much variation there is, including time to see if heat has any impact. Subject to this all being ok, then my quest is almost complete ...

so going back to my original goal, to measure parasitic drain over extended periods, we can adapt a fuse/holder, to provide easy 4mm banana lead access, to bridge the earth cable and allow testing without ever loosing memory of the car, and if we can achieve the resolution needed, 5-10ma detail, then using a 20-30amp fuse will mean we can trigger using door locking without fear! (I guess using a proper suitable Resistor to do this might be even better, anyone got an RS Part number?!)

However having an inline testing tool with impressive resolution, in the tool box, would be very handy, firstly you'd save a channel on the scope, which might be important, second the resolution and accuracy may end up being far better. I like options, and not having any inline device I can use with Pico is a gap I'd like to fill (though it might be used a lot less than I think if all goes well)!

I'm really impressed with the forums here, having people share/come up with solutions that I would never have found on my own, due to blinkered approach more than anything, I should have shared these issues years ago!

I guess some testing, prove the accuracy and reliability of the fuse method, and maybe checking all the fuses we buy have the ability to be probed! In Jan we'll do some tests, build some maths channels and post some results here.

Thanks for all the input and help, wishing you all a great Holiday Season and for to a Prosperous Year to come!

Richard
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Re: Accurate and Stable low amps measurement

Postby hexibot43 » Wed Dec 26, 2012 7:18 pm

Back to work. :?

I already ordered some parts including the allegro chip with an adapter to dip for breadboarding for now. I think the simplicity of the resistor had escaped me. I was looking into getting a 1 ohm resistor from Digikey. I was a little surprised at how expensive they could get. I was calculating for a 30 amp draw. I know it will only be momentary but shouldn't I get one based on worst case? IR 12.6 volts x 30 amps = 378 watts. So I need a resistor that can dissipate that much wattage right? Picking one at 400 watts costs around $32. I was thinking of putting two of the allegro chips in parallel, and then summing the output. It would be cheaper. I found an SD card reader I had lying around and a Pic Chip with 10 bit resolution ADC. I also just got a sample for a new electric ink lcd numerical displays. Same type display hardware they use in the Nook readers I believe. I should have something together quickly for testing. I should be able to set this thing up for pretty good battery life putting everything to sleep most of the time. I'm hoping to have something together that will take accurate readings without drifting, handle momentary 30 amp draw, have an output for the PicoScope, and also be able to record on it's own and have a way of displaying it. Haven't done much of this in a while. Should be fun.

Question: If I were to simply use the resistor method. Ground to Earth of car / battery negative....and then use two channels with some math for the IR drop of the resistor? I can do some actual math in there too right? Convert the voltage into amps for easy of displaying and reading in the PicoScope. That way I'm only measuring the resistance of the resistor and not any of the wiring.
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Re: Accurate and Stable low amps measurement

Postby KimAndersen » Wed Jan 02, 2013 5:40 pm

Hello again and a Happy New Year.

To continue this thread. I have been testing the resistor method, where i have placed a 0.01 Ohm 50W 1% resistor in serie with the negative battery cable and the battery´s negative pole.

The first thing i did, was to make a custom probe in the Automotive Picoscope software where it is possible to make your own scale.

I chose to make scale that goes from 0 milli amp to 40000 milli amp - which i thought was enough to cover the range - which I was sure that i had plenty of space for high amp readings - but i were surprised.

When i turn the alarm on and off - will it pull a lot of current, but i haven´t expected that it will go that high.

Alarm_On_Off.psdata
(2.68 MiB) Downloaded 367 times


The first time you open these Picoscope files it looks like there no current flow at all. You must use the zoom button to get those small milli amp readings and then the horizontal ruler to get a accurate milli amp reading.

It is possible to see the where alarm is flashing when you have zoomed in on the trace from the signal.

Alarm_Led_Flashing.psdata
(1.09 MiB) Downloaded 356 times


One last thing - when your are in capturing mode i discovered that it´s very sensitive - when you touch the keyboard !!!

I would say - ít´s accurate within +/- 10 milli ampre in compared to my Fluke 88 DMM or maybe less.

What are your opinion about the this method to measure current on !

Kim
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Re: Accurate and Stable low amps measurement

Postby FioranoCars » Wed Jan 02, 2013 10:50 pm

Looks good! Well done.
I am not surprised by 4amps (or I am having a senior moment again 40k ma = 4amps?) for the door locking cycle, as we see 20-30amps on 4 door cars without any special self closers etc, and I guess in future these figures could climb as more fancy closing measures are added!

What car is this from? 5 seconds for it to completely shut down seems very quick most cars take 30secs-10minutes ...?

Are you able to add a trace from a Pico amp clamp (the 20amp mode) along side to compare? (from a reaction time and detail point of view as well as the obvious accuracy)

How is the keyboard affecting the readings, I assume the PC is powered from the mains? Any info on this would be interesting.

Did you try a wider ranging probe setup to see if your could read the total amps used during door locking cycle?

And also can you leave it in collection mode for several hours (say 3-4) (not needing the file - just to know if the resistor gets hot or starts to wander in any way - clearly 40-80ma should not give the resistor any issues?!).

Pleased that others are trying things, I like the resistor idea, as it gives the ability to use a more scientifically calibrated tool rather than a mass produced fuse, which may or may not be made to any standard whatsoever!

thanks
Look forward to see any further news!
Richard
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