Accurate and Stable low amps measurement

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Accurate and Stable low amps measurement

Postby FioranoCars » Sun Dec 16, 2012 6:18 pm

Hi
We currently use the Pico PP264 60amp (switchable to 20amp) amp clamp, in 20amp range using 10 windings to increase resolution, but find the clamp (we have 2 so it's not one clamp being in need of repair!) to wander over long periods. It wanders regardless of windings, the windings are used purely to increase the accuracy.

We use it to measure parasitic current drain, so once the car doors are locked it is 1amp or less we are measuring with the interested area being 20-150ma in 5ma steps max.

Our cars should have a residual drain of 40-55ma max, it's easy to see a 150ma drain if it's there all the time, but we see cars that have long term (ie after 24hours) intermittent system activations causing parasitic drains. being able to accurately monitor the drain for 2-3 days (using mains powered clamps if needed to maintain power).

Knowing accurately how big the drain is and for how long is helpful in finding the fault. An extra 100ma for every 4th hour verse a 2amp burst for 15 minutes once every 6 hours. It's not a daily requirement but we get half a dozen or more cases a year, with 2-3 needing very extended testing times.

We have been told about the AEMC K110 MicroProbe, which appears to be more suitable than the Pico unit, but I'd like to know if anyone has used it, but especially for long continuous periods at the 40ma levels? The wandering and/or stability of the inductive loop is key ...

Any advice of alternatives or feedback on the K110 MicroProbe would be appreciated!

Thanks
Richard
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Re: Accurate and Stable low amps measurement

Postby Autonerdz » Mon Dec 17, 2012 4:13 pm

Richard,

There is no alternative to the K110 for micro amps measurements. There is a Chinese one that we purchased and tested here. Total junk. The K110 has a relatively slow response time but is extremely accurate and stable over long periods.

We have sold a lot of these stupidly expensive probes. They are used for AFR current, magneto resistive WSS testing, and many other uses that don't require a fast response time.

The jaws are very tiny so not sure how you would use this for parasitic draw testing without breaking the circuit. But you must be doing that anyway with your inductive loop method?
Tom Roberts
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Re: Accurate and Stable low amps measurement

Postby hexibot43 » Mon Dec 17, 2012 4:33 pm

I had much the same problem....the inductive probes have a tendency to drift over the course of time you are trying to take measurements. What I've done instead is use an ES 595 Automotive meter. It has the ability to log to a computer. And instead of going inductive I go series. Very stable and accurate over long periods of time. I use a wall wart battery eliminator to ensure I don't have a problem with the 9v battery in the meter. The software that comes with it works on a pc. I had to use a computer set up for windows 98. It didn't like running on a newer computer. It will log basically forever. BMWs are notorious for crazy drains. And I would agree that knowing for how long and how much really does help figure out the root cause.
I'd be very interested to see if anyone comes up with some better ideas. I get a lot of cars with these problems. I've thought about putting together something with a pic chip and an SD memory card in place of the pc. Output a file that would be easily read into a spreadsheet onto the SD card. Keeping it very simple to do. I could probably fit the pic into the case of the meter. I wouldn't have to drag out the old pc. To many projects and not enough time.

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Re: Accurate and Stable low amps measurement

Postby FioranoCars » Mon Dec 17, 2012 6:35 pm

Oddly enough I have 2 DMM's that look identical to the ES 595 Automotive meter, they are from Sealey in the UK called teh TA203, the reason for them is they have a 20amp rating for measuring current, rather that the usual 10 or 15amps, which is the difference on door locking sequences to blowing the fuse in the meter and working fine!

I have dug out the lead and CD and will play, never thought of the PC link!

Ideally would like to use Pico as the recorder, so the other channels can be used to see key components, and if they are turning on at the same instant as the drain, but it's a start.

thanks

Richard
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Re: Accurate and Stable low amps measurement

Postby Avdr » Mon Dec 17, 2012 6:50 pm

Have a look at the NVE AAV003-10E,

Its a piece of integrated circuitry that has a upto 600mA current capability, which is monitored via hall effect, and processed into a proportional voltage output.

datasheet here: http://www.hy-line.de/fileadmin/hy-line/sensorik/hersteller/datasheets/nve_aav-current_sensor-serie.pdf
I bought mine from that firm (Hy-line in germany) and while its not an 'out of the box' solution, I think its a serious contender to these micro amp probes. It is of course an 'in circuit' device but its not a perfect world, I've yet to actually get mine set up in an enclosure etc and havn't even tested yet (had in the box for about a year). I can see it being useful for looking at everything from Pump cell currents in wideband sensors, to magneto resistive ABS set-ups, and indeed parasitic drain.

See what you think,
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Re: Accurate and Stable low amps measurement

Postby hexibot43 » Mon Dec 17, 2012 8:21 pm

That is exactly what made me choose this meter in the first place. 20 amp rating for the Ammeter. I have had an OTC 500 multi-meter for forever that came with a 20 amp rating. In anticipation of OTC unit failing I picked up a second meter from bluepoint at a steal and realized with a 15 amp rating it was annoying. Next thing I know I've bought a third meter, and yet my OTC is still plugging along.
I just did a bit of reading on the net and see quite a few pic based voltmeter/ammeters. Shouldn't be too big a deal to put something together that outputs a voltage based on an ammeter reading thought could be easily used with the PicoScope. One in particular ' "the lightening bug" which only will function up to about an amp. But something like this would work great.
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Re: Accurate and Stable low amps measurement

Postby FioranoCars » Tue Dec 18, 2012 12:33 am

Andy
Sadly even at 600ma range it's too low, but reading the spec sheet I think (or Fear!) the working range may be -80 to + 80 ma, and a max overload of 600ma ?? but either way we would need the ability to cope with say 2-3amps, probably safer to be more than that, as some items that cause a drain (alarm with flashing lights etc) can draw quite a bit ... which probably means sticking to an inductive setup, reducing the risk of damage to the kit ... it's a tough set of compromises!

Tom
Thanks for confirming my fears!! Please let me know if you come across any others, I think it will go on my Santa wish list, but it's a heap of money for something we'll only "NEED" 2-3 times a year, but boy would it help! Maybe the next car with a serious issue will mean splashing the cash!

Yes we do split the battery cable to measure the parasitic drain, as few cables are small enough for the PP264 jaws, so we use a power-saver elsewhere (OBD socket, cigar lighter, fuse board, where ever there is a +30 and chassis) while sorting out the earth cable, putting inline our custom cable set, which has a dual cable (one thick battery style with kill switch and a small cable with fuse link, and with optional 10 coil winding, which bridges the kill switch) the thick cable can be used to lock the doors or even start the car ... once the doors are locked the kill switch can be used, to put all drain via the thin cable. Using the thin cable on it's own can lead to unexpected fuse blows (and 20amp fuses in the DMM!!) hence now the more elaborate cables, but next to no chance of unexpected battery loss, no need to self learn the ECU's, radio codes and all the other grief from that happening (some of our cars take about an hour of technician time to fully relearn, not much fun!

So we can get away with a small cable for the K110 (maybe even 3-5 windings? if we needed the extra resolution).

Any other ideas gratefully received
Thanks
Richard
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Re: Accurate and Stable low amps measurement

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

Hi Richard,

The K110 will (with filtering switched on in PicoScope) measure sub 1mA - the main issue is as Tom say they are "stupidly expensive".

There are a few work arounds. One is pretty much what you are doing by using multiple turns of wire through the jaws of the 20A/60A clamp.

I have used one of these clamps

clamp.jpg


Which cost a few pounds / dollars. With a bit of modification the wire to the fuse can be extended with some thin wire and wrapped through the jaws of the clamp many times. Once the doors are locked etc the switch can be opened so all the quiescent current goes through the fuse / clamp.

The 20A range the standard clamp can resolve / spot a change of about 50mA without filtering in PicoScope and about 20mA with. Adding 10 turns of wire through the jaws makes it sensitive enough for battery drain (spot a 5mA change) but if you can find some really thin coated wire then 100 turns is better - the 20A range is now a 200mA one and you have sub 1mA resolution.

In terms of drift you have a couple of considerations - time and temperature. The auto zero feature when you press the button on the clamp works by charging up a capacitor, this holds it charge for a few hours but gradually leaks away so the signal drifts. My advice for this test is to switch on the clamp but NOT auto zero it if you want to measure for more than an hour. You will have an offset of a few mA all the time and have to subtract this (use a maths channel if you want). Note that the capacitor discharges over time regardless of whether the clamp is on or off so this trick only works if the clamp has not been auto zeroed in the last 12 hours or so.

The second issue is temperature - small standby currents draw by circuits in the car vary a lot with temperature as does the performance of the battery and clamp. If you start the test with a warm engine and then the vehicle cools down over several hours this can effect the results.

A couple of other tips - be careful of keyless entry, systems wake up as keys come into range of the vehicle even if not the correct keys. Also if testing over many hours put in a fresh battery into the clamp - a good battery will last for a couple of days of use.
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Re: Accurate and Stable low amps measurement

Postby KimAndersen » Tue Dec 18, 2012 11:29 am

Hi FioranoCars

There is also another way of measuring parasitic current drain in a car today.

The way Volkswagen does it on - is by measuring the voltage drop across a known fuse and then convert this value to milli ampere from a conversion table - its called a (Current Consumption Matrix).

What i don´t know is - wheater this conversion table is a general standard that can be used across all automotive industry or its only specfic for Volkswagen cars.

I´m well aware - that this method can only be used in conjuction with a measuring of a parasitic current drain from a fuse in circuit.

I don´t see why - it cant be used - when volkswagen can use it - so can other car makers also be using it.

For further study - read this document.

Diagnosis for Excessive Static Current Draw.pdf
(111.77 KiB) Downloaded 585 times


Regards
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Re: Accurate and Stable low amps measurement

Postby Alan » Tue Dec 18, 2012 11:48 am

Hi,

Measuring the voltage across a resistor is certainly an option, I have not tried it myself though.

Use something like the battery disconnect switch above on the negative terminal of the battery and put a 1 ohm power resistor in series with the fuse. You can then measure the voltage drop across the resistor on the scope.

Using ohms law

1V = 1A (resistor might get very hot!)
100mV = 100mA
10mV = 10mA
1mV = 1mA

Obviously you can use a maths channel to read directly in mA.

If anyone wants to give this a try please let us know how you get on!
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