Accurate and Stable low amps measurement

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Re: Accurate and Stable low amps measurement

Postby Alan » Thu Jan 03, 2013 6:16 am

Hi,

Thanks for sharing these results with us!

Is perhaps 0.01 Ohm a but low for looking at mA signals? A higher value resistor such as 0.1 ohm will give better resolution but might struggle with the power dissipation if the doors are locked (say 30A as mentioned above) and possibly cause too much of a voltage drop.

V=IR = 30 * 0.1 = 3V drop for 30A
P=V*I = 90W

That is however a pulse of current so you would not need to use a 90W rated resistor - this is the sort of thing I have in mind http://uk.farnell.com/te-connectivity-c ... dp/1259452 or http://uk.farnell.com/welwyn/wh100-r10j ... dp/1768194

Both are under £5 / $10, not read the data sheet but I am guessing that they need a heatsink to take a continuous 50W / 100W but would be ok with a short pulse of 30A.

Having said that if you can switch a resistor in after the doors have locked then you can go higher in value as we are looking at currents in the mA range.
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Re: Accurate and Stable low amps measurement

Postby KimAndersen » Thu Jan 03, 2013 10:18 am

Hi Alan

With my first test where I used a 1 OHM resistor the door locks responded very slow - due to the higher voltage drop across the resistor.

So i decided to use a lower resistor to avoid this problem with the door locks. But maybe i could have used a 0.1 Ohm resistor as i discovered that 1 ohm resistor i to high and 0.01 is to low.

They are not that expensive so i may consider ordering this resistor just to prove the difference in the measurement.

I will make some further test today with the Pico Amp Clamp in 20 Amp mode just to see how it react compared to the resistor.

Kim
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Re: Accurate and Stable low amps measurement

Postby KimAndersen » Thu Jan 03, 2013 7:33 pm

Here are the results from the comparison between the resistor method and the Pico current clamp in 20/60 amp mode.

All measurements are taken on a VW BORA TDI without Canbus system installed - that explains the quick shut down.

According to Volkswagen shall the nominal sleep current be: 40mA

From this capture i can conclude that with the Pico current clamp in 20 amp mode and the scale set to -500 mA to 500 mA is showing a current flow of 17.3 mA and where the resistor method shows 42.9 mA.

Sleep_Current_PicoProbe.psdata
(2.06 MiB) Downloaded 373 times


I had a bit problems with Pico 60 AMP current clamp - after only a couple of minutes of use, the milli ampere measurement dropped to around 4 mA. So what could be the reason to this drop in current flow !

I replaced the 9 Volt battery in the Pico 60 current clamp despite there was no light for low power. With a new battery in place - the milli ampere measurement rose to 17.3 mA.

This capture is taken with 60 Amp current clamp and shows current flow when you activate the alarm and as shown have i expanded the milli ampere scale to 60000 mA = 60 A

Alarm_Pico_60AMP.psdata
(1.8 MiB) Downloaded 373 times


The big question is what method is the best !!!

Kim
Last edited by KimAndersen on Sat Jan 05, 2013 10:13 am, edited 1 time in total.
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Re: Accurate and Stable low amps measurement

Postby Robski » Thu Jan 03, 2013 8:07 pm

Can't you compare to a DMM reading between them all to see which is closest ?

Maybe one you can hook up to a PC & record ?
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Re: Accurate and Stable low amps measurement

Postby FioranoCars » Thu Jan 03, 2013 9:52 pm

Looking at the comparison data, the alarm led flashes are about half the amplitude via the clamp vs the resistor, showing the downside of the clamps lack of responsiveness, so the resistor looks both more accurate and having a faster response time. The clamps reading is too low and I'd trust the resistor reading, and this is our trouble that 20ma difference could be a good car from a bad car, ie 50ma and 70ma, if you can trust the reliability and repeatability of the kit (the clamp) you end up using an in line meter or doing something else to ensure confidence. So here I have no problem believing the resistor as better ...

The drift in the clamp readings is standard and due to the circuit design, the battery power reducing and also if you have the push button zero style, the capacitor discharging (so I've been told), which is why we need a long-term (2-4days) reliable method, and even the expensive alternative clamp suffers the same basic issues ... that's inherent in the clamp design.

On the door lock sequence it's amazing how the clamp is showing massively lower results in the peaks (about half), is this down to the clamps slower response times? I would like to see these validated against something else to know which is the true value, you might just manage to not blow the fuse on a 20amp meter! Sadly all the readings we've ever taken (over teh last 6 years) have been with a clamp, now I wonder about all those readings!

I would probably say the resistor (provided the maths and other stuff is setup correctly!) is always going to be a better method than a clamp, especially based on what I've read here and elsewhere on the web since starting the thread! Some way to valid this would be good, however a 3v drop in the circuit could itself start to affect things too (if using a 0.1ohm) so need to be cautious.

Clamps have their place, but have many "features" (issues!) that you need to understand to prevent yourself from making poor measurements. I guess I should re-read the response times etc and then re-compare.

Will place an order this week for some sample resistors both 0.1 and 0.01, maybe even 0.001 if I can find any at a reasonable price for measuring higher amp's? Any sources in the UK appreciated (Farnell / Rapid / RS ...)
Richard Lukins
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