Pigtail Resistance

BundyRoy

Dedicated elf
Joined
Apr 9, 2014
Messages
1,026
Out of curiosity I got the multimeter out and measured the resistance through my 4 pin 0.75mm2 20cm long pigtails. I was getting readings of around 0.6 ohms. Now even for 14/0.20 cable which is only small the resistance is around 0.043 ohm/m. So on a 5m run (each way) this adds up to a resistance of 10 x 0.043 = 0.43 ohms.

So if my multimeter is even close to right the resistance through a pigtail at each end is 1.2 ohms. I would be better off ignoring the wire resistance and using the pigtail resistance rather than calculating the wire resistance and ignoring the pigtails like I have been so far. The pigtails seem to provide more resistance than the wire.

Is my multimeter way off, am I doing something wrong or is there a lot of resistance in the pigtails. This sort of resistance could lead to significant voltage drop if the strip is using 1.8A per 3m length.

Thanks
 

fasteddy

I have C.L.A.P
Global moderator
Joined
Apr 26, 2010
Messages
6,644
Location
Albion Park NSW
What type of multimeter are you using, most people in this hobby will just use a cheap one and these are not very accurate when taking very low resistance readings
 

BundyRoy

Dedicated elf
Joined
Apr 9, 2014
Messages
1,026
Sorry did mean to mention that. Part of my query was meant to be the accuracy of my multimeter. I used one from work but I don't think it is a very expensive one. I don't know the details though. It did seem a bit odd the resistance of such a short length of wire being higher but I thought I had better check. I don't have enough experience to trust my gut feelings on expected values.

Has anyone ever measured the resistance through a pigtail or have an expected range.
 

fasteddy

I have C.L.A.P
Global moderator
Joined
Apr 26, 2010
Messages
6,644
Location
Albion Park NSW
BundyRoy said:
Sorry did mean to mention that. Part of my query was meant to be the accuracy of my multimeter. I used one from work but I don't think it is a very expensive one. I don't know the details though. It did seem a bit odd the resistance of such a short length of wire being higher but I thought I had better check. I don't have enough experience to trust my gut feelings on expected values.

Has anyone ever measured the resistance through a pigtail or have an expected range.
This added resistance may be coming from the cheap cables that are supplied with the multimeter, one way to test this is to short the test wires on the multimeter and see what reading you get.
 

BundyRoy

Dedicated elf
Joined
Apr 9, 2014
Messages
1,026
Like zeroing a set of crappy scales. I didn't think to do that. Will check this arvo.
 

BundyRoy

Dedicated elf
Joined
Apr 9, 2014
Messages
1,026
Just checked the resistance in the meter by shorting out the leads. It was 0.4-0.5 ohms. So not much in the pigtails, especially considering my multimeter only reads to 0.1 ohms as well.

Thanks Eddy.
 

BundyRoy

Dedicated elf
Joined
Apr 9, 2014
Messages
1,026
How important is it to get a super duper multimeter. Can I get through with the basic model.
 

multicast

Senior elf
Joined
Jul 13, 2013
Messages
715
BundyRoy said:
How important is it to get a super duper multimeter. Can I get through with the basic model.

Really not that important. They all suffer the same things to some extent. They are more of a "indicator" tool than something you make precise measurements..
 

ChristmasInKaralee

Senior elf
Joined
Feb 15, 2014
Messages
581
Location
Karalee
I have an expensive multimeter and it makes no difference.


I just did a test using both leads and it reads 0.25 ohms.
Using just the Red gives 0.05 and just the black 0.2


Either get good leads or just check before measuring
 

David_AVD

Bite my shiny metal ass!
Community project designer
Generous elf
Joined
Jun 12, 2010
Messages
4,065
Location
Victoria Point (Brisbane)
Any 2-wire resistance measurement will have an error due to the resistance of the meter leads. The error is only a real issue when you're measuring really low resistances.

The options for measuring low resistances with no error are:
  • Take the meter lead resistance into account (subtract from reading). This will only get you so far.
  • Use a meter that can do a 4-wire measurement. This required a special meter and leads.
 

multicast

Senior elf
Joined
Jul 13, 2013
Messages
715
Theres other ways as well.. If you have a precision current source, you can measure the voltage drop ( even cheap multimeters can do this reasonably well ),.. knowing the voltage and current, you can calculate the resistance.


Saw a couple of LCR meters at the SEG, but was a bit dubious.. I'm tempted next time to take a couple of 'sample' parts with me, that i've already measured, and see how they stack up..
 

fasteddy

I have C.L.A.P
Global moderator
Joined
Apr 26, 2010
Messages
6,644
Location
Albion Park NSW
The meter i use at work makes you short the leads first so then it measures the resistance of the leads first before doing any testing, but this is a multi thousand dollar meter made for motor circuit analysis
 

AussieDoug

Full Time elf
Joined
Dec 1, 2012
Messages
375
I had some great pigtail resistance last night. I wasn't getting any power through it at all, after 30 minutes of searching & changing pixels out I eventually found the problem. The 2 pins in the plug were recessed back so far they wouldn't connect with the female plug.
The fun & games of fault finding.
 

BundyRoy

Dedicated elf
Joined
Apr 9, 2014
Messages
1,026
I noticed on some of mine the female part was a reasonable distance in and was wondering if there was ever problems with non connections.
 
Top