Odd current shunt voltage measurement

[HwAoRrDk]

A quick query on something that is making my head hurt this evening.

On a breadboard I have a current shunt resistor of 0.15R, and I'm passing approx. 110mA through it. Connecting my meter directly to the leads of the resistor shows me a reading of around 17mV, as one would expect.

Now if I connect two short jumper wires adjacent to each lead of the resistor (i.e. in parallel) and then instead attach my meter to the other ends of those wires, I get a voltage reading several times higher! It has varied too: one time it was around 80mV, another time it was 150mV.

I don't get it. What's causing this?

[BrianHG]

Is the shunt an SMD resistor?
Can you provide a photo?
Did you damage the shunt with too much heat right at the legs which may be soldered inside the device if it is an axial component?

[Rudane]

How long are the wires? Could they be acting as antennas?

[HwAoRrDk]

No, it's an axial leaded 0.5W resistor. I don't think there's anything wrong with it. Seems to check out okay; reads 0.1R on the lowest range of my meter.

The wires are only your usual breadboard jumper wires, nothing special. About 4"/100mm long. I checked their resistance on the meter too - about 0.1 for both.

[coppice]

Maybe you breadboard has poor contact resistance. Try soldering your two wires onto the leads of the resistor, so you have well defined contacts with 150milliohms between them,

[edpalmer42]

Maybe you breadboard has poor contact resistance. Try soldering your two wires onto the leads of the resistor, so you have well defined contacts with 150milliohms between them,

The breadboard doesn't need to have poor contact resistance, just normal contact resistance is enough to mess up the reading for a 0R15 shunt.

HwAoRrDk, you've got two more resistors in the circuit, specifically the contact resistance between the leads of the shunt and the connectors in the breadboard.  The fact that the voltage is changing suggests that one or both connectors might be a bit loose, as Coppice said, or the leads of your shunt resistor might be a bit oxidized.  However, even if the leads are shiny and the contacts are perfect, you'll never get down to anything approaching your original 17 mv.

Coppice's suggestion is good.  Solder two wires onto the leads of the shunt and then use those wires as the voltage monitor leads.  Make them 10 or 15 cm long so you can plug them in where ever you want on the breadboard.

Ed

[HwAoRrDk]

I managed to sort out a workaround. I found some micro test probes I forgot I had, and placing the hooks of those on the shunt resistor's leads gives me a proper reading (despite the wires attached to the probes being about 1ft long).

You know, I think you're right about it being crappy contacts on the breadboard. I took just the shunt resistor and connected a jumper lead in an adjacent hole to each lead (so that everything is in series). I then measured the resistance of the entire chain, from the loose end of one wire to the loose end of the other. I initially found it to be around 3R, but found that nudging the resistor even slightly caused the measured value to vary wildly - all the way from 0R6 to 10R! It doesn't actually feel loose though, and there is significant friction when trying to pull it out of the holes, so I can't imagine how it's failing to make proper contact. The leads of the resistor are perfect, no oxidation at all.

Thing is, I swear I was doing a similar circuit (with the very same shunt resistor even) just a week or so ago with this breadboard, and I had no problems. Oh well. Maybe I shouldn't be buying my breadboards from eBay...