don't resist!

[Volter]

Is there a way to measure small resistance like 0.33 ohms or 0.15 ohms, how? Can I make resistance this small, how?
Thanks.

[Robert Reed]

Yes- run a known current thru it and measure the voltage across it, then use Ohms law.
Parrellel small value carbon film resistors (1 ohm?) or use resistance wire. You did not mention wattage needed.

[Bigglez]

Is there a way to measure small resistance like 0.33 ohms or 0.15 ohms, how?

The key to making reliable and repeatable low-R measurements
is the use of a Kelvin four terminal connection to the
unknown resistor. The two outer contacts provide current,
the two inner ones measure voltage. This eliminates the
contact resistance of the high current terminals.
More info here

[haklesup]

While I would have to agree that the Kelvin method is easier to understand and implement it is not the only method available. (FYI, the Kelvin 4 wire method not only eliminates contact resistance, it eliminates the resistance of the test leads. This is important becasue for very low R, you need to apply a fairly large test current so you can get a measureable test voltage. This can be a problem with low R, Low Wattage components.)

A variation of the Wheatstone Bridge, The Kelvin Double Bridge is also very accurate. Potentially allowing you to make the low R measurements with lower test currents. However this method is more complicated and requires a fair amount of manual math calculations and some resistors of accuraely known value.

http://en.wikipedia.org/wiki/Wheatstone_bridge
http://en.wikipedia.org/wiki/Kelvin_Double_bridge

Be sure to survey all the external wiki links, there is some good stuff in there

[Robert Reed]

Volter
Since you noted your resistances at 0.15 & 0.33 ohm, I assumed you were only concerned with two decimal place accurracy or maybe three at the most. While the kelvin method can be quite accurrate, it sounds like you don't require the fuss involved for the degree of accuracy you are looking for. If The DMM probe make firm contact right at the resistors terminations the probe resistance is insignificant due to the extrmely low current flow in that test loop. The DMMs typical 10 meg input impedance insures that. Now if you were measuring very low milliohm resistance which would require a considerable amount of current, then the kelvin/bridge method is far superior. I guess it all comes down to what you are measuring and what degree of accuracy you wish to obtain.

[Volter]

Well,
They call these small resistances Rsence and they are not offered by suppliers unless I would want the resistor to be of a size nearly AA battery! These resistances are shown on some of the circuit boards and the size of the resistors is like a wheat grain!
That is why I wondered if I can homemade something like that.
I did find one site where they offer these type of resistors: http://www.ohmite.com.
Thank you guys.

[sofaspud]

As said, quite a bit depends on your application. I'm not certain whether your need is low ohm resistors,
a measurement technique for low R, or both. You could surely find a "milliohm adapter" for a DMM on the
internet or in back issues of Radio-Electronics, Popular Electronics, and/or Nuts & Volts (most likely a
bench-type, rather than lab-type, instrument).
If you are in need of those resistor values, just parallel three 1-ohm resistors. That will give you 0.333
ohm. Six 1-ohm resistors in parallel will get you very close to 0.150 ohm.

[Bigglez]

They call these small resistances Rsence and they are not offered by suppliers unless I would want the resistor to be of a size nearly AA battery!

You haven't told us all of the specs yet. What
power rating is required? What current and voltage
is expected in your project's circuit?

[MrAl]

Hi,


I like a lot of the ideas i have read already here and i'll just add a little comment too now...


A long time ago i needed to repeatedly measure low resistances and get a good idea what
the resistance value was without super accuracy, but yet still reasonable. One very
easy way to get there was to build a very very simple constant current generator using
a widely available 7805 which is a 5v 1 amp regulator. After you read this you will note
that many other regulators can be used in place of the 7805, such as the LM317.

The circuit is so simple too: Run a 5 ohm 10 watt resistor from the 7805 output terminal
to the 7805 ground terminal, then supply this with a 10v power supply. The circuit
makes up a fairly decent constant current regulator that puts out 1 amp dc (depending
on the load too of course).
Now the unknown resistor connects between the ground terminal of the 7805 and system
ground (the power supply ground), and you measure the voltage across your unknown
resistor. The voltage read is equivalent to the value of the unknown resistor in ohms.

For example, if you read 0.15 volts then your resistor is 0.15 ohms. It's that simple.

This is especially good when you only have to do one or two measurements or
a bunch quickly, as the 7805 will heat up a bit when the circuit is connected to
an unknown resistor and so will the 5 ohm resistor.

Other notes:
This circuit is good up to about 2 ohms with a 10vdc power supply.
Keep in mind that the resistor under test may heat up with 1 amp flowing through it,
so you may wish to change the 5 ohm resistor to a 50 ohm resistor which will put only
100ma through the unknown resistor, but then the ohmic value of the resistor under
test will be 10 times the voltage measured...example: measure 0.015v and the resistor
value is 10 times that which is 0.15 ohms.

[Volter]

I would include the *.jpg file showing the circuit but cannot do that or do not know how so here is the file you need to look in.
It is the first schematic on the first page: NCP3065MNTXG.pdf
To MrAl, reading the second paragraph I new what you talking about but still I need to make the resistor but I guess that is not that easy task.
Thanks.

[Bob Scott]

Well,
They call these small resistances Rsence....

Ah, you mean "R Sense", commonly in a current sensing circuit. It does not surprise me that it is misspelled. It is always a good idea to replace these resistors with the exact original replacement part.

I've seen "current miler" printed in a Sony diagram where the correct description would be "voltage mirror".

What type of product or circuit is this in? Is a schematic available?

I have used small lengths of the thin iron wire that is used inside garbage bag ties to make a resistor. Also fuses have been used. They have resistance. Just make sure the current in the circuit doesn't get anywhere near the fuse current value, or the resistance will change when the filament temperature rises.

Come to think of it, I used a length of coat hanger wire to make a current shunt for a 10 amp analog meter.

[Robert Reed]

I would include the *.jpg file showing the circuit but cannot do that or do not know how so here is the file you need to look in.
It is the first schematic on the first page: NCP3065MNTXG.pdf
To MrAl, reading the second paragraph I new what you talking about but still I need to make the resistor but I guess that is not that easy task.
Thanks.

I am sorry to say this Volter, but I am getting more confused as this topic carries on. If the quote refers to what you are building and the sense resistor is your target, MOUSER deals in just about any size and/or value you could ever want. Just google up their website and plug "Sense resistor" into the search box. The surface mounts are quite small.

[philba]

Well,
They call these small resistances Rsence and they are not offered by suppliers unless I would want the resistor to be of a size nearly AA battery! These resistances are shown on some of the circuit boards and the size of the resistors is like a wheat grain!
That is why I wondered if I can homemade something like that.
I did find one site where they offer these type of resistors: http://www.ohmite.com.
Thank you guys.

Uh, then you haven't looked very hard. check out this Mouser page for KOA. They have exactly what you are asking for in SM packages from 402 to 2512 including the eminently hand solderable 805 and 1206 packages. 1% accuracy.
http://www.mouser.com/catalog/637/622.pdf