Help with 4-pin current sensing resistors - useful or fizzers ?

[nick mulder]

Hi,

The other day I heard mention of stepper motor and servo control cct's in an advert for some surplus current sensing resistors which were priced well below the cost of getting them in to New Zealand (where I live) - so I bought them like the true monkey I am and now need some info as what they actually do and more on how to actually get them goin...

quick specs:

"
* 10W ( on suitable heatsink )
* 10mohm
* Low Inductance
* Four terminal configuration
* Tolerance to 0.1%
* Temperature coefficient: tcr ( R > 20mOhm ) <10ppm/K ( 20C to 60C )
* Applicable temperature range: -55C to + 125C
* Load capacity: 10W with heatsink provided / 3W free air
* Dielectric withstanding voltage 500VAC
* Inductance (R = 10mOhm) < 10nH
"

more and picture: http://www.rhopointcomponents.com/pr...uctWebCode=AH2

the full spec pdf : http://www.rhopointcomponents.com/isa/pdfs/ah2.pdf

I am currently building the UHU DIY servo system for use in motion control camera work (and good ol' CNC as well) which is well in excess of 10W, but thought I may make a small stepper system running from a UCN 5804 unipolar chip also...

Would these resistors help me get more torque from such a system ?

there is no typical application and circuit diagram to show how these kelvin connections work and I'm not too flash with electronics yet ...

any help appreciated!

cheers

nick

[ViperTX]

Well....these are current sense resistors....so they are designed to be used in parallel with some other component....usually a load or current limiting resistor....you measure the voltage drop across the sense resistors to determine the current flowing through the load....

[nick mulder]

Originally Posted by ViperTX Well....these are current sense resistors....so they are designed to be used in parallel with some other component....usually a load or current limiting resistor....you measure the voltage drop across the sense resistors to determine the current flowing through the load....

i'm not an electronics expert and have so far gotten by looking at spec sheets and using logic chips which I find easier to understand - especially if there is a 'typical application' diagram in the spec sheet -

Any idea of a simple pin-out ? which pin where ?

[pastera]

On a low value resistance the interconnection resistance is very significant. If you use the same leads to sense the voltage drop of the resistor as used for main current flow, you will end up measuring the interconnection drop also (depends on solder joint, pcb plating, etc) To combat this, resistors are made with two extra leads which connect directly to the resistive element. Very little current is drawn from these leads, therefore the interconnection resistance (device to PCB) is of little significance.

Simply use the "I" leads to carry the load current and measure across the "U" leads.


Aaron

[nick mulder]

ah... ok! thanks guys - they are simpler than i thought, and now i know what to do - an ok purchase after all considering i got them for about a 1/4 of usual price ...

thanks again,

nick

[AbSat]

Originally Posted by nick mulder Hi, Would these resistors help me get more torque from such a system ? nick

Hello Nick.

I am sorry for you Nick but I dont think these resistors will help you to get more torque from a stepper.

Very nice resistors, but with a resistance of 0.01 Ohm only useable in very high current applications I think. As a current limit resistor in a chopperless system the resistance is much to low, and even as a current sense resistor in a chopper system normally 0.1 till 1 Ohm is used to get enough voltage for the comparator at currents till about 4 A.
These resistors are ment to measure the current in a system with very low loss over the sense resistor but they will give you only 10mV/A. So you will need a dc amplifier to bring this 10mV/A in the range a choppercontroller can work with.

AbSat.

[NC Cams]

THe low resistance is good to prevent voltage drop from added circuit resistance.

If you put a simple op amp (or more precise instrumentation) amp across the "K" terminals, you can amplify the voltage to pretty much whatever level you want. This type of current sense resistor is used just with that sort of precise ancillary amplification in mind which is why the Kelvin connection is used

That is also why it is heat sunk - heat changes the resistance which affects the E from E=I*R relationship in play here.

[apollonono]

I have one opinion.
Because English cannot understand it, I use an interpreter.
I cannot express it definitely.
According to the calculation, the electric current reaches 32A.
In this case there are many cases that I apply car Lent converters (a hall element), and is measured.
If an electric current is interchange; car Lent transformer and a full-wave rectification circuit.
If it is direct current, it is a reluctance element and a hall element, active feedback.
The remainder is calculated if I measure two lines if it is three aspect circuits.