Milliohm resistors and 6th band Temperature Coefficient

Discussion in 'General Electronics Chat' started by harrison2015, Apr 29, 2015.

  1. harrison2015

    Thread Starter Member

    Apr 22, 2015
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    When measuring milliohm resistors like 0.33ohms you need to use a milliohm meter. Also milliohm resistors drift very easy with temperature. How do you measure milliohm resistors besides using a milliohm meter? I put my milliohm meter across the 0.33ohm resistor and use a hairdryer or cooling spray and watch if the resistor drifts up or down. If I replace the 0.33 ohm resistor with a higher wattage the temperature drifts less. But I don't know which temperature coefficient to use at a lower watter when I resistors with a 6th band. I heard that military resistors milliohm resistors don't drift at all why is that?

    Resistance Temperature Coefficient

    Resistors values can change with temperature. The 6th band represents the temperature coefficient or tempco and is represents the amount the resistance value will change with temperature. It is in units of ppm/degree C. The band colors represents the following:

    brown – 100 ppm/ºC
    red – 50 ppm/ºC
    orange – 15 ppm/ºC
    yellow – 25 ppm/ºC
    blue – 10 ppm/ºC
    violet – 5 ppm/ºC

    Is brown band better or is violet band better for temperature changes?

    I never understood this temperature chart because carbon comp resistors change differently , metal film , carbon film

    So i don't know which type to use for milliohm resistors, which type to use so they don't drift in temperature or in voltage?

    The wattage rating in milliohm resistors makes it stable for voltage coefficent?
     
  2. harrison2015

    Thread Starter Member

    Apr 22, 2015
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    They are different types of resistors metal film, carbon film, metal glazed, wire wound, ceramic metal, high power metal film

    Which works better for milliohm resistors?
     
  3. Reloadron

    Active Member

    Jan 15, 2015
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    I would have to vote for Violet - 5 ppm degree C because smaller is better. Think about it. :)

    These forums have a writeup on the subject Temperature coefficient of resistance, which can be found here.

    When selecting resistors for a circuit at design time the designer will research certain resistors and find those best suited for his or her design. Not just for temperature coefficient but also the accuracy. In many cases for example a 1.2 K Ohm +/- 5% carbon resistor is just fine. However, if that 1.2 K Ohm resistor is part of a precision divider network or gain circuit we may want to use a higher quality resistor, less likely to drift. It just depends on the application and the environment the end circuit will be used in.

    Ron
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Low values of resistance (actually ANY value of resistance) may be measured with a power supply and a multimeter, though two meters are by far preferable.

    Using an external power supply a driving force (voltage) is applied to the device with a meter in series to measure the current. Then a second meter is used to read the voltage across the device. Use points as close to the device as possible to measure the voltage, and these points should be equal or closer to the device than the current from the external power supply.

    That forms a basic “Kelvin” or 4 point measurement the same way a milli ohmmeter would perform the same measurement. The resistance may be calculated thru Ohm’s law.

    If you use a single meter your current may change (sometimes drastically) with the meter in or out of the path and spoil the measurement.

    There is a huge array of resistors on the market and which one to choose will depend on what you are doing: how much current is being measured, how accurate you need to measure it being the two most important items to lead you on a search for a device. Physical space and mounting methods also come into play. Thus I never worry about how the resistor is made, all I concern myself is with the overall ratings and treat the device itself as a black box.
     
  5. harrison2015

    Thread Starter Member

    Apr 22, 2015
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    What type of material they use for military resistors?
    I know that military resistors milliohm resistors don't drift at all why is that?

    What Can I use to heat up a resistor? I just use a hair dryer
    What Can I use to freeze a resistor?

    I watch the voltage meter and current meter when I'm heating up and cooling down the milliohm resistor to what the voltage meter and current meter any type of meter needle fluctuations. If I get any current or voltage fluctuations the milliohm resistor is bad?
     
  6. Reloadron

    Active Member

    Jan 15, 2015
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    If you want to better understand mil-spec resistors I suggest you start here. The fact that any product or service is Mil Spec simply means it meets or exceeds a prescribed set of published specifications, it does not mean something is "better". There is no set material used in making Mil Spec resistors and that includes low value resistors like below 1.0 Ohm. The resistors are made to meet a given specification and that is all there is to it.

    ErnieM mentions above how low precision resistance values are measured. Commonly called a 4 wire measurement or Kelvin measurement. A known precision current is passed through a resistance and the subsequent voltage drop across the resistance is measured. This reverts back to basic Ohms Law. I suggest you Google DLRO (Digital Low Resistance Ohmmeter). There are also a few other things to consider. When measuring accurate precision resistance values, as mentioned, we pass a current through the resistance. We also make sure that selected current does not cause "self heating effect" of the resistor being measured. You may want to Google "resistor self heating effect" to understand what that is all about.

    Yes, you can heat a resistor using a blow drier, heat gun or any other heat source. You can chill a resistor using freeze mist, an ice cube in a plastic bag, dry ice or any other method that makes cold. Unless you can accurately measure temperature and temperature change you have nothing.

    Measuring or better put accurately measuring resistances in milli-ohms is not all that easy. While change in K Ohms is easy to see because the change can generally be easily resolved measuring change in PPM of a milli ohm resistor is hard to see.

    Why would you say that? All resistors drift and anything having resistance drifts.

    Ron
     
  7. tindel

    Active Member

    Sep 16, 2012
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    I use these resistors regularly in high-reliability military-like applications. http://www.vishay.com/resistors-fixed/list/product-30133/

    5ppm/degC is 20 times better than 100ppm/degC. About 95% of the time when you use these type of resistors, you're using them to sense current... in conjunction with an amplifier of some sort - usually to feedback into a control loop or to provide current telemetry. What ppm you use is completely up to the designer, and is a function of the requirement of the circuit.

    My experience has been that the temperature coefficient for this type of circuit is such a small percentage of the error in the circuit that I can use just a slightly better op-amp to get a much better precision than using a very good tempco resistor. What is really important in a current amplifier is the ratio of the input voltage to the op-amp offset voltage. The higher the ratio, the better your precision.

    a 5ppm resistor will also be more expensive than a 100ppm resistor. whereas a 10x better op-amp may be only 10 cents more. of course you have to balance these things. Will the 5ppm resistor be $2 more? then use a better amplifier. If the 5ppm resistor is only 2 cents more... get the better resistor, and still consider the better amplifier to boot! That what makes engineering challenging and fun... making all these things work to maximize profits!
     
  8. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Having spent most of my career building things for the military I can assure you there is no magic inside military resistors or any military part whatsoever. "Mil Spec" simply means some parameters have been tested to military specifications.

    There is a spec for marking permanency. You can make a box out of wood, brand it with a branding iron, and if the branding is still visible after an alcohol rub then it passes the mil spec marking permanency test and you can sell it as a mil spec part.

    When I was starting out my career the engineer mentoring me told a story about mil grade transistors: from low to high they come in JAN, JANTX, and JANTXV grades and denote the ammount of testing that goes into each.

    These grades are said to mean "may work," "probably works," and "no useful life left."

    We would need to see what specific part you are describing to comment any further.
     
    Reloadron likes this.
  9. Reloadron

    Active Member

    Jan 15, 2015
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    That was good, really good. Not to mention totally on target. My entire working career was Military Specifications. I even had a favorite I would share with the technicians, MIL-T-FD41 which was Make It Like-The-Friggin Drawing For Once. :)

    Seriously, people fail to understand that a Military Specification is just that, a specification. While a part meeting a military specification can be good it does not always mean it is a superior part.

    Ron
     
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