Need Help for Wire Wound Resistor

With the information you've give I doubt it.

find the power of resister by using I^2R formula, usually current taken to determine power is double the rated current flowing through an alloy made resistor.Use the formula R=ρL/A Where R= Resistance,ρ= Resistivity of wire, A= Area,L= Length. So more is the length more resistance,less area more resistance.

You need to state what resistance value you require, its current, voltage and power rating and also how much its resistance may vary to do the job you need.

For low resistance, power and voltage with wide limits on resistance, plastic, cotton or enamel insulated copper wire wound by hand with multiple layers on a wood rod may serve.

For higher resistance, special metal alloys with much higher resistance and similar insulation may be needed.

High power, as for an electric "fire-bar" may use a ceramic former with a spiral to keep not-insulated turns of "red hot" wire apart.

If low change of resistance with temperature is needed special alloys, such as "constantan" are available.

If the value is more than 50,000 ohm, wire will probably not be practical - wire too thin and difficult to wind or insulate.

So what is your requirement??

Well the kind of resistor i am trying to make is wire wound of the value 50K Ohms. The purpose for this resistor is to be used in calibration of moving iron (AC) meters.

Calibration of moving iron meters.....

Well, if you are thinking 50 kohm, fixed value, then you are talking voltmeters - most moving iron voltmeters are used for AC mains - which is 250 volts rms 350V peak or maybe 440 - 600 V rms. Also 50k at 250V is 5 milliamps and 1.25 watts power.

Moving iron meters are not noted for their precision - about 1% is good enough?

But if you are to give "1%" with confidence, all your test equipment errors must total to less than 1% between THEIR calibrations e.g. over a year. So I do not suppose less than 0.1% initial error can be allowed for the resistor.

I must warn you that - even supposing you have resistance test equipment with 0.01% error to measure your resistor and its errors over temperature and time with confidence and accuracy - making a reliably precise wirewound resistor of 50k and such power/voltage is a major operation in your time, learning and effort. There must be 200 years history of lifetimes of effort in making resistors and tens of thousands of patents involved.

The good news is that there are companies out there who know all the methods and problems and can sell you an assured reliable precision product.

To mention a few effects, which folk often do not realise change resistance by as much as half the initial tolerance...

Soldering. Humidity. Temperature cycling. Storage. Applied voltage. Time.

However, for 50k, 1.5 W and 250V rms I think you can buy 0.1 % grade for less than 15 British pounds.

I recommend :-

1. Look at the website www.vishay.com. Their business is precision resistors. Look at www.farnell.co.uk for ranges of precision resistors in stock.
2. Get the detail specifications of their products. These tell you a lot about what will change the value of your 50k after you put it to use.
3. Ensure the voltage rating and power rating are ten times what you need e.g. by putting 10 resistors in series. Low temperature rise and voltage will minimise resistance changes due to operating stress.
4. Protect your test stand with voltage limiting resistors [VDRs] with high rating and peak voltage close to test peaks to ensure measurement resistors are never overstressed causing value change. Ordinary mains can see 5 kV voltage spikes.
5. Looking round for the values and ratings which can be put in series or parallel to meet your need at lowest cost. There can be good offers on unusual values which are surplus e.g. "non ROHS" - which may not affect you because you are not marketing your test equipment. I found that, nowadays, for 0.1% or even 0.05% tolerance, surface mount components are cheaper than wire ended - provided you do not mind the small size and need to mount on PCB ["Veroboard" copper strip board will do!]. The SM resistors smaller than 1206 size are restricted in voltage and more difficult to handle. If the total value is not correct, it can be made up by adding one or two resistors of a chosen value (note that if you adjust the value 1% with a 1% tolerance resistor, its tolerance only affects overall tolerance by 0.01%).
6. Learning about the probability mathematics of errors. Two 1% resistors in series are more likely to be close to nominal than 1 resistor, because their errors may be in opposite directions to each other.
   

I should have said, for your application, wirewound may not be the best solution, you should consider all options. Wirewound are only generally available for high powers, and usually with just +/- 5% tolerance. Wirewound "aluminium clad" types e.g. type HSA,, 25 Watt do have a good temperature coefficient 25 ppm/degree and stability/power/voltage, so if you could measure an assembly's resistance accurately, you could put several in series and adjust the value to 50k with series or parallel metal film resistors.

If 1.5W is enough...

May be it will be easier to buy two 100 kOhm, DC/AC, 1W Resistor Standards, and connect them in parallel. Usually Standard Resistors have accuracy <= 0.1%.

Or AC/DC (bifilar wirewound) Decade Resistor with 10 x 10 kOhm decade. They have usually 0.25W/resistor, and have 0.1% accuracy.

Or just buy two 27.826k resistors:

http://www.ebay.com/itm/Vishay-S102-High-Precision-Foil-Resistor-27-826K-ohms-0-005-/120791779434?pt=LH_DefaultDomain_0&hash=item1c1fc0486a

and six 24.048 resistors:

http://www.ebay.com/itm/Vishay-S102-High-Precision-Foil-Resistor-24-048K-ohms-0-005-/120791779185?pt=LH_DefaultDomain_0&hash=item1c1fc04771

connect them 3 x 24.048 + 27.826k in series , then two series in parallel, and You will have excellent 49.985 kOhm 0.01% @ 1.5W resistor