Resistor Characteristics Help Please

[SirGus]

Some resistors just say the resistance (10 KOhm for example), where other says some Wattage as well (10 KOhm. 2W).
Why is that?

[Kat Manton]

Hi,

I assume you mean on a circuit diagram or parts list?

A manufacturer will use resistors of a "standard" power rating for most of them. I think they're usually 1/4 W, sometimes the diagram might say "all resistors 1/4 W unless otherwise stated" somewhere.

When 1/4W isn't enough and would burn up in a particular part of the circuit, then they fit 1W, 2W, 10W or whatever's needed - and mark that on the diagram.

Cheers, Kat

[Station X]

Resistors up to about 2 watts are colour coded and there is no indication of power rating. You have to deduce it from the physical size of the resistor.

Above 2 watts resistors are generally wirewound and will have their resistance, power rating and tolerance written on them.

You must choose a resistor capable of handling the power being dissipated in it. This power is given out as heat.

If in doubt measure the voltage across the resistor and multipy it by itself, then divide by the value in ohms of the resistor. This gives the power dissipated.

[SirGus]

Thanks Graham. Yes I remember P=V*I, where I=V/R.

I am a physics teacher..but no knowledge on electronics, on the practical way.
Just theory!

[Ed_Dinning]

Hi, as a practical approach to resistor sizing you can replace with a modern similar size part without problems. For various reasons most modern parts have a higher "power for size rating". As a double check, you should not have any small resistors running so hot you cannot keep your fingers on them (with the set switched OFF !) after it has run for an hour or so.

Ed

[Radio_Dave]

Although not often specified I'm sure all resistors have a voltage rating?

Does voltage rating effect the physical size of a resistor? Could something like a 500V 1/4W resistor get confused (physically) with a 250V 1/2W ... I hope you see how I'm thinking

David

[Station X]

That's always puzzled me a well. Given a resistor of specified ohms and watts, then the voltage across it at full power dissipation is fixed. Why then is it necessary to specify voltage?

For valve radios I always use 2 Watt Flameproof resistors which I think are rated at about 500 Volts.

[Ed_Dinning]

Hi Gents, a look at the Welwyn website will explain about the voltage rating of resistors. www.welwyn-tt.com . It also has data on colour codes and the characteristics of resistors manufactured by different methods.

Ed

[SirGus]

Any way to check a 3M resistor when the meter measures only up to 2M?

The only way I can think is connecting parralilel parallely another smaller resistor and check both?

[ronbryan]

Lift one end of the resistor, apply 18V from 2 x 9V batteries in series with a DC currrent meter. Measure current. Divide 18 by the current.
This will give a good approximation to the resistor value as the 18V is large compared to the voltage dropped across the current meter.

Ron

[SirGus]

Here you have a resistor I must change.

there is no Wattage on it and the lenght is 1,4 cm (200kohm resistor)

-Which wattage would you use?

-If you use more wattages, is there any risk for conflicts?

[Ed_Dinning]

Hi Costas, no problems going to a bigger wattage other than it may not fit. In most vintage radio applications there is no problem with the type of resistor used (carbon, film, wirewound etc) but in modern electronics you need to be more careful as fitting a wirewound can in some cases cause instability.

Ed

[paulsherwin]

You can always use a higher wattage resistor that the original, unless the resistor is serving some special purpose such as a fuse (this is very uncommon).

Most resistors in valve radios were 0.25W rated components. These can be replaced with modern 0.5W or 1W parts which are about the same size.

There are a few resistors around the output valve which are 2W, and one between the 2 smoothing capacitors which is sometimes 5W (though usually 2W).

If you replace a resistor with one with too low a wattage rating, it will run hot, smell bad, and eventually fail open circuit.

Paul

[SirGus]

Then why someone would use a 1W resistor and not 5W resistor?
Not only on vitage restorations. Generally speaking.

I mean which is the criteria for a smaller wattage?

[Kat Manton]

Originally Posted by SirGus

Then why someone would use a 1W resistor and not 5W resistor?
Not only on vitage restorations. Generally speaking.

Two things, price and size. Higher power resistors are bigger than lower power ones. They also cost more. So, the designer calculates what power the resistors are dissipating then specifies the next power rating up.

But, modern resistors of the same power are physically smaller than they were; so in restorations it's common to fit resistors which look the same size, but happen to handle higher power. Small modern ones just look silly under an old valve radio/TV chassis...

Remember, the power rating of a resistor is the maximum power it can dissipate without burning up. The actual power it dissipates in circuit is the product of the voltage across it and the current through it. P = V x I

Cheers, Kat

[SirGus]

You are right Kat.
That's something I did't think...price!

You see when you are doing a restoration you don't care if you spend some euros more or less...but that's not the same with handling a company!

Originally Posted by G4ILN

If in doubt measure the voltage across the resistor and multipy it by itself, then divide by the value in ohms of the resistor. This gives the power dissipated.

Just did it!
I found that the resistors are *about* 0.5 W

So you are all right that 1 W is the perfect choice!

[Ed_Dinning]

Hi Costas, as a general rule a resistor will have a surface temperature of 70C when dissapating nominal power in still air at 20C.
(In case you feel like doing some experiments)

Ed

[SirGus]

so...the wattage for a resistor is just the maximum wattage that the resistor can handle.

right?

[Ed_Dinning]

Hi Costa, Yes.
Note that some resistors are made to run at higher temperatures, but these are usually wirewound and covered in vitreos enamel.
If you were going to put a resistor next to a hot valve, it would be good practice to under-run it. That is, if the circuit needed 0.2W, fit a 0.5W resistor.

Ed

[Sideband]

Originally Posted by Kat Manton

Two things, price and size. Higher power resistors are bigger than lower power ones. They also cost more.

Quite so!

Manufacturers will save a fraction of a Euro (or whatever currency they work in) in order to cut total cost of a finished product. Imagine producing a TV, Hi Fi system or anything like that and using .25 watt resistors that are (say) €0.5 then find that the same product can be produced with .1 watt resistors that only cost €0.2 saving €0.3. Now multiply that €0.3 by the number of units produced (say 500,000) and you see that the €0.3 Euro saved works out to a significant sum!

Rich.