[ARC5] Resistor Wattage

[Richard Knoppow]

    There were quarter watt resistors in the 1940s. See old 
catalogs. However, the stability of carbon comp resistors is 
dependent partly on not heating them too much. This was well 
understood back then so larger ones were used to extend life. 
Modern film resistors, either carbon or metal, are far more 
stable and tolerant of heating. They are also smaller for a given 
dissipation. Check sizes of current products. Its generally good 
practice to use parts with some margin so check what the required 
dissipation really is before using half value replacements, make 
sure the originals were not marginal.
     There are few applications where carbon composition has any 
advantage over film resistors, mainly in certain kinds of pulse 
circuits. Carbon film is less expensive than metal film (but not 
by a lot) and are very stable but metal film are the best where 
temperature stability is critical and where minimum excess noise 
is desired. Note that all resistors generate noise due to thermal 
agitation but certain types, particularly composition resistors 
produce noise in excess of the "Johnson" noise, carbon film 
resistors very little and metal film none.
     Also, deposited film resistors generally have better RF 
performance then composition resistors although that is often not 
well understood. When I had a working Boonton RX meter (looking 
for another) I confirmed this by measuring a great many resistors 
of various types and values.

On 3/28/2019 9:44 AM, Robert  Eleazer wrote:
> In rebuilding the GRC-9 receiver I have found a number of 
> locations where it would be both easier and more convenient to 
> use 1/4 watt resistors rather than the presumably 1/2 watt 
> originals. I'd have to do some more investigation and take some 
> measurements to determine if that would be feasible.
> 
> But it led me to wonder.  Were 1/4 watt carbon composition 
> resistors even in existence as an design option back in the 
> 1940's and early 1950's?
> 
> Wayne
> WB5WSV


-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL