They can be inserted by the same machines that insert resistors- note
the tape. They are also easy to insert by hand if you buy the bulk
kind, if you keep to the usual spacing. I designed the things into
dozens of products..
Of course the "1/4W" is an indirect reference to the body size.
If the amount of current that results in dissipating 1/4 watt into the
"resistor" does not significantly heat the copper foils that it is
soldered to, then it appears to me that this is a safe current.
Than again, in typical usage this is .7 inch of wire, and it appears to
me that the leads of a 1/4 watt resistor are 24 AWG (or .5 mm dia.) tinned
copper, resistance at 100 degrees C is approx. 1.8, maybe 1.9 milliohms...
Current that results in 1/4 watt dissipation here is around 11-11.8 amps
- keep in mind guidelines for what the foil traces can hangle - I suspect
that will be the limiting factor.
Also - power ratings of mil-spec resistors are often half that of
commercial grade resistors of the same size. This limits a "1/4 watt"
"zero" to somewhere around 8 amps (or the amount of current that the
circuit board traces can handle, whichever is less).
**********************
Power rating of a resistor may depend on heatsinking effects of either
the leads or *near-resistor-body* circuit board traces that are oversized
for the amount of current actually being conducted. This may mean that a
1/4 watt zero ohm resistor may only safely pass 8-11 amps when its ends
are in close contact with conductors that don't have much of a temperature
rise at this amount of current.
Keep in mind that my "measurement" of the wire size of a "1/w4
watt" resistor might be "off by a size", and that I am not especially
certain that 1/4 watt zeros aren't made with AWG 26 (.4 mm dia.) wire. A
..7 inch length of that dissipates 1/4 watt at 100 degrees C at somewhere
around 9-9.5 amps, and 1/8 watt at 100 degrees C at somewhere around
6.25-6.7 amps.
Also, unless you have circumstances unusually favorable to 24 gauge
wire (such as heatsinking of a very short piece by something much
thicker), the safe current carrying capacity will be closer to whatever
the "norm" is for 24 gauge wire. I do not know this figure for sure, but
I believe it is somewhere around 2 amps.
(Transformer windings with AWG 24 may be safe only at much lower
currents like around .5 amp due to a large assembly of adjacent turns all
producing heat, heat from other windings, and heat from the core.)
- Don Klipstein (Jr.) (
[email protected])
