21 Feb
2011
21 Feb
'11
3:16 p.m.
> But... electronic ballasts are switching devices
and the current
> is not steady-state. If twice the current flows for half the time,
> then the power dissipation doubles (Remember, I-squared-R, you doubled
> I, so I squared goes up by a factor of four.)
Let's do the math. Suppose the current went from
0.5A to 4A.
Probably, the 2A fuse in series with this thing would blow. If that
didn't happen, the resistor would be called on to dissipate about 5W--
on a transient basis, the resistor could handle this; on sustained
basis, the resistor would overhead and discolor and the PC board
would also be discolored. That didn't happen.
The resistor is a plain-Jane 2W light-brown painted body with orange-
orange-silver-gold markings. No letterings or other notations, no
discoloring--just two paint chips missing from the body.
I can post photos if you want.
You might have heard the rule of thumb that "in the case of an overcurrent
fault, the most expensive component in the circuit will burn out to protect
the fuse". That rule is only slightly facetious. In reality almost any component
(no matter what the $ value) will burn out to protect the fuse :-). You might
think I'm being facetious but I'm not!
I still suspect that, for example, bad bulbs or some other ballast failure had
taken out the resistor.
Fluorescent ballasts and CFL's are not known as places where manufacturers
spend many dollars on high quality components to increase lifetime and
decrease the possibility of failure.
Resistor wattage ratings are not always mathematically related to the maximum
voltage or current that you should put on/through the device. At the high ohm end
(say a 22MOhm 1/2W resistor, which would imply 3300 volts) the calculated voltages are
more than the bodies are rated for, and at the low ohm end (especially for
lowest-bidder resistors) there can be weak spots in film resistors that become
"hot spots" in transient overcurrents you might think the resistor should be
able
to survive.
Good metal films can survive massive overload, and all they do is glow bright orange
and burn off their outer coatings. But I strongly hint above, that such good components
will not generally be found in ballasts or CFL's.
Tim.