Brian Gaff (Sofa) wrote:
Fuses do not blow for no reason, so be careful shove a bulb in series with
it when you put it on!
Brian

Fuses *do* blow for no reason.

Electrical loads have transient current flow level
and steady state current flow level. There is more
to selecting a fuse, than "just meeting steady state".

https://www.littelfuse.com/~/media/a..._fuseology.pdf

"I2t [I*I*T] is an expression of the available thermal energy resulting
from current flow. With regard to fuses, the term is usually
expressed as melting, arcing, and total clearing I2t. The units
for I2t are expressed in ampere-squared-seconds [A2s]."

During the startup transient, the I2t can be quite large.

Slo-Blo fuses are typically used when you want a rugged response.

On a motorized appliance, all that adding a light bulb will
do, is cause the bulb to come on full brilliance, while
the motor remains stalled (shaft stall equals high current state).

I would start by "ohming" the load, to see if it reads
a low number of ohms, or see whether it reads as a dead short.
Due to semiconductors being present in the circuit, it
may not be possible to get "honest" readings, depending
on where you're probing. You could pull the motor connector
and ohm the motor - but then that doesn't tell you anything
about an upstream PWM thing (or whatever form the control
might take). With switching semiconductors, some of those will
fail in dead short state, and it ends up burning other,
harder-to-replace, analog components (powdered core inductor,
maybe a capacitor, the MOSFET itself burned to a crisp). Ohming
that section might not tell you anything, whereas a visual inspection
may already spot incinerated devices and a burnt smell.

I've had one semiconductor, where the failure
was violent enough, the silicon die inside the device
was blown off its anvil. And the tell-tale sign was
a "clink" noise when the ON switch was pushed. That
was the silicon die bouncing around inside the
packaging. I never applied an ohmmeter to the
device after removal - just shaking it and hearing
something flopping around inside was enough.

I don't have an answer for every DIY circuit repair
you attempt, but I can tell you that the light bulb
method may not give a meaningful result. If there
is a semiconductor control in the way, then behaviors
can be quite non-linear. In the old days, you could
pull some of the devices and put them on a curve tracer,
but curve tracers were almost "out-of-style" when I
got my first job after university. Later jobs, there
just wasn't one of those on the premises. I think I've
run the curve tracer, only once or twice in the lab.
(You can get a shock off a curve tracer, be careful
to not become part of the circuit.)

Paul