FUSE AND
METER SAVERS
b y B i l l S t i l e s
Don’t Blow Fuses or Your Meter
Here are three circuits which
can save time and prevent
frustration. They go well with “Don’t
Blow a Fuse” by Ronald Newton
(Nuts & Volts, August 2006, page
40). When I built his circuit breaker
box, I added two test leads with
insulated alligator clips for fuses in a
fuse post holder.
My first circuit — shown in Figure
1 — is connected in the AC input of
the equipment being tested, with S1
set to “lamp.” Blown fuses in the
equipment are bridged with Mr.
Newton’s substitution box (or
replaced). If Lmp1 and Lmp2 are
correctly chosen, new fuses will not
blow, even with shorts in the power
supply of the equipment.
If the lamp or lamps light
brightly, there is probably a short.
The lamps are dependent on the
power used by the equipment. The
two lamps shown ( 40 Watt and 60
Watt in parallel) are suitable for
equipment drawing 50 to 70W. For
smaller power use, one of the lamps
can be unscrewed.
I also use a 200W lamp and a
25W lamp, for large and small power
usage. If the lamp(s) light only dimly
— if at all — full voltage can be
applied to the equipment by switching S1 to “direct.” The “off” position
of S1 is used to completely remove
power from the equipment while
you are working on it. I used a
double-pole switch because I have
seen several cases where the neutral
and hot wires are reversed.
SO2 is added for very large
loads. Several years ago, I had a 1/4
horsepower motor with burned
insulation in the start switch. It would
blow a fuse as soon as it was turned
on. I plugged a 1,000W electric
heater into SO2, which prevented
the fuse from blowing and allowed
me to see arcing at the burned insulation inside the switch. Fortunately,
the switch was mounted on the
outside of the motor, where it could
be easily replaced.
I also use this circuit to check for
shorted turns in a power transformer.
Disconnect the output of the
transformer, and use only one 40W
or 25W lamp. A large transformer
may light the lamp dimly from
internal losses; smaller transformers
should not light the lamp at all.
Measuring the Fuse
Current
hot - black
F1
direct
off
S1A
PL1
Lmp1
Lmp2 SO2
lamp
direct
NE1
neutral - white
off
As Mr. Newton mentioned, it is
desirable to connect an amp meter
in series with the fuse substitution
box. A single meter could be used
but a multi-meter with several
current ranges is better. However,
there may be a problem at higher
currents.
Many digital multimeters have a
10 or 20 amp range, but the duty
cycle is limited to something like
“ 30 seconds on and 15 minutes
off.” With Figure 2, a DMM can be
used for high current with no time
limits.
Set the meter to measure AC or
DC voltage, as appropriate. Connect
PL2 to one of the banana jacks on
the fuse substitution box, and one of
the leads from the fuse
adapter to J3. For current
of 10A, the meter will
read 1.0 volts; for 1A, the
reading will be 0.1 volts
(100 millivolts).
Figure 3 is a circuit
similar to Figure 2, which
SO1 provides an easy way to
connect a DMM to measure the AC input current.
S1B
ground - green
lamp
FIGURE 1. A Current Limiter.
70
April 2007