Nuts & Volts - April 2007

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).