POWER MONITOR TESTS--Electronic Test & Alignment Techniques (from early 1970s)


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The power monitor is a relatively simple piece of test equipment in comparison to the multimeter , oscilloscope , signal generator, etc. Yet, in the busy electrical appliance service shop, it is one of the most used units. It provides basic tests and can be used on every appliance that passes through the shop.

For instance, every appliance draws a certain amount of current. That current rating is specified in the service notes and on the appliance itself. Amounts vary from a few milliamps in a transistor radio up to 20 or 30 amperes in an air conditioner as it kicks on.

Current ratings should be correct, not too low or too high.

When they are too low, some function of the appliance is not working. When they are too high, a leak or short is occurring, which signals danger.

The test on a power monitor, like the Sencore PM157, is easy. The monitor is plugged into an AC outlet. The range switch is set on the highest range, which in this case is 10 A. The function switch is set on Amps-Watts. The meter switch is set on Line Cord. The equipment being tested is plugged into the AC outlet on the face of the power monitor marked AC Out.

The appliance is then fed current through the power monitor. The amount of current it draws is shown on the amp scale. The amount is matched to its current specifications and a determination is made as to its correctness. That 's all there is to it.


Appliances are also rated in watts. For instance, an electric iron can be rated as 1200 watts , or a TV can be rated at 500 watts. Here , again, low wattage readings mean the appliance is not "firing up" properly, and higher than usual wattage readings mean an appliance is burning up too much power.

The wattage reading is an instant and accurate indication of the working quality of the appliance.

Most AC lines measure around 115 volts. As a result, power monitors, such as the Sencore PM157, are set for and are accurate at 115 volts AC RMS. If you are somehow checking an appliance on a drastically higher or lower line voltage, you have to convert the reading to the different line voltage.

With 115-volt AC RMS, the actual voltage test is identical to the AC line current test and, in fact, as you read the line current, all you have to do is look at the wattage scale and at the same time get the wattage.

When the line voltage is other than 115 volts, you must use a power-voltage conversion chart. Thus, you can get the power used by an appliance when the line voltage is not 115 volts. For instance, suppose you have a 100-watt light bulb being used with a line voltage of 127 volts. How many watts will be drawn ? Find 100 watts on the normal 115-volt line. Then follow the vertical blocks until you get near 127 volts. The wattage reads about 110 since more voltage is being applied.


Most appliances are designed for the familiar 115-volt input. During service work, the question arises , is the appliance causing problems because 115 volts is not available? Or is the problem in the power supply section of the appliance.

The power monitor produces the answer quickly and accurately. The power monitor is plugged into the suspect AC line and the function switch placed on Line Volts. The line-volt scale is ca librated from 65 to 135 volts because a line voltage under 65 volts is almost impossible to find. The meter reads the voltage directly on the scale.

The line voltage might read 115 volts right on the nose.

You might find such a reading on a cool morning in a trailer camp. That evening, though as the sun goes down and as the trailers became warm from dinner cooking , the lights and the TVs are turned on, the air conditioners started up, and so forth , the 115-volt line could drop drastically to as low as 90 volts. The TV picture shrinks, the electric iron does not get hot enough and so forth. The power monitor , if it is plugged in and a reading taken every hour or so, will reveal the pattern the AC line is following. It tells the servicer that the appliances are probably OK, while the trouble is on the AC line. It better be corrected or the wires can become too hot and possibly cause a fire.


The field serviceman often encounters an open fuse, circuit-breaker or fusible resistor. The question is , has the fusible unit opened due to its own defect or a short circuit in the electrical appliance? The usual reply is a replacement of the fuse or resetting of the circuit breaker. Then the unit is turned on again. If the fuse blows quickly , there is a short in the unit. When the fuse does not blow, it is assumed that there is no further trouble.

Repeated fuse burn out is costly. Just because the new fuse doesn't burn up, it does not mean there is no short. It could be an intermittent tube that shorts as soon as the technician drives away.

It would be much better to measure the amount of current the appliance is drawing across the fusible unit. If a current reading could be made, the following wiser service moves could be conducted. Suppose a stereo radio is found with a blown 5-amp fuse. A reading is taken and the unit is found to be passing about 8 amperes through the fuse connections. Immediately, the fact is known that a bad leak has developed and the cause of the leak should be located before any further tests or fuse replacements are made. Another case in point could be a color TV that is popping its circuitbreaker. The breaker is reset and the TV comes on for a minute or two, then trips the breaker again. A current reading taken across the open breaker may reveal a current of only 5 amperes. Yet it is a 7 ampere rated circuitbreaker.

Obviously, the breaker itself is at fault. It cannot carry its rated current. A new breaker must be installed for the repair. A third common trouble is the fusible resistor. Assume a burned-out resistor is found in an appliance. Instead of trying a new resistor, the current flow through the resistor is checked across the terminals in the appliance. It is a small resistor with a 4-watt rating. The appliance is turned on and the current is shown in the low milliamp range. That means the resistor died of old age and you can be safe putting in a new one.

If, on the other hand, the current was considerable then the resistor had been burned out. The reason for the high current drain would have to be found before a new fusistor was installed, or, the replacement would go right up in smoke.

In a power monitor like the Sencore PM157, fuses are tested by performing the AC line current test, except that the meter switch is set at Test Leads instead of Line Cord. This simply switches the AC output connection to a more convenient position for this test.

Fig. 23. A good power meter can test the exact current that an electronic unit draws to verify fuse capability.

The test leads are applied across the open fuse and the power monitor takes the place of the fusing unit (Fig. 23). The appliance could still be attached to the AC output for its current supply, but the meter will not read unless the test leads are used. The meter is set on the 10A scale. This should be sufficient for most fuse tests.

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