Current meters

Editor’s note: A recent article by Nigel Calder (“Electrical Tests,” Issue No. 51) discussed the use of a multimeter in analyzing and troubleshooting electrical problems. The following builds on that article and provides further operating details of both analog and digital multimeters.

Measuring actual current flow is one of the things that makes having a multimeter so valuable. For example, a multimeter can be used to run down the cause of the slow depletion of charge in a vessel’s battery bank. The other functions, like measuring voltage and resistance, are also useful, but measuring current (amps) can yield answers to many vexing electrical questions.

Most analog multimeters are equipped with a means for measuring DC flow. For example, the meter shown on page 83 of Issue No. 51 has three current measuring ranges: 500 microamps (500 mA, 500 millionths of an amp), 30 and 300 milliamps (mA, thousandths of an amp). These ranges, however, are likely too low to be of much use in working on many of the circuits found in a typical 12-volt electrical system.

To obtain usable current ranges, two methods are generally used: 1) Multimeters are available that offer a 10-amp current measuring range. This range will allow direct measurement of the actual current flowing in many of a vessel’s circuits. Or, 2) an external shunt can also be made for a multimeter, which will allow measurement of relatively high DC flows.

The current that will flow through a masthead, tricolor light equipped with a 25-watt, 12-volt bulb will be approximately two amps. To measure the current in the circuit, it’s necessary to break into the circuit and place the ammeter in series with the load. As shown in figure 1, this is most easily done by bridging the switch or circuit breaker which controls the circuit, or removing the fuse and bridging the fuse with the meter leads. The polarity of the test instrument leads must be as shown in the sketch. In the event an incorrect polarity is selected, the meter needle will move to the stop at the left, or zero end of the scale. (This won’t damage the meter – the current flowing in the meter’s coil is within safe limits.) Multimeters are generally equipped with an internal fuse which protects the meter movement. These fuses are not the same as typical circuit fuses, they are fast acting. Meters that offer a high current measuring range, such as 10 amps, will likely have an additional fuse that protects the high current shunt (a shunt is a precision, low-value resistor placed across the terminals of an ammeter to increase its range). When purchasing a meter, check to see which fuses are used and carry spares. It is usually possible to tape a few spares inside the meter enclosure.

When a partial short circuit exists, or a low-current consumer is inadvertently left on, even a very small current flow can drain the batteries. If such a condition is suspected, turn off all consumers, turn the master switch to the off position and bridge the open switch contacts with the ammeter. Any current flow will be immediately apparent. Should a current flow exist, the next step might be to open all branch circuit breakers, or remove all branch circuit fuses and then bridge each breaker or fuse with the ammeter. The meter will identify which branch circuit is carrying the unwanted current. Connections for this test are shown in figure 2. External shunts

Since a 10-amp, full-scale reading will likely be too low for direct measurement of some current drainsandmdash;for example, that of an electrically-powered head pumpandmdash;it can be useful to adapt a multimeter to measure higher current flows. To do this, an external shunt is needed which, in conjunction with the most sensitive current scale in the meter, will allow direct measurement of high DC currents. As shown in figure 3, a DC ammeter operates by allowing most of the current to flow through a piece of fairly heavy wire, while a small portion of the current flows through the sensitive meter. The flow of direct current in the circuit shown in figure 3 will be in inverse proportion to the ratio of the resistance of the two paths, that of the shunt (very low resistance, therefore, high current) and through the meter (high resistance, therefore very low current).

If we need to have a 50-amp full-scale capability for some measurements, but only possess a meter with a 500-andmicro;A, full-scale current sensitivity, a shunt is required that will allow 49.99995 amps to flow through the shunt, while 0.00005 amps (50A) flow through the meter. As an alternative to using a relatively expensive multimeter when measuring high currents, it’s possible to build one’s own ammeter for less than $15.00. Since great precision is not needed when making most high-current measurements, an inexpensive panel meter, such as the 15-volt DC meter sold by Radio Shack stores can be used. This meter is, in reality, a milliammeter with a full-scale current of 0.001 A (1 milliamp). The meter is supplied with a 15,000-ohm resistor, which, when placed in series with the meter movement, will provide full-scale deflection when 15 volts is applied. (Using Ohm’s law, I = E/R; 15/15,000 = 0.001A.)

To properly design the shunt, we need to know the internal resistance and the full-scale deflection current rating of the meter. This information is usually stated on the data sheet that accompanies the instrument. The data for the Radio Shack meter mentioned above says the internal resistance is 80 ohms. Since the current flow will divide inversely to the resistance, as shown in figure 3, the greater current will flow through the circuit having the least resistance. Thus, the ratio of shunt resistance should be equal to the inverse of the ratio of the desired current flows. Figuring capability

For a full-scale capability of 150 amps, the current flowing through the shunt must be 150 amps minus the 0.001 amp that will flow through the meter movement. This ratio of current flows is 150,000:1. Since the resistance of the meter is 80 ohms, the resistance of the shunt must be 1/150,000 of 80 ohms, 0.0005333 ohms. Don’t go to the local electronics parts store and ask for an 0.0005333 ? resistor, they won’t have one. Fortunately, such a resistor is readily available if one has a short length of no. 12 AWG solid copper wire. This gauge wire has a resistance, at 20anddeg; C, of 1.588 ohms per 1,000 feet. Since we only need a length whose resistance is 0.0005333 ohms, we need 0.3358522 feet, or 4.03 inches of wire. For practical purposes, a 4-inch length of wire will suffice. This is one instance where the use of solid conductor wire on board a vessel is preferred. The actual length of wire required will be slightly longer, to allow for terminals at the ends with which to connect the shunt into the current-carrying circuit. While the total length of the wire used to make the shunt is not critical, the distance between the taps on the shunt for the multimeter must be reasonably precise. The shunt connections are shown in figure 4.

When built, the shunt can be simply a straight piece of wire, with the connections as shown, or it can be built into a small plastic box, with four terminals, two for the main current flow and two for the meter connection. If the shunt is placed in a box, avoid the temptation to wind the shunt wire into a neat, little, multi-turn, coil. If wound into a multi-turn coil, an inductor will be created, where it is not needed or desired.

There may be times when an ammeter with a full-scale reading of 75 is needed. Making the shunt twice as long as required for the 150-amp full-scale instrument will result in a shunt with twice the resistance. Twice as much current will flow through the meter. By using a two position switch to connect one of the meter leads to either the four-inch point or to the eight-inch point (actually 8.06 inches) on the shunt, a two-range meter results. Obviously, by using a shunt with three times the resistance, three times the length, and using a three-position switch, a meter with full-scale ranges of 50, 75 and 150 amps will result. A 15-inch length of bare copper no. 12 wire can easily be formed into a two turn coil having a diameter of less than three inches. No. 12 solid copper wire is reasonably stiff, and it can be formed into a suitable coil and will be self supporting, so that it doesn’t short from one turn to the other.

If precise calibration of the shunt/meter combination is desired, it can be connected in series with another meter, of known accurate calibration, and one of the taps moved along the shunt wire until the current reading on the meter being calibrated matches the standard instrument’s reading. When the 150-amp, full-scale meter is in use and when measuring the full 150-amp current flow, almost 12 watts will be dissipated in the shunt wire. With this dissipation, the wire will become quite warm after only a few minutes. It will be best to limit the time of use at full current flow to only a minute or two. This should present no problems since most high current loads, such as anchor windlasses, only operate for a relatively short time.

If a 450-amp shunt were desired, the shunt resistance would have to be 1/3 that of the 150-amp shunt. To obtain a resistor having a value of 0.0001777 ohms, use a length of no. 6 wire, with the taps for the meter spaced 5.4 inches (5.396″) apart. When measuring the full 450-amp current, the voltage drop across the shunt will be 0.0796 volts; the power dissipation, 36 watts. Although current measurements in this range are not often required, the capability to make such measurements, using materials from the vessel’s electrical “junk box” can allow troubleshooting of items such as anchor windlass motors, starting motors, etc. Since operation of a 450-amp load for more than a short time is unlikely, the heating of the shunt by the 36 watts shouldn’t be a problem. Doing things digitally

The digital multimeter is an extremely valuable part of the equipment on board any vessel. It complements the analog multimeter, allowing some important measurements to be made with higher accuracy and ease. Not long ago, digital multimeters were large, expensive, laboratory devices. (The first digital voltmeter I used, in 1957, occupied two cubic feet, weighed more than 60 pounds, cost 1.5 times the price of a new Cadillac Fleetwood and made one measurement every three to five seconds.) The development of large-scale integrated circuits, which can incorporate the entire meter circuitry on one or two chips, plus a liquid crystal display, have made very valuable instruments available at prices between $20 and $100.

One of the most useful on board capabilities of a digital multimeter is measurement of battery bus voltage, and the voltage of each individual battery in the bank. The voltage range between a fully charged lead-acid battery and an effectively discharged battery is only 0.8 volts: 12.6 volts fully charged, 11.8 volts when totally discharged. Accurately measuring this relatively small range of voltage on an analog meter, using the typical 25- or 30-volt full-scale range, is difficult. The accuracy of most low-cost analog multimeters, operating on the DC voltage scales, is on the order of +/- 2 to 3% of full scale. Were the 25-volt scale in use, the inaccuracy could amount to 0.5 volts for the 2% case, 0.75 volts for the 3% meter. Obviously, depending on readings with this much possible inaccuracy for determining the absolute state of battery charge is a questionable practice.

However, simply because a digital meter provides specific numerical readings, don’t believe what it says without question. All measuring instruments have accuracy limits, or inherent inaccuracies, depending on whether one is an optimist or a pessimist. Read the following and then carefully examine the data sheet for the multimeter in use. Virtually any digital multimeter will provide DC voltage accuracy on the order of andplusmn;0.1%, +/- 1 digit. The typical meter will read to two decimal places, for example, 12.60 volts. A voltage reading may be expected to be accurate to 0.1% of full scale, 0.001 x 25 = +/- 0.025 volts, +/- 1 digit on the display. An actual voltage of 12.60 volts might actually be as low as 12.60 andndash; 0.025 = 12.57, – 1 digit = 12.56 volts. The corresponding high reading might be 12.60 + 0.025 = 12.63, + 1 digit = 12.64 volts. In either of these extreme cases, the accuracy of the measurement is much improved over what can be expected from the analog meter. Somewhat less accuracy will be achieved when making measurements of other than DC voltage. These increased errors will not present problems in shipboard use of the meter.

The typical digital multimeter will also offer AC voltage scales, resistance scales, and DC current scales. A few may even offer temperature measurements, using extra cost, plug-in test probes. Temperature measurements which can be made remotely, such as checking temperatures at alternators on engines, can be very valuable. You might even hang the probe over the side when navigating the Gulf Stream, where water temperature readings provide very useful navigation information. Measurement without contact

Some digital multimeters also offer a special form of AC current measurement capability called a clamp-on ammeter. This type of meter will allow measurement of current flowing in AC circuits, such as those powered from an on-board genset or shore power. The ability to directly measure the current in such circuits can be of great value in troubleshooting AC-powered refrigeration and air conditioning systems, microwave ovens, and other AC devices. Where two 30-amp shore power inputs are provided, or where a single 50-amp, four-wire system is used, maintaining a balance of the AC loads is important.

The use of the clamp-on ammeter is simplicity itself. The only requirement is that the two wires which constitute the AC circuit must be at least a few inches separate from each other. The measurement is made by opening the jaws of the clamp-on part of the instrument and allowing them to close around one of the wires in the circuit. It makes no difference which wire is chosen, so long as the wires go only to the consumer being checked. Where genset output or shore power is being checked, select the “hot” wire, usually color-coded black or red, not the neutral, which may be colored white. The wire should be insulated. It is not necessary and it is unsafe to clamp the meter around a bare, current-carrying wire. The insulation on the wire will have no effect whatever on the current reading. The clamp-on element is actually an electrical transformer. The wire around which the clamp is placed becomes the primary winding of the transformer. A multi-turn coil of wire within the instrument is the secondary winding. The flow of alternating current in the load-carrying circuit wire induces a small magnetic field in the metal laminations of the clamp, which is the core of the transformer. In turn, this varying magnetic field induces a higher voltage in the multi-turn coil which constitutes the secondary winding of the transformer. The resulting AC voltage is then delivered to the measuring circuits in the multimeter and provides a reading of current flow.

Many of the meters which offer this type of AC current measurement also have a sample-and-hold feature. This allows the meter to be placed where it must be to clamp on the current-carrying wire, for example behind the switch panel, where simultaneously reading the display might be impossible. When the hold button is pressed, the reading is frozen on the display and can be read when the meter is withdrawn. Even though it is not necessary to break into or directly contact any uninsulated parts that are carrying AC voltage, it is vital to observe all normal precautions when working around energized “line voltage” (120 or 220 volt) AC circuits. These voltages are high enough to kill!

Digital multimeters with clamp-on AC current capability can be purchased for less than $100. As with most other products, when one spends more money, one can obtain higher quality instruments. The likely differences will be somewhat more accuracy, additional measurement ranges and more robust construction. For most on-board purposes either an analog unit, costing between $20 and $50, or a digital model, costing between $20 and $65, (perhaps up to $100 for one with the clamp-on AC current feature) would be fine.

A word of caution. It is okay to buy one, three, or more digital meters, but never two. When one makes simultaneous measurements with two meters one will never know which unit to believe. Using three or more models, one can take the two meters with the closest readings and choose one of their outputs, or average all the readings. For all practical purposes, however, one meter will suffice.

Contributing editor Chuck Husick is sailor and pilot based in the Tampa area.

By Ocean Navigator