ANALOG AND DIGITAL METERS - Autoshop 101

1 ANALOG VS. DIGITAL METERSU ltimately, your diagnosis of vehicleelectrical system problems will comedown to using a voltmeter, ammeter, orohmmeter to pinpoint the exact location ofthe problem. There are two types of eachmeter ANALOG and DIGITAL . ANALOG METERS use a needle andcalibrated scale to indicate values. DIGITAL METERS display those values on adigital display. This chapter will help you understandhow these METERS work as well as theadvantages and disadvantages of each. Before using a meter, read themanufacturer's operating ANALOG METERS usually requiressimple mental calculations. For example,a meter might have three voltage V, 20 V and 40 V, but only two V and 20 V.

2 In order to use the 40 Vrange, you need to multiply the needlereading on the V scale by 10 (or forthat matter, the 20 V scale by 2). DIGITAL METERS are usually simpler toread and many will adjust to the properrange required for the circuit or devicethey are connected to. These METERS are known asauto-ranging METERS . Other digitalmeters require the operator to select theproper range. In any case it is importantto learn the symbols used in a digitalreadout so you can interpret the electrical units of measure symbolsare: M for mega or million K for kilo or thousand m for milli or one-thousandth u for micro or one-millionth The three types of METERS voltmeters,ammeters and ohmmeters connect tothe circuits or devices in different is necessary to get accuratemeasurements and to prevent damage tothe METERS .

3 VOLTMETERS ANALOG AND DIGITALV oltmeters measure voltage or voltagedrop in a circuit. Voltage drop can beused to locate excessive resistance in thecircuit which could cause poorperformance or improper operation. Lackof voltage at a given point may indicate anopen circuit or ground. On the otherhand, low voltage or high voltage drop,may indicate a high resistance problemlike a poor connection. Voltmeters must be connected in parallelwith the device or circuit so that themeter can tap off a small amount ofcurrent. That is, the positive or red lead isconnected to the circuit closest to thepositive side of the battery.

4 The negativeor black lead is connected to ground or thenegative side of the circuit. If a voltmeteris connected in series, its high resistancewould reduce circuit current and cause afalse reading. ANALOG AND DIGITAL METERS Page 1 Toyota Motor Sales, , Inc. All Rights voltmeters are always hooked toa circuit in parallel, they become part ofthe circuit and reduce the total resistanceof the circuit. If a voltmeter has aresistance that is too low in comparison tothe circuit, it will give a falsemeasurement.

5 The false reading is due tothe meter changing the circuit bylowering the resistance, which increasesthe current flow in the circuit. The effecta voltmeter has on the circuit to which itis attached is sometimes referred to as"loading effect" of the meter. The loadingeffect a voltmeter has on a circuit isdetermined by the total resistance of thecircuit in relation to the impedance of voltmeter has an impedance,which is the meter's internal impedance of a conventional analogvoltmeter is expressed in "ohms per volt."The amount of resistance an analogvoltmeter represents to the circuitchanges in relation to the scale on whichit is placed.

6 DIGITAL voltmeters, on theother hand, have a fixed impedancewhich does not change from scale to scaleand is usually 10 M ohms or is the biggest differencebetween ANALOG and DIGITAL most DIGITAL voltmeters have 50times more impedance than analogvoltmeters, DIGITAL METERS are moreaccurate when measuring voltage inhigh resistance example, if you are using a lowimpedance (20,000 ohms per volt) analogmeter on the 20 volt scale (the voltmeterrepresents 400,000 ohms resistance to thecircuit) to measure voltage drop across a1,000,000 ohm component in a circuit, twoand a half times as much current isflowing through the meter than throughthe component.

7 You are no longermeasuring just that component, but thecomponent plus your meter, giving you afalse reading of the actual voltage dropacross the component. This situationmight lead you to believe the voltage at thecomponent is low or that there is highresistance somewhere in the circuit orthat the component is defective when it isjust the meter you are you use a DIGITAL meter with 10 millionohms of impedance to test the samecomponent, only 1/10 of the current willflow through the meter, which means ithas very little effect on the circuit AND DIGITAL METERS Page 2 Toyota Motor Sales, , Inc.

8 All Rights ANALOG AND DIGITAL Ammeters measure amperage, orcurrent flow, in a circuit, and provideinformation on current draw as well ascircuit continuity. High current flowindicates a short circuit, unintentionalground or a defective component. Sometype of defect has lowered the circuitresistance. Low current flow mayindicate high resistance or a poorconnection in the circuit or a dischargedbattery. No current indicates an opencircuit or loss of power. Ammeters must always be connected inseries with the circuit, never in is, all the circuit current must flowthrough the meter. It is connected byattaching the positive lead to the positiveor battery side of the circuit, and thenegative lead to negative or ground side ofthe circuit, as shown.

9 CAUTION: These METERS have extremely lowinternal resistance. If connected in parallel, thecurrent running through the parallel branchcreated by the meter might be high enough todamage the meter along with the circuit the meteris connected to. Also, since all the current willflow through the ammeter when it is connected besure that the circuit current will not exceed themaximum rating of the meter. There is not a great difference betweenanalog and DIGITAL ammeters. Digitalmeters are often capable of measuringsmaller currents, all the way down tomicroamps. They are easier to usebecause they give a specific value,eliminating the need to interpret theanalog meter's needle on its speaking, most digitalammeters are combined with a voltmeter.

10 OHMMETERS ANALOG AND DIGITAL An ohmmeter is powered by an internalbattery that applies a small voltage to acircuit or component and measures howmuch current flows through the circuitor component. It then displays the resultas resistance. Ohmmeters are used for checking continuity and for measuringthe resistance of components. Zeroresistance indicates a short while infiniteresistance indicates an open in a circuitor device. A reading higher than thespecification indicates a faulty componentor a high resistance problem such asburnt contacts, corroded terminals orloose connections. ANALOG AND DIGITAL METERS Page 3 Toyota Motor Sales, , Inc.