A N A L O G A N D D IG I G IT I T A L M E T ER ER S
ANALOG VS. DIGITAL METERS
U ltima tely, your dia gnosis gnosis of vehicle vehicle electrical system problems will come down t o using a voltmeter, voltmeter, a mmeter, or or ohmmeter t o pinpo pinpoint int th e exa exa ct locat locat ion ion of th e problem. problem. There There a re tw o ty pes of of each m e t e r —a —a n a l og og a n d d i g i t a l . An a l o g m e t e r s u s e a n e e d l e a n d calibr calibr ate d scale to ind ind icate values. Digi Digital tal m eter eterss d isplay isplay th ose values valu es on a digital d isplay.
conn conn ected ected t o. These meters a re known a s a u t o -r -r a n g i n g m e t e r s . O t h e r d i g i t a l meters require t he operat operat or t o selec selectt t he p r op op er er r a n g e . I n a n y c a s e i t i s i m po por t a n t to lea lea rn t he symbols symbols used used in a digital readout so you you can int erp erpret ret th e reading. The electrica electrica l unit s of measu re sym bols bols are: M for for m eega ga or million million K for for kilo or th ousan d m for f or milli mill i or one-th one-th ousan dth u for m iicro cro or on e-mill e-millio iont nt h
The three types of meters—voltmeters, a m m e t e r s a n d o h m m e t e r s — co co n n e c t t o th e circuits circuits or device devicess in different w a ys. This is n ecessary ecessary to get a ccura ccura te m e a s u r e m en en t s a n d t o p r ev ev en en t d a m a g e t o th e meters. VOLTMETERS— ANALOG AND DIGITAL
This chapt er w ill help help you you und ersta nd how th ese meters work as w ell a s the a d v a n t a g es es a n d d i s a d v a n t a g es e s of of ea ea c h . B efore efore using using a meter, rea rea d the manufacturer's operating instructions. R e a d i n g a n a l og og m e t e r s u s u a l l y r e q u i r es es simple mental calculations. For example, a m e t er e r m i g h t h a v e t h r e e vo vo l t a g e r a n g e s : 4.0 V, 20 20 V a nd 40 V, but only t w o sca sca les: 4.0 V an d 20 V. V. In order t o use th e 40 V ra nge, you need need to multiply t he needle reading on the 4.0 V scale by 10 (or for th a t ma tt er, the 20 V sca sca le by 2) 2). Digita l meters meters a re usually simpler simpler to read a nd ma ny w ill a djust djust to the prope properr ra nge requir ed for for th e circuit circuit or device device they are
Voltmeters m easure voltage or volta ge drop in a circuit. Volta Volta ge drop can be used t o locat locat e exce excessive ssive resista nce in th e circuit circuit w hich could cause poor poor performa performa nce or or improper opera opera tion. La ck of volta volta ge at a given given point point ma y indica indica te a n open open circuit circuit or gr ound. On t he other ha nd, low low volta volta ge or or high volta volta ge drop, drop, m a y i n d i c a t e a h i g h r e s i s t a n c e pr pr o b bll em em like a poor conn conn ect ect ion. Voltm oltm eters must be connec connected ted in par a lle llell w ith t he device device or circui circuitt so tha t t he m e t eerr c a n t a p o ff f f a s m a l l a m o un un t o f current. Tha Tha t is, t he positive positive or red lead is conn conn ected ected t o th e circuit circuit closest closest t o the positive positive side of of th e bat tery . The The nega tive or black lead is connected connected t o ground ground or th e nega tive side of of th e circuit. circuit. If a voltm voltm eter is connected connected in series, its i ts high r esista nce w ould reduce reduce circuit circuit current a nd cau se a false reading.
A N A L O G A N D D IG IT A L M E T ER S
B eca use voltmeters ar e alw ay s hooked t o a circuit in par a llel, they become par t of th e circuit a nd reduce th e total resista nce of th e circuit. If a voltmeter ha s a resistan ce tha t is t oo low in compar ison t o th e circuit, it will give a fa lse mea surement. The false read ing is due to th e meter cha nging t he circuit by l ow e r i n g t h e r e s i s t a n ce , w h i c h i n c r ea s e s
th e curren t flow in t he circuit. The effect a voltmeter ha s on th e circuit t o which it is a tt a ched is sometimes referred to a s " loadin g effect" of th e meter. The loa ding effect a voltmet er ha s on a circuit is determined by the tota l resista nce of the circuit in rela tion to th e impeda nce of th e voltmeter. Every voltmeter ha s an impeda nce, w h i ch i s t h e m e t e r 's i n t e r n a l r e s is t a n c e. The impedan ce of a conventiona l a na log voltmet er is expressed in " ohms per volt." The am ount of resista nce a n a na log voltm eter represents t o the circuit chan ges in relation to the scale on w hich it is placed. Digita l voltmeters, on the ot h e r h a n d , h a v e a f i xe d i m pe d a n c e w hich does not cha nge from scale to sca le a nd is usu a lly 10 M ohms or more.
Impeda nce is th e biggest difference betw een a na log an d digital voltmeters. Since most d igita l voltmeters ha ve 50 t i m e s m or e i m pe d a n c e t h a n a n a l og voltmeters, digita l meters ar e more a c cu r a t e w h e n m e a s u r i n g v ol t a g e i n h i g h r e s i s t a n c e ci r cu i t s . For example, if you are using a low impeda nce (20,000 ohm s per volt) a na log meter on th e 20 volt scale (th e voltm eter represent s 400,000 ohms resist a nce to the circuit) to measur e volta ge drop a cross a 1,000,000 ohm component in a circuit , tw o a n d a h a l f t i m es a s m u ch c u r r en t i s f l ow i n g t h r o u g h t h e m et e r t h a n t h r o ug h th e component . You a re no longer measu ring just th a t component, but th e component plus y our meter, givin g you a false reading of the a ctua l volta ge drop across the component. This situation might lead you to believe the volta ge at the component is low or t ha t t here is high resista nce somew here in th e circuit or th a t t he component is defective w hen it is just t he meter you are using. If you use a digita l meter wit h 10 million ohms of impeda nce to test t he sam e component , only 1/10 of t he curr ent w ill f l ow t h r o u g h t h e m et e r , w h i ch m e a n s i t ha s very little effect on th e circuit being measured.
A N A L O G A N D D IG IT A L M E T ER S
AMMETERS— ANALOG AND DIGITAL
Am m e t e r s m e a s u r e a m p er a g e , or current flow, in a circuit, an d provide i n f or m a t i o n on c u r r en t d r a w a s w e l l a s circuit cont inuity . High current flow i n d i ca t e s a s h o r t c i r cu i t , u n i n t e n t i on a l ground or a defective component. S ome ty pe of defect h a s lowered t he circuit r e s is t a n c e. L ow cu r r e n t f l o w m a y indicate h igh resista nce or a poor connection in th e circuit or a dischar ged bat tery. No current indicat es an open circuit or loss of power.
Ammeters m ust a lwa ys be connected in series w ith t he circuit, never in para llel. Tha t is, a ll the circuit current m ust flow th rough th e meter. It is conn ected by a tt a ching t he positive lead t o the positive or ba tt ery side of th e circuit, a nd t he nega tive lead t o nega tive or ground side of t h e c i r cu i t , a s s h o w n . C A U T I O N : T h e s e m e t er s h a v e ex t r em e ly l o w interna l resistance. If conn ected in parallel, the current ru nn ing through th e parallel branch created by the m eter m ight be high enough to dam age the m eter along with th e circuit the m eter is connected to. Also, since all the current will fl ow th rou gh th e a m m et er w h en it is con n ect ed be sure tha t th e circuit current will n ot exceed th e m a x i m u m r a t i n g of t h e m e t er .
There is not a grea t difference betw een a n a l og a n d d i g it a l a m m e t e r s . D i g i t a l meters a re often capa ble of measur ing s m a l l er c ur r e n t s , a l l t h e w a y d o w n t o microa mps. They a re easier t o use beca use th ey give a specific va lue, eliminat ing th e need t o interpret t he a na log meter's needle on it s scale. G e n er a l ly s p e a k i n g , m os t d i g i t a l a mmeters a re combined wit h a voltmeter. OHMMETERS— ANALOG AND DIGITAL
An ohmmeter is powered by a n int erna l batt ery tha t a pplies a sma ll voltage to a circuit or component a nd m eas ures how m u c h c u r r e n t f l o w s t h r o u g h t h e c ir c u it or component. I t then displa ys t he result a s r e s i s t a n ce . O h m m e t er s a r e u s e d f or
c h eck i n g c on t i n u i t y a n d f o r m e a s u r i n g th e resista nce of components . Zero resista nce indica tes a sh ort w hile infinite resista nce indicat es an open in a circuit or device. A read ing higher th a n t he specification indicates a faulty component or a high r esista nce problem such a s burnt cont a cts, corroded termina ls or loose connections.
A N A L O G A N D D IG IT A L M E T ER S
Ohmmeters, beca use they a re selfpow ered, must never be conn ected t o a pow ered circuit a s th is ma y blow a fuse in t h e m e t er a n d d a m a g e i t s b a t t e r y . U n l es s th e circuit being mea sured cont a ins a diode, polarity (a tt aching t he leads in a par ticular order) is inconsequent ial. An a n a l og o h m m e t e r s h o u l d b e ca libra ted regularly by connecting t he tw o lea ds together a nd zeroing the meter with th e ad just knob. This compensa tes for cha nges in the sta te of cha rge of the internal batt ery. C A U T I O N : A n a l o g oh m m e t er s m a y a p p l y a higher voltage to a circuit t han a d igital ohm m eter, causin g dam age to solid state components. Use analog ohm m eters with care. Digital m eters, on th e other h an d, ap ply less voltage to a circuit , so d a m a g e i s l es s l i k e l y .
M a n y a n a l og oh m m e t e r s w i l l , w h e n sw itched t o the ohm function, reverse the polarity of the t est leads. In other w ords, the red lea d ma y become negative and the bla ck lea d ma y become positive. The meter w ill function properly a s long a s you are aw a re of this a nd reverse the leads. This is especially importa nt w hen w or k i n g w i t h d i od e s or t r a n s i s t o r s w h i c h a re polarity sensitive an d only a llow current t o flow from th e positive to the nega tiv e end. To check for polar ity reversal, set th e ohmm eter in ohm function an d connect it s leads t o the lea ds of a v oltm eter (red t o red, black to bla ck). If th e voltmeter shows a negat ive value, t h a t p a r t i c ul a r oh m m e t e r r e v er s e s polarit y in ohm function. Most digita l meters do not r everse pola rity .
Ana log meters can a lso bias, or turn on, semi-condu ctors a nd cha nge th e circuit by a llow ing current t o flow to oth er port ions of the circuit. Most digita l meters ha ve a low volta ge setting w hich will not bia s semiconductors an d a higher volta ge sett ing for test ing semiconductors. The informa tion displayed on a d igital meter in t he diode test function differs from one meter brand t o a nother. Some digita l m e t e r s w i l l d is p la y a v a l u e w h i ch represents t he perceived resista nce of th e diode in forw a rd bia s. Other meters w ill display the forw a rd bias voltage drop of the diode. Digita l ohmm eters do ha ve one limitat ion. Due to the small a mount of current t hey pass t hrough th e device being tested, t hey can not check some semicond uctors in circuits , such a s a clam ping diode on a r elay coil.
You should note tha t ohmmeters d o little good in low resista nce, high current ca r r y i n g ci r cu i t s s u c h a s s t a r t e r s . Th e y cann ot find points of high resista nce beca use they only use a sma ll amount of cu r r e n t f r o m t h e i r i n t e r n a l b a t t e r i es . I n a la rge conductor (such as a ba tt ery cable), th is current meets little resista nce. A volta ge drop test durin g circuit opera tion is much more effective a t loca tin g point s of high r esista nce in t his t ype of circuit. Ta ken w ith permission from the Toyota Advanced Electrical Course#672,
A N A L O G A N D D IG IT A L M E TE R S
A S S IG N M E N T
NA ME:
1.
E x pl a in h ow r ea d i n g a n An a l og m et er d if fer s fr om a D i gi t a l m et er.
2.
E x pl a i n t h e f ol low i n g el ect r i ca l u n i t s of m ea s u r e s y m bol s ( M , K , m , u ).
3.
List th ree t ypes of met er s.
4.
D es cr ib e h ow v olt m et er s a r e con n ect ed t o a cir cu it .
5.
E x pla i n how “ m et er loa d in g” a ff ect s t he cir cu it .
6.
D es cr ib e “ m et er im ped a n ce” a n d h ow i t ef fect s a ci rcu it ?
7.
L is t t h e f ixed im ped a n ce v a l ue of a d ig it a l vol t met er.
8.
E x pl a in h ow t h e im ped a n ce of a d i gi t a l met er d if fer s fr om a n a n a l og m et er .
9.
D es cr ibe how a m m et er s a r e con n ect ed t o a cir cu it .
10.
E x pl a i n h ow a n a l og oh m m et e r s d if fe r f r om d ig it a l oh m m et e r s i n s et u p.
11.
E x pl a i n w h a t p reca u t i on s on e sh ou ld t a k e w h i l e con n ect i n g a n oh m m et e r t o a circuit.
