Chapter 3.2 DC ammeters

Transcription

1 Chapter. DC aeter generic D ronval oveent i liited to eauring whatever current produce a. How can we increae the range of current that can be eaured? Conider the following circuit: V We have odelled the aeter a having the PMMC oveent in erie with a reitance which repreent the coil reitance of the particular aeter. f we add a reitor in parallel with the eter we can now develop the following: + V V ; V ; Therefore a value for exit that can be calculated baed on the total current, that we wih to eaure. We call a hunt reitor. t purpoe i to bypa or hunt exce current greater than around the eter oveent. Exaple: Deign an aeter to eaure 00 uing a 50µ PMMC eter oveent with internal reitance kω. Solution Uing the ae circuit arrangeent a before: EE Handout Chapter. 00

2 kω Ω Ω 000 Ω Note: Since the total eter current i o uch higher than the capability of the PMMC oveent, we ut bypa a ignificant aount of it. We would expect that the hunt hould therefore be of all reitance in coparion to the PMMC. Multi-range eter n practical ter, aeter with a ingle range are not very ueful; Soe exception Marine eter-voltage, fuel Power tation eter - voltage, frequency utootive eter - aeter, tachoeter ll of which have one ueful range How can we now ake an aeter to eaure everal range at once? One approach i to have a eparate hunt reitor for each range and we can calculate each reitor value of the hunt a we jut did. Conider the following circuit: 4 4 co By the poition of the witch, one of to 4 would be connected a a hunt acro the eter. Can you identify a proble with the above arrangeent? What happen if a the witch i oved fro poition to poition ay, it oentarily loe contact with both and while till carrying current? EE Handout Chapter. 00

3 t that point, the PMMC oveent will be forced to pa a current that ay be >>. Thi will ot likely detroy the eter, or at bet blow a fue. Thi can be olved in way ake-before-break witch Univeral or yrton Shunt 4 To hunt reitor bank The ake-before-witch etablihe contact with the next contact poition before loing contact with the exiting connection. n thi anner, the hunt reitor are never reoved fro the circuit and the PMMC oveent i alway protected. The Univeral or yrton hunt. Conider the following circuit arrangeent: co Here, the hunt reitor,, and, are all in erie and collectively in parallel to the eter oveent. Thu: S + + Let u connect input : EE Handout Chapter. 00

4 co We can then ay that: S ( + + ) where S i the hunt current for thi witch poition Note that thi i identical to our previou calculation. Let u now analye the aeter in witch poition B co Note that for thi ituation, i now in erie with the eter and and for the hunt. Therefore: VB ( + ) S + + ( ) where S i the hunt current for thi witch poition n witch poition we have: ( ) EE Handout Chapter. 4 00

5 co Therefore, S ( + + ) ( ) where S i the hunt current for thi witch poition Diadvantage The ain diadvantage of the Univeral hunt concern the hunt reitor. What happen if, or change value or fail? The overall accuracy of thi type of arrangeent i entirely dependent on all of the hunt reitor. Precaution uing an aeter Never connect an aeter acro a ource of ef. Why? Think of the value of and Oberve polarity when connecting the aeter in circuit When uing a ulti-range aeter, tart with the highet range, then decreae to get the highet reading for a particular current. The nearer the, the better the accuracy. EE Handout Chapter. 5 00