electronic circuit (diode)?
Monday, May 18th, 2009 at
11:17 pm
i'm struggling with this question ,i couldnt find the value of R1 and R2 and if u have any info about how i can use pspice program plz tell me
the question details is inside the file below.
http://www.freewebs.com/radicalfunction/Questions.doc
plz help me
Filed under: Electronic Circuit Diodes
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The meter will have 2 volts across it (20 mA * 100 Ohms)
The zener diode node (between R1 and R2) will be at 4.7 volts with 0 current flowing through the diode (according to the way the problem is stated).
So… R2 will have 4.7volts on one side, and 2volts on the other side with 20 mA flowing. (4.7 – 2)/0.02 = R2
Same thing applies to R1. One side has 25 volts, the other side has 4.7 volts, and 20 mA is flowing (because none is flowing through the zener). (25 – 4.7) / 0.02 = R1
For Vs >= 50volts, change the value of R1 to: (50 – 4.7) / 0.02 = R1
As for PSPICE, there isn't a whole lot of room in this answer box to give you a lot of help. But, here's a MicroCap SPICE netlist for you:
D1 0 2 1N750
R1 1 2 2265
R2 3 2 135
R3 0 3 100
V1 1 0 DC 0 AC 50 0 SIN 0 50 1000 0 0 0
*
*** 4.7V 500mW Zener Diode
.MODEL 1N750 D (IS=5.72393F BV=4.7 IBV=20M RS=19 CJO=159.081P VJ=750.016M
+ M=490.331M)
*
.OPTIONS ACCT LIST OPTS ABSTOL=1pA CHGTOL=.01pC DEFL=100u DEFW=100u DIGDRVF=2
+ DIGDRVZ=20K DIGERRDEFAULT=20 DIGERRLIMIT=0 DIGFREQ=10GHz DIGINITSTATE=0
+ DIGIOLVL=2 DIGMNTYMX=2 DIGMNTYSCALE=0.4 DIGOVRDRV=3 DIGTYMXSCALE=1.6 GMIN=1p
+ ITL1=100 ITL2=50 ITL4=10 PIVREL=1m PIVTOL=.1p RELTOL=1m TNOM=27 TRTOL=7 VNTOL=1u
+ WIDTH=80
*
.LIB "C:\MC8\LIBRARY\NOM.LIB"
*
.TEMP 27
.TRAN 0.0002 10m 0
.PRINT TRAN V(3)
.PLOT TRAN V(3) -0.75,3
.PROBE
.END
The options section refers to many things in MicroCap ver8, as does the library — you'll have to take the basice netlist at the top and the model for the 1N750 and apply them to your PSPICE program.
.
Are you talking about the R1 and R2 in the ps controller, if you are looking for them you'll find at the southeast in the controller.
Use Ohm's law to find R2 as follows: The zener limits the voltage to 4.7v. So, full scale has to happen when voltage is at 4.7v. Current to have full scale deflection is 20ma. The internal meter resistance is 100 ohms. (by the way, which is it for full scale, 15 or 20ma? the paragraph says 20, the diagram says 15 and there is a smaller 20 next to the text). So, E=IR says: 4.7v=0.02a(100+R2 ohms), now solve for R2. I leave the math as an exercise for the reader. Once you have figured R2, use Ohm's law again to find out what R1 must be for the voltage divider to have 4.7v. This should get you started on finding the solution to your homework. Hint: at 20ma and 25v supply, there is a 4.7v drop across R2 and the meter and 20.3v drop across R1. From this, you should be able to calculate the values of R1 and R2. For the 2nd part, where they change the voltage to 50v, simply substitute and solve for R1. The voltage drop across the meter at full scale current is always 4.7v and the current is always 20ma, so only the value of R1 changes with a different voltage at the supply terminals while R2 remains a constant. Remember, the zener is a voltage limiter. It conducts nothing until you reach the breakdown voltage at which point it starts to conduct. The higher the voltage attempts to go, the more current is shunted around the meter, so in essence, the voltage is and current are constant across the meter.and R2.
I have no idea what PSpice is, so I can not help you there.
The key to the problem is in the zener diode. No current will flow in the diode until the diode sees more than 4.7 volts.
To get full scale on the meter apply 25 volts and have 20mA flow. At this point the diode should not be conducting but have 4.7Volts across it. Any less voltage applied to the input and you will have less voltage at the diode (still no current flow through diode) and less current through the meter.
Apply more voltage to the input and the zener will start to conduct shunting the extra current through it and keeping 4.7V across R2 and Rm.
So to get full scale on the meter you need 20ma current which will be applied with no more than 4.7Volts (limited by the zener)
Use Ohms Law using 4.7V and 20mA to compute a total resistance then subtract the meter resistance to get R2.
Now that you know R2 compute R1 again using Ohms Law and Kirchoffs law ( Series circuit current is the same and voltage divides across resistors)
You have 25 Volts applied at the input (0mA through the zener and 20mA through the meter at 4.7 Volts) So R1 drops the difference in voltage at full scale current.
You should be able to compute both resistance values.
Hint: since the diode is not conducting when 25V or less is applied you can ignore it and consider the schematic as a simple series circuit. With 4.7V 20mA across R2 + Rm
And 25V Across R1 + R2 + Rm