Hey guys,
My portion of the circuit has been trying to use a simple idea of using a full bridge rectifier and an AC voltage source to charge a capacitor, and to have a zener diode connected to it so that when the charged voltage of the capacitor reaches a certain value that I want, the zener will let the current through and therefore supply the output resistor. Refer to the screenshot I put as reference(values I used are different though).
Now, in order to implement the zener diode set up, I connected two 1n5337B zener diode in series (Vbr of 4.7V). The Izt indicated in the datasheet showed that it carries a current of 260mA each, so I figured that I will need:
(4.7*2 = 9.4V)/ (260mA*2)=18 ohm resistor as my resistor value (R5 in the diagram). I didn't have exactly 18 ohms available, so I substituted with two resistors in series that established 14.7 ohms and I changed the calculation accordingly to see the new voltage:
14.7*260mA*2=7.644V.
I supposed that after I hooked this circuit up, as I ramp up the amplitude of the voltage source, I shouldn't see voltage on the output Resistor(R5) until the AC source reaches 7.644V.
however, when I hooked the circuit up, two issues came up:
1. The zener diode was already conducting at 7.6V- it was giving about 200mV through the resistor. As I dialed down the voltage amplitude to see when it started conducting, it was at about 5.6V when it started to conduct.
2. Also-as you can see in the oscilloscope readout, the capacitor and diode readouts have a big spike, in the same phase as the positive ramp of the voltage source. I don't understand- isn't the whole point of the bridge rectifier to use both the plus and minus side of the voltage source? Why would this be happening? I found out that substituting the charging capacitor with some huge value (like 1000 uF) made the spikes disappear, but the max voltage, capacitor voltage, and the zener diode voltage all went down in magnitude by about 1V or so. Besides, having a 1000uF cap might be too big(in terms of size)- i was hoping for nothing bigger than 100uF.
I looked also at this post
http://forum.allaboutcircuits.com/showthread.php?t=7209
to look for answers, but I couldn't find the answer to my issue at the moment.
Right now, one thing that comes to mind is that changing the resistor value might have altered the Vbr- however, should it change it that drastically?(from 9.4V to 5.6V)
another thing that I noticed is that the diodes itself hold a Rz value(as indicated in the linked post), and it said in the datasheet that it was about 2 ohms each. However, would that have also changed the results that drastically? I actually substituted the resistor to a 20 ohm to reflect this (18 ohms + 2*2 ohms = 22 ohms- matched it as much as possible), and it raised the conducting voltage by about 1V (so 6.6V), but still not at the rated voltage value of 9.4V.
Any suggestions or help will be appreciated, as always.
Thanks for your help!
My portion of the circuit has been trying to use a simple idea of using a full bridge rectifier and an AC voltage source to charge a capacitor, and to have a zener diode connected to it so that when the charged voltage of the capacitor reaches a certain value that I want, the zener will let the current through and therefore supply the output resistor. Refer to the screenshot I put as reference(values I used are different though).
Now, in order to implement the zener diode set up, I connected two 1n5337B zener diode in series (Vbr of 4.7V). The Izt indicated in the datasheet showed that it carries a current of 260mA each, so I figured that I will need:
(4.7*2 = 9.4V)/ (260mA*2)=18 ohm resistor as my resistor value (R5 in the diagram). I didn't have exactly 18 ohms available, so I substituted with two resistors in series that established 14.7 ohms and I changed the calculation accordingly to see the new voltage:
14.7*260mA*2=7.644V.
I supposed that after I hooked this circuit up, as I ramp up the amplitude of the voltage source, I shouldn't see voltage on the output Resistor(R5) until the AC source reaches 7.644V.
however, when I hooked the circuit up, two issues came up:
1. The zener diode was already conducting at 7.6V- it was giving about 200mV through the resistor. As I dialed down the voltage amplitude to see when it started conducting, it was at about 5.6V when it started to conduct.
2. Also-as you can see in the oscilloscope readout, the capacitor and diode readouts have a big spike, in the same phase as the positive ramp of the voltage source. I don't understand- isn't the whole point of the bridge rectifier to use both the plus and minus side of the voltage source? Why would this be happening? I found out that substituting the charging capacitor with some huge value (like 1000 uF) made the spikes disappear, but the max voltage, capacitor voltage, and the zener diode voltage all went down in magnitude by about 1V or so. Besides, having a 1000uF cap might be too big(in terms of size)- i was hoping for nothing bigger than 100uF.
I looked also at this post
http://forum.allaboutcircuits.com/showthread.php?t=7209
to look for answers, but I couldn't find the answer to my issue at the moment.
Right now, one thing that comes to mind is that changing the resistor value might have altered the Vbr- however, should it change it that drastically?(from 9.4V to 5.6V)
another thing that I noticed is that the diodes itself hold a Rz value(as indicated in the linked post), and it said in the datasheet that it was about 2 ohms each. However, would that have also changed the results that drastically? I actually substituted the resistor to a 20 ohm to reflect this (18 ohms + 2*2 ohms = 22 ohms- matched it as much as possible), and it raised the conducting voltage by about 1V (so 6.6V), but still not at the rated voltage value of 9.4V.
Any suggestions or help will be appreciated, as always.
Thanks for your help!
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