# Clamper circuit

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#### sammie

Joined Feb 2, 2006
1
I read the previous topics about clamper circuits but i am not quite content.

My problem is:

Take a positive clamper circuit for an example. now, starting with the negative cycle. if our voltage source generates 5v sine wave so , during negative cycle diode is forward biased so capacitor will start getting charged. As the charging time of the capacitor is less it gets charged very fast. When the voltage source reaches maximum of -5 v then capacitor is also charged at 5v. Now voltage source's value decrease so for an example it is -3v and capacitor is at 5v . So at the cathode of the diode 5v is there and at cathode there is 3v so it is reverse biased and capacitor doesn't get discharged easily bcoz now it's rc time constant is very large so it almost retain its 5v . Now the reverse biased resistance of the diode is very large so ideally it can be considered open circuited. so when we measure voltage across resistor it should be 2v and then should increase... but instead it remains -0.7 for whole negative cycle. So tell me where am i going wrong? am i lacking some basic concepts?

thank you in anticipation.

Sammie

#### chesart1

Joined Jan 23, 2006
269
Hi,

I once had a similar problem. While troubleshooting a circuit, I disconnected a power supply [consisting of a bridge rectifier and a capacitor] from the rest of the circuit. When I measured the output of the power supply, I found a sine wave instead of rectified DC. The problem was the lack of current through the rectifier. After I added a load resistor to the power supply, all waveforms were correct.

A diode is forward biased at 0.6 volts according to most text books. If you look at the voltage-current characteristics of the diode you are using, you may find your answer. Add a load resistor to your circuit and see if it's operation is closer to what you expect.

John

#### JoeJester

Joined Apr 26, 2005
4,390
Sammie,

Do you have a clamping circuit in mind? If you do, post the schematic.

A basic clamp circuit is discussed in UIUC Passive Circuits towards the end of the document.

If you attach a capacitor, parallel to the diode, and you still want a clamp circuit, the Xc needs to be high compared to the frequency your working with.

Attached is a basic clamp circuit ... the second attachment is one with a capacitor in parallel. The transient analysis shows the output for a 10 nF or a 1 uF capacitor.

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