I am trying to get a zener diode to work with no interruptions off rectified AC mains, using this schematic:
**broken link removed**
The problem is although there is a large 100uF filtering cap, when the positive side of the zener is close to negative and the negative side of the zener is very negative the zener shuts off and the zener voltage goes to zero. However, the way I am measuring it (+ minus -), the different voltage of the zener cap is still large. Hard to explain with words but here is what the waveforms look like:
Node 1 (CH1, CH2, and Math which is CH1 - CH2):
**broken link removed**
Node 2 (CH, CH2, and Math @ 50V/div):
**broken link removed**
Node 2 at 20V/div (now you can see the zener turning on and off):
Actually by positve I meant the annode of the zener, yes confusing I know, but what I really mean is where you put the positve voltage on a zener.
As for my reference point, my oscilliscope is earth grounded so the ground is unconnected and the trigger is set to the AC line (as it shows in the pics). CH1 is what I use to probe and CH2 is connected to the negative side of the bridge rectifier. The Math waveform is CH1 (probe point) minus CH2 (negative side of the rectifier).
I hope that clears it up, if not please let me know.
In your third picture; When the traces go off the screen the math does not work. While the math is working it looks like the Zener is 18 to 20V.
In the 2nd picture the two traces are about 20V apart.
You are having a hard time seeing the 18V on the power line. This project is connected to the power line.
I'm building an offline boost mode smps, I'm using this experiment to debug a problem so this is the actual application in which I will be using this in, without the isolation transformer.
Is there anything actually wrong with the circuit? As in, as per the schematic above, should I be seeing the zener go in and out of operating at 18V when both the + and - side of the bridge rectifier is low?
In your third picture; When the traces go off the screen the math does not work. While the math is working it looks like the Zener is 18 to 20V.
In the 2nd picture the two traces are about 20V apart.
You are having a hard time seeing the 18V on the power line. This project is connected to the power line.
Are you saying that the only reason why I am seeing that dip in the 3rd pic is because CH1 and CH2 go off screen? And that the 2nd pic is accurate since both CH1 and CH2 don't clip? I thought the 2nd pic was inaccurate because it was zoomed out too far.
Picture 3; 20V/div, at -80V both traces go off the screen. The scope can not read beyond -80V. So -90 looks like -80. -100 looks like -80. At that point (-80V)-(-80v)=0
Picture 3; 20V/div, at -80V both traces go off the screen. The scope can not read beyond -80V. So -90 looks like -80. -100 looks like -80. At that point (-80V)-(-80v)=0
I'm building an offline boost mode smps, I'm using this experiment to debug a problem so this is the actual application in which I will be using this in, without the isolation transformer.
Is there anything actually wrong with the circuit? As in, as per the schematic above, should I be seeing the zener go in and out of operating at 18V when both the + and - side of the bridge rectifier is low?
If it isn't clear, the problem is that the zener common is not the line input common, due to the bridge rectifier. That is why you need to reference the scope common to the zener common, not the mains common. But if you do that without isolation you can blow your scope due to high ground current.
Alternately you can use the scope in a differential mode (A-B with both inputs set to the same sensitivity), and connect one probe to the zener common and one probe to the cathode (plus) side of the zener. That should give the correct waveform (a DC voltage).
A 0.1uF cap after the bridge can give significant ripple unless the DC load current is very low.