Bridge Rectified AC mains and zener diodes

[FusionITR]

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):

**broken link removed**

 

[ljcox]

Your explanation is confusing.

Diodes have an anode & a cathode.

So when you say "positive side of the zener", I assume you mean the cathode. Is that true?

Your waveforms are meaningless since did not say where you connected the Scope 0 Volt line?

Did you connect it to the anode of the Zener? If so, I would expect the waveform at the cathode of D5 to be a line with very little ripple.

 

[ronsimpson]

Is the ground lead on the scope connected? To what?
Connecting the ground lead will probably blow the fuse!
Is you Zener 20 volts?

 

[FusionITR]

Your explanation is confusing.

Diodes have an anode & a cathode.

So when you say "positive side of the zener", I assume you mean the cathode. Is that true?

Your waveforms are meaningless since did not say where you connected the Scope 0 Volt line?

Did you connect it to the anode of the Zener? If so, I would expect the waveform at the cathode of D5 to be a line with very little ripple.

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.

 

[FusionITR]

Is the ground lead on the scope connected? To what?
Connecting the ground lead will probably blow the fuse!
Is you Zener 20 volts?

Scope ground is unconnected as the oscilliscope is earth grounded.

The zener is 18V.

 

[ljcox]

I would use a transformer to isolate the mains & connect the 0 Volt line of the Scope to the anode of the Zener.

The positive is connected to the cathode of a Zener if you want it to regulate the voltage.

 

[ljcox]

Scope ground is unconnected as the oscilliscope is earth grounded.

The zener is 18V.

That is why you're seeing such confusing results.

I suggest to use a transformer as above and connect the Gnd of the Scope to the anode of the Zener.

Then you won't need the maths function.

 

[ronsimpson]

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.

The traces look right to me.

 

[FusionITR]

That is why you're seeing such confusing results.

I suggest to use a transformer as above and connect the Gnd of the Scope to the anode of the Zener.

Then you won't need the maths function.

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?

 

[FusionITR]

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.

The traces look right to me.

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.

 

[ronsimpson]

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

 

[ronsimpson]

You can not build a PFC with a 100uF on the bridge!

 

[FusionITR]

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

So it's just a measurement error? Man I look really stupid So theres must be something else going on with my real PFC circuit.

 

[FusionITR]

You can not build a PFC with a 100uF on the bridge!

This is just a test circuit, my real PFC circuit has a 0.1uF cap after the bridge.

 

[ljcox]

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 you don't want an isolation TF for the circuit, then you could use one for the Scope.

You need a proper reference point for the Scope.

What you will see if you connect the Scope Gnd to the anode of the Zener is a DC voltage of about 18 Volt.

 

[crutschow]

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.