140V on a 18V transformer's secondary!!!???

[Bricolo]

I assembled my gainclone in it's new case today, and before connecting the whole thing, I tested the diode bridge (without load)

On the scope, between the + and - of the positive bridge, I got a sine wave, 140V p-p !!!! And 50Hz instead of 100. And the signal is a sine, instead of a rectified sine
I attached a picture of the diode bridge. I measured between the left and the center point.


I don't even know where can 140V come from! It's a 2x18V trafo, so 25V p-p


help!

 

[till]

No load? you measured with a voltmeter with high resistance? You measured something capacitively coupled to the mains?

 

[Bricolo]

No load, only the trafo connected to the diode bridge (and they are only 100V diodes!, can they be hurt?)

I measured with the scope

 

[Bricolo]

one thing: the point connecting the 2 bridges (the 0 point) isn't grounded or earthed yet. Can this change something?

 

[till]

Didn´t you ground it to the scopes ground?

 

[Bricolo]

yes I did, so this isn't the problem


I can't imagine how I can have 140V!!!

 

[till]

please measure with a load, even a small one, a LED and some k or so, that ensures ther IS really a little current flowing.

 

[Bricolo]

I tried, without hoping any good result

I added a 9.5K resistor between 0 and +, and the same for the - side.

Now I have a clean full wave rectified sinusoid, 100Hz, 25.2V
It works OK now, thanks Till



But I'm very affraid of the results I first got
140V p-p, 50Hz sinusoid (not rectified), but the secondaries were clean 50Vp-p (18V RMS) sinusoids

I only have 100V shottky diodes, I wonder if they could have been damaged.
Imagine if I had the caps connected my 50V caps would have exploded


I'm seriously thinking about keeping those 9.5k resistors on the diode bridge, to always have a load.


the strangest thing is that the voltage across each diode was much higher that the secondary, but each diode didn't have the same voltage graph (on the scope). each one was veeery different from the other

 

[till]

my 100V schottkys have no prob with a 87V power supply in my BZLS. Dont you think the caps are resistive enough? or do you use only filmcaps and no electrolytics?

 

[Bricolo]

my 100V schottkys have no prob with a 87V power supply in my BZLS. Dont you think the caps are resistive enough? or do you use only filmcaps and no electrolytics?

Nice, I use the same diodes as you
The caps could have been a high enough load, too.
But I take care of my panasonic FCs, expensive stuff compared to what I usually use

 

[Bricolo]

Am I the only one who had this problem?
This seems so strange, I'm looking for a scientifical explanation but can't find one


Can this come from my shottky diodes? I don't remember having had this problem with my old basic bridge

 

[till]

In case there is no real connection and no current flowing, the scope probe may be connected to something capacitively coupled to the mains?

 

[peranders]

If you measure direct to a rectifier bridge with 10 Mohms you must have REALLY good diodes with low leakage etc. Those diodes don't exist...in power version.

You must have 100 uA- 1 mA at least in order to get a clean signal.

Note also that your bridge is capable of dissipating 2-4 W only if you don't have any cooling. That's not much current, 2-3 A.

 

[Bricolo]

If you measure direct to a rectifier bridge with 10 Mohms you must have REALLY good diodes with low leakage etc. Those diodes don't exist...in power version.

You must have 100 uA- 1 mA at least in order to get a clean signal.

Note also that your bridge is capable of dissipating 2-4 W only if you don't have any cooling. That's not much current, 2-3 A.

Ok, but how can I get 140V!!??

 

[peranders]

If you have 10 Mohms inimpedance of your voltmeter and measure you must now how the whole electric circuit look like. If you count in stray caps between the primary and the secondary windings the circuit you measure on don't look like you think.

Non-conduction diodes are high impedance elements, don't forget that.