Hi Guys
It’s a Christmas Miracle! ... and an unwanted one!

I’m looking at a scope connected to an independent output winding through a toroid, and expecting a negative going pulse every six seconds.
This is working fine.
How is it possible that if I reverse the connection to the scope, the pulse is still negative going, and looks exactly the same when I changed nothing else?

Cheers, Brek.

 

On your scope are you using AC or DC coupling for the vertical input?. Is the scope vertical input single ended like most of differential input?

Ron

 

The secondary output of a transformer has no apriori GROUND reference. Both sides are floating and it will look like what ever is happening on the primary. Without a ground reference you can't really tell what is positive and what is negative -- can you? Unless your probe GROUND is internally connected to the Scope's Chassis GROUND, then you have defined one side or the other of the secondary output as GROUND. This can be dangerous on a completed circuit and should be avoided. I've seen whole scopes go up in smoke from doing this.

 

Hi
I mean the end of the transformer winding connected to the centre pin of the probe looks negative with respect to the
other end of the transformer winding connected to the outer sheath of the probe (the alligator clip).
Then if I reverse the probe connections for the same winding, the pulse still goes negative.

It makes no difference to what I see if the channel is AC or DC coupled.
The circuit is battery operated and shares no reference with the scope other than the transformer winding.

To get it to look positive I just have to make another toroid with ordinary ferrite and make the two windings
turn in opposite directions don’t I? I still don’t understand though.

 

My shift register is eight cores (4 bits).
When I send positive current to the dot side of the shift winding a the bottom,
I see a pulse out of the right side if the core was set to one. All of this works well,
but I expect the dot side of the output winding to be positive with respect to the non dot side.

The unwanted effect is I can’t see it with my sense circuit (a 2N3904).

and what do you mean floating? For there to be a potential difference on the output
winding, there has to be a positive and negative for the moment the pulse occurs.

 

Floating means "not referenced to GROUND". Connecting a scope probe to a circuit may or may not create a GROUND reference. It depends on the scope. For a true differential look at the secondary connect two different probes to the secondary winding, invert one channel and add them together as a single trace.

DANGER! If your scope has probes that are grounded to the scope chassis which is connected to earth GROUND and you connect a ground clip to +800V you should be prepared for smoke and flame and death (of the scope) and destruction. Know what you are connecting to! I cannot stress this point enough.

 

What is the duration of the post? Does the pulse still exist if you load the transformer with a 1 k to 10 k resistor?

I am guessing that you are seeing capacitive coupling from the primary to the secondary of the tranformer and not the output from the transformer winding.
If it is then the pulse will likely be shorter in duration than the input to the the transformer and it will go away if the transformer secondary has a load on it.