AC rectification trouble.. voltage on open circuit

[d4n13l]

Hello,

So I´m taking electronics as a hobby, so I´m pretty new to this stuff. I´m working on making my own power supply. The circuit starts with the AC rectifier part.




T1 is a 24v, 5A transformer. So when the switch (S1) is closed I can measure the 24V between A and B. What I don't understand is that when it is open, I still can measure 6.5v between A and B.

How could this be explained?

[WaveyDipole]

Although I'm logged into my Dropbox account I can't seem to view the image? All I get is:

Error (403)
It seems you don't belong here! You should probably sign in. Check out our Help Center and forums for help, or head back to home.

[d4n13l]

Sorry about that, the image should show now.

[Towz]

My guess is capacitive coupling between primary and secondary, it should disappear after a load is connected.

[Brumby]

What reading do you get if you unplug the power cord, coil it up and sit it next to your project?

[d4n13l]

Ok so I hadn´t notice this before. I shorted the terminals after disconnecting the circuit. I made a new reading between A and B and got 0v. The 6.5v are present in the open circuit only after I turned it on and off once. So I guess the voltage is stored in the transformer when I close the circuit. Is Capacitive couple the name for it?

[Cerebus]

My guess is capacitive coupling between primary and secondary, it should disappear after a load is connected.

Indeed. It's worth mentioning that the meter the OP is using will have either 10Mohm or possibly several Gohms input impedance so although he's measuring 6.3 volts it's at a negligible current. The easy way to test this is to use his meter on a current range and measure the current being output when the switch is open - I'll bet it's bugger all.

Just tried it with a loose toroidal 0-230V/0-25V transformer that was handily sitting two feet away and into a 10 meg meter it registered about 5Vrms to ground on one leg of the secondary with just one side of the primary connected to live. Same setup but current range recorded 31 uA to ground, which by my calculations is 429pF capacitive coupling and my dodgy capacitance meter (1% accurate on a good day) says 387pF - close enough for jazz.

[d4n13l]

Cool. Just learned a new thing. Thank you all.