Electronics > Beginners
The big mystery...
Belosguardo:
Hi there, this is my first post. So be gentle. I just starded electronics.
For my first real project, i’m building a power supply. I'm doing tests before going further in my project.
Here is my problem. I’m recycling an old computer power supply cage and connector to the main (120v 60hrz). The green wire (earth) is connected to the cage, the hot wire to a fuse then a switch before it is connected to a toroidal power transformer. The other wire (neutral) is connected directly to the toroidal connector. When i test the voltage from the output of my toroidal transformer, i get 13.6v RMS when the switch is on, and 0 volt when the switch is off.
So everything seems normal. Then, i want dc from that power source, so i connected the two AC output to a simple bridge rectifier (4 diodes). Here is where the magic happens: I get 12volts dc if the main switch is on, and 5 volt dc if the switch is off!! How is this possible?
Thanks for your help. I'm stuck!
ArthurDent:
Try placing a 1K ohm or some low value resistor across where you measure the 12VDC. See if the 5VDC disappears when the switch is now turned off. You could also unplug the power cord and see if it disappears.
It probably is just stray capacitive coupling and the high input resistance of the meter isn't low enough to discharge the small leakage that you are probably seeing.
MarkF:
F.Y.I.
A PCB layout for 12V @ 1A.
It has protection diodes and minimum_load/power_LED.
Audioguru again:
You showed a far away photo of a bunch of wires on a solderless breadboard instead of a clear schematic so we do not know how you connected the diodes.
Without a filter capacitor then your "12VDC" is jumping up to about +18.5V and down to 0V at 120Hz and the average is about 12V.
Belosguardo:
Thanks everyone for the quick input. First, i have to say that when the main source is not connected, there is no voltage. So i don't think the 5 volt when the switch is at off comes from capacitance (i may be wrong). So far, i did not put any capacitor in the circuit to filter de dc variations . I will later on.
I'm just trying to understand the basics before i continue my circuit.
right now, when the switch is on, i get 13,7 DCV RMS, 20 volts max peak at 120hz (58% duty cycle)
When the switch is at off, i get 5.6 volts RMS DC at 60 HZ (23% duty cycle)
And when i unplug the main, i get no voltage!.
The circuit is simple for now, but i really don't get why i have 5 volts when the switch is at off.
thanks
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