The big mystery... - Page 1
Forum section: electronics / beginners

author: Belosguardo
author_rank: Newbie
timestamp: August 06, 2019, 12:40:22 pm
content: 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!

author: ArthurDent
author_rank: Super Contributor
timestamp: August 06, 2019, 03:43:44 pm
content: 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.

author: MarkF
author_rank: Super Contributor
timestamp: August 06, 2019, 04:26:29 pm
content: F.Y.I.A PCB layout for 12V @ 1A.It has protection diodes and minimum_load/power_LED.

author: Audioguru again
author_rank: Regular Contributor
timestamp: August 06, 2019, 04:48:59 pm
content: 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.

author: Belosguardo
author_rank: Newbie
timestamp: August 06, 2019, 08:39:39 pm
content: 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

author: rstofer
author_rank: Super Contributor
timestamp: August 06, 2019, 09:04:30 pm
content: Plug it in, turn it off and measure the primary voltage (carefully). Is there a chance the switch is defective?In theory, the transformer should have the same primary voltage when switched off as it does when unplugged. Why isn't that happening?

author: CatalinaWOW
author_rank: Super Contributor
timestamp: August 06, 2019, 09:22:58 pm
content: Quote from: Belosguardo on August 06, 2019, 08:39:39 pmThe capacitance referred to is parasitic capacitance. One of the things you will quickly learn is that though we have components called resistors, capacitors and so on, those are just the names for the primary characteristic of the part. They all have capacitance, resistance, inductance in varying quantities. Sometimes those minor characteristics can safely be ignored. You have clearly hit on one of the cases where they cannot.It is still not clear which minor characteristic you are coming across. It could be leakage resistance across the switch, it could be capacitance across the switch or a couple of other possibilities. Depending on the physical arrangement your body capacitance could even be involved. Putting a load on the circuit (1K ohm was suggested) is a way of understanding how big this minor characteristic is. You should try the suggestion, and perhaps try a couple of other values (maybe 10K and 100K).You can also do things like measuring the resistance across the switch when it is open and measuring the capacitance.

author: ArthurDent
author_rank: Super Contributor
timestamp: August 07, 2019, 12:08:14 am
content: As I said, and CatalinaWOW said, the effects you are seeing are very likely from stray, or by another name, parasitic capacitance. The capacitance is quite small so that any real load like a 1K ohm resistor will cause the voltage you are seeing on the output of the bridge on the secondary with the switch off to drop to almost zero. My suggestion to unplug the cord verifies that the voltage you are seeing is coming from the power line coupling.Here is a video that does a good job of explaining this effect in detail.

author: Brumby
author_rank: Unknown
timestamp: August 07, 2019, 03:59:58 am
content: As stated above, capacitance is a property that exists outside of the component called a capacitor. In fact, it exists between any two conductors in the universe.For the most part, however, the separation distances give rise to a capacitance value which, while it could be calculated, is unmeasurable and completely ignorable.It's when things get closer that you can start observing some strange phenomenon. Capacitive coupling between the primary side and the secondary side of a transformer isolated circuit is just one such example. The voltage detected will have a very high impedance, but with 10M ohm (or greater) input impedance of your typical DMM, there is still enough EMF to register.This phenomenon is fairly common and there are many DMMs (not all) that will have a low impedance voltage range (Lo-Z) that will add enough of a load so as to cause this "ghost" voltage to be effectively shorted out. (These loads are not so low in value to cause major current flow if the voltage is a real one from a comparatively low impedance source.) The above suggestion of using a 1K resistor** is intended to provide just such a load when using a DMM without a Lo-Z range.** Please note that this simple resistor tip has been suggested with some basic knowledge of what your circuit looks like - and while the principle is much the same, there are circumstances when a 1K resistor would not be appropriate - or safe. Until you are more conversant with dealing with electricity at this level, I would suggest you not start choosing such resistors on your own.