acmultimeteroscilloscopetransformer
I have a 220 – 14 V transformer, but when I hook it up to oscilloscope or DMM they show different values. DMM shows 11.8V and oscilloscope shows 17.2.
Ok, this is cheap Chinese transformer, so it can possibly be not accurate, but what the hell with readings? I know my equipment kinda low-class and low-accuracy, but how can it have such a big difference? What is the reason and what reading is more accurate?
It's a little unclear what exactly you are trying to accomplish. If the point is just to experiment with full wave bridges and see all the signals, then do it at a low voltage like JonnyBoats said. You can probably find a old transformer somewhere that is rated in the range 6-12V AC at 1 Amp or so. That's a good size to put a full wave bridge after, and then you also get a useful DC voltage to do lots of other things with.
As Jonny mentioned, there are also wall wart type transformers that are rated for less power but you can do mostly the same things with. The advantage is that these things are cheap nowadays. You can probably find something in the 3-5 Watt range for $5. Jameco has a broad selection of such things. That's a good place to look around.
If you really want to experiment with a full wave bridge driven by a function generator, then you should power the function generator from a isolation transformer. These are 1:1 transformers meant to take line power in and put line power out, except that the output can float. These will cost a bit more since they are usually intended for 100 W or more. Sometimes they even come in a box with a line cord for the primary of the transformer and a regular output connected to the secondary. You simply plug the cord into a wall outlet, and the thing you want isolated into the outlet on the box.
One gotcha you have to be careful of with these things is that they may not come wired up fully isolated. I bunch of years ago I bought a 500 W "isolation transformer" that was just as I described above. I used it to float a device under test so that I could hook up a grounded scope to it at various places. The first time I touched the scope ground clip to part of the power supply there was nice spark and the fuse blew. It turns out there was actually a deliberate ground wire inside the isolation transformer box connecting the ground from the line cord to the grounds on the output sockets. That's not what I consider "isolation", but someone else apparently does. Once I disconnected the two sides of the transformer and carefully verified there was no conduction path, it worked as intended.
Here's how you read the oscilloscope waveform. I took the time to edit your waveform picture and annotate it to show which bit is which. The microcontroller sends 10 bits per character; START, which is always 0, 8 data bits and STOP which is always 1. The line also rests at 1, so the first START falling edge alerts the receiver that a bytes is coming. The bits are sent LSB first, so if you want to "take a look" at them aritmetically, you need to mirror them horizontally for them to make any sense. The width of each bit is determinated by the baudrate, and the transmitter and receiver must both know what the baudrate is.
From the below picture you can see that it sends three characters: ASCII character 'A', a carriage return and a line feed.
Best Answer
The oscilloscope shows peak values and the DMM shows RMS or average values.
$$\frac{17.2V}{\sqrt{2}} = 12.162237 V$$