I have it soldered together now. It seems to work…
What I don’t quite understand is, why I am only getting 5.2 Volts on the output side? This transformer should output 8V when wired in parallel.
Is it possible that I wired the voltage switch the wrong way? When I set the switch to 230V, I get about 10V. (My AC mains is 120V).
Here is the datasheet of the transformer:
Maybe I am just not understanding something? Sorry, I am new to electronics!
How are you measuring the output voltage?
With a multimeter (Amprobe 37XR-A)… just connected to the AC out terminals.
Which pins, of the device, are you probing between?
The transformer is soldered in place, everything is soldered in place actually, just like in the diagram here
So I just screwed the test probes of the multimeter into the “AC Out” connection, and wired 120V to the “AC In” connection. The switch was set to 115V, which should give 8V AC output, but gives about 5.2V AC. When I set the switch to 230V, the output is ca. 10V AC.
I am pretty certain the switch is wired the wrong way around…
I’ve never seen a test lead with “threads”…
I asked for “pin numbers of the device”, and those are not on the linked drawing.
Your switch is wired correctly.
I assume he is referring to the screw terminals of the AC connector, but is it really relevant?
Also irrelevant.
Your meter is measuring RMS, the transformer specs are Vp. 5.2 Vrms = 7.35 Vp. Perhaps your mains voltage is a little low.
More importantly, what DC voltage do you get after rectification and filtering?
I have not put together the rectifier yet.
But I still believe the switch is wired wrong, e.g. when it is set to 230V, the primary is set to run in parallel, when it is set to 115, it runs in series. @1.21Gigawatts, wouldn’t you expect the RMS voltage to be higher when the primary runs in parallel as opposed to serial? Because that is what I am measuring now:
switch set to 230V: 10V
switch set to 115: 5V
So while the switch is technically not wired wrong, it is showing the wrong voltage. I would need to turn it 180° so that the correct voltage would show.
I hope I am making this clear… I could take a picture of the board and components, if that helps.
From your PCB image that you posted there is no silkscreen indicating which side of the switch you think should be the 115Vac position but it would appear you have that backwards. You don’t mention if you are using a toggle switch or a slide switch. For 115Vac you want the primaries in parallel, for a slide switch you want it in the position closest to the AC In connector. 10 V does seem like the correct no-load value. Your multimeter is an RMS meter but I’m not familiar with that particular one, does it have the option to turn off RMS and measure only peak values? If so, are measuring peak or RMS?
Image is worth a thousand words:
The lever for these kind of switches usually is on the opposite position vs. to which terminals are connected with the center pins.
[quote=“1.21Gigawatts, post:27, topic:8353”]
Your meter is measuring RMS, the transformer specs are Vp. 5.2 Vrms = 7.35 Vp.[/quote]
The voltage values for the mains voltages are RMS on the other hand. That’s a mixed state of affairs. 
… and that is not the case. I guess a picture is worth a thousand words:
I guess I will have to get the desoldering tools out!
Ahhh! One of those slide switches. Then you indeed have it backwards. 
Yep, that’s what I thought. And getting about 10V RMS when in the (wrong) 230V position is then to be expected, I suppose?
The thing is, I had a really hard time reading the datasheet, figuring out which way to orient it. All the pin-outs are there, numbered etc., but they don’t make it obvious in what position what pins are active… at least I didn’t see it!
Here the datasheet of the slide switch, it’s the “MS” one:
Yes, You are correct. I suppose it’s too late to take that statement back. 
( I couldn’t find an emoticon with a dunce hat)
There did not appear to be anything wrong with his circuit or PCB, a backwards switch didn’t occur to me, so I assumed it was an RMS vs peak measurement error. I’ll go stand in the corner for 5 mins. 
Don’t worry, I was typing a transformer 101 reply earlier and had to delete all of it as I was way off track.
The RMS vs Peak voltage is the only thing that makes sense to me now, but I have no clue how one could get that from the datasheet alone.
What is the full part number of the switch you have used?
The part number is C&K V80212MS02Q
OK, I gave the voltage switch a 180°, and now things appear to be correct:
120V AC -> switch at 115V -> 10.2V
220V AC -> switch at 230V -> 8.8V
I used a 125V to 220V step-up transformer to for the 230V setting.
Still, getting 10.2V is quite a bit more than the spec’d 8V, but I suppose it will be different under load?
I will now start to put together the voltage regulator. I will report back when that is done… hopefully with a successful 5V DC ±0.1 output!
Thanks for all your help
please be aware that the mains voltages of the world are harmonized, so every device does work the world over.
Before if was 220Vac/240Vac/120Vac… now it’s 230Vac/115Vac as the average RMS voltages that are expected.
And then there is ‘jitter’ to those due to where you are on the grid and what devices are connected to it (solar grid-tie inverters, big loads, far out in the bush, …). Can be higher or lower by I think 10% easily.
So it can vary a lot.
Yes, that makes sense, of course.
I am just a little worried about supplying too much voltage to the LV Regulator; just don’t want to overheat things.
