7815 Regulators. Voltage margin?

[nonost]

Hi guys! I'm building a BK Butler tube overdrive. Actually the Bajaman version. It's almost the same, the only different it's the tone control (2 pots vs 1 pot) and the power supply. I have a question regarding the latter. Here the power supply section:



The circuit needs 16V AC, ok. As you can see there's a 1N4007 diode in series with the 16v AC, so the voltage drops to around 15.5V. Well, after that diode there is a 15v regulator, and here's where my doubt comes in. Wasn't regulators supposed to have a little margin, let's say 1v or 2v, in order to operate properly? Here it barely has 0.5v. Plenty of people have built this and the "author" is very reputed so I'm sure it's something I'm missing here...

Cheers!

[strewel_peterson]

I think the 16v rms AC will give you something closer to the peak voltage of the AC sinewave (16 * 1.41 ish v) after the diode and reservoir cap.

[nonost]

what??? how is that? I thought it should be lower after the diode...Is it because AC thing? In the DC world after the diode it will drop 0.5v.

Thanks!

[strewel_peterson]

16v AC is a sine wave: it goes from about -22.5 to +22.5 volts: if you take the average of the voltage squared (which cunningly makes the negatives positive, then take the square roiot of that, you get16.

What this means is that at peak positive on the incoming AC you get 22.5 volts, the diode takes 0.5 v or so, and the reservoir cap could therefore charge up to 22v.

So long as the circuit isn't trying to drain the reservoir cap faster than it charges from the AC, you would see a voltage somewhere between 16 and 22  at the input of the regulator - should be high enough to guarantee 15v out.

At least, I think that's what happens!

Cheers.

[nonost]

Wow, I didn't know about that.

How is that a 16v ac goes from -22.5 to + 22.5? I mean, why +-22.5 and not another number?

Thanks!

[strewel_peterson]

The maths-y bits come from assuming the AC voltage is a pure sine wave (which for mains and mains-derived electricity -eg from a transformer psu* - is more-or-less true). The difference between average value and peak value for a sine wave is about 1.41.

* all bets are off as to what waveform you'll get out of a switched mode psu.

The link below probably explains things way better than I'm managing to! I'm in danger of giving you my grossly simplified, and probably not really correct explanations otherwise.

https://electronics.stackexchange.com/questions/175474/why-do-rectified-voltage-boosts-after-adding-a-capacitor 

[nonost]

I wasn't aware of rms and peak values!

Thank you very much, I've learnt a lot today :) And now everything make sense.

[nonost]

I'm getting the same voltage at V+ and V++, is it ok? Since the V+ feeds the opamp, it looks too much.

The circuit isn't connected to anything, though...

Cheers!