# Maximum Supply Voltage Output by Transformer - Sanity Check

First off, I am not an electrical engineer by trade, or even by education. Pretty much everything I know about EE is self-taught, and I know enough to make a hobby out of it, but may be missing basic, core concepts.

I have found an article on how to build an audio amplifier, linked here. I am confused by one of the calculations it presents, under the heading "Find the Maximum Supply Voltage Output By a Transformer" (the link takes you directly there). Here is an excerpt:

Keep in mind that a transformer’s voltage rating only tells you it’s AC voltage output. The DC voltage will be higher after the bridge rectifier diodes on your power supply convert the AC voltage to DC.

The article then goes on to include 1.41 in its calculations, which I recognize as how to deal with peak vs. RMS measurements, I understand where that's coming from, but my question is: is that at all necessary?

Is AC in general by-default expressed in RMS values? Is the "120VAC" here in the US actually closer to 170V peak-to-peak? I was under the impression that all AC expressions where peak-to-peak, and that if a transformer was outputting 25VAC on its secondary, then a full bridge rectifier (and capacitors for smoothing) was just going to output something close to 25V DC. Is that not correct?

Am I missing something else that would be blatantly obvious to an officially-educated engineer?

Is AC in general by-default expressed in RMS values?

Yes. An unqualified AC voltage is usually understood to be RMS. Otherwise, it would be specified as peak-to-zero (Vp) or peak-to-peak (Vpp).

Is the "120VAC" here in the US actually closer to 170V peak-to-peak?

120VAC mains is about 170V peak-to-ground. It's actually double that, close to 340V peak-to-peak, as it swings from -170V to +170V.

... that if a transformer was outputting 25VAC on its secondary, then a full bridge rectifier (and capacitors for smoothing) was just going to output something close to 25V DC. Is that not correct?

No; the rectified voltage of a 25VAC (RMS implied) input will be just over 35V, minus a bit for the voltage drop in the diodes. About 33-34V.

And potentially much more, the output voltage of a mains transformer is usually specified under its rated load. When open circuit, it commonly outputs a significantly higher voltage, perhaps as much as 150%, so at no-load your rectified DC could be as much as 50V.

Is AC in general by-default expressed in RMS values?

Yes. AC voltages are usually in RMS. So when we say the wall outlet is 120VAC, it means 120Vrms, which is 170Vpk or 340Vpk-pk.

The reason is because it's more straightforward to do power calculations with RMS. If I tell you the wall voltage is 170Vpk, you have to jump through some extra hoops in power calculations. AC current is in RMS for the same reason.

Is the "120VAC" here in the US actually closer to 170V peak-to-peak?

Not peak-to-peak. Just peak, as in zero to either the positive peak or negative peak.

I was under the impression that all AC expressions where peak-to-peak, and that if a transformer was outputting 25VAC on its secondary, then a full bridge rectifier (and capacitors for smoothing) was just going to output something close to 25V DC. Is that not correct?

Not correct. See above. Of course, if a transformer outputs 12Vrms for 120Vrms input, then it will output 1.2Vpk for 12Vpk input. Since the ratios factors are linear.

• "then it will output 1Vpk for 12Vpk input" Can you confirm this? Is it 1Vpk, or 1.2Vpk? Feb 17, 2020 at 2:29
• @RussellUhl Typo. Feb 17, 2020 at 2:45

Well AC voltages are usually rated in RMS, because if you put 10VAC or 10VDC into a resistor, it dissipates equal amount of power and thus generates equal amount of heat.

But it is approximately right that 25 VAC can approximately be converted to 25 VDC. First of all, there is voltage loss at the rectifier diodes, if roughly 1 V per diode, the drop on full bridge is 2V. Then if you have a traditional linear voltage regulator, it needs about 3V more input voltage to work properly. If 25 VAC has 35 peaks, there will be only 33 V max at the bulk reservoir caps, and it can go down to 28 V. That leaves only 5V of ripple voltage, and that with bulk capacitor rating determines how much current you can draw.