# Confirmation needed

25 views

### Bob Engelhardt

Aug 29, 2021, 8:16:26 PM8/29/21
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I'm going to make a 12v DC power supply with a linear reg. I will
half-wave rectify 12v AC and have a smoothing cap. The load is only 10ma.

So: 12v RMS is 17v peak, minus the diode drop of 0.7 is 16.3v peak.
Using a 47uF cap, the ripple will be 3.5v p-p. So the min voltage into
the regulator will be 16.3-3.5 = 12.8. Ripple calculated from Vpp =
i/fC (.010/(60*47e-6).

Am I missing anything?

Thanks, Bob

### Phil Allison

Aug 29, 2021, 8:48:25 PM8/29/21
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bobenge...@gmail.com wrote:
=======================
** Yep two:

1. The drop out voltage of a standard 12v reg IC is not 12.8 - more like 13.8.
2. The AC supply varies and may be be 10% low on occasion.

So, you need a 15VAC tranny.

OR you can make a "voltage doubler" supply with two diodes and two electros.

That will give about 33V DC - minus the same deductions.

..... Phil

### Bob Engelhardt

Aug 29, 2021, 10:41:50 PM8/29/21
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Thank you - that is helpful in 2 ways: pointing out what I missed &
was wrong about, and even more helpful is not having to correct my analysis.

### Phil Allison

Aug 30, 2021, 2:31:12 AM8/30/21
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bobenge...@gmail.com wrote:
>
> > ** Yep two:
> >
> > 1. The drop out voltage of a standard 12v reg IC is not 12.8 - more like 13.8.
> > 2. The AC supply varies and may be be 10% low on occasion.
> >
> > So, you need a 15VAC tranny.
> >
> > OR you can make a "voltage doubler" supply with two diodes and two electros.
> >
> > That will give about 33V DC - minus the same deductions.
> >
>
> Thank you - that is helpful in 2 ways: pointing out what I missed &
> was wrong about, and even more helpful is not having to correct my analysis.

** If your 12v tranny is really small, like say 100mA - it will output about 15V to 16v AC off load.

So would do at a pinch with a puny 10mA DC load .

....... Phil

### Bob Engelhardt

Aug 30, 2021, 2:56:06 PM8/30/21
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Thanks again - I will take some measurements on the tranny.