# Confirmation needed

Started by August 29, 2021
```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
```
``` bobenge...@gmail.com wrote:
=======================
>
> 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).
>

** 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

```
```On 8/29/2021 8:48 PM, Phil Allison wrote:
>   bobenge...@gmail.com wrote:
> =======================
>>
>> 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).
>>
>
> ** 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
>
>
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.
```
```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

```
```On 8/30/2021 2:31 AM, Phil Allison wrote:
> 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
>
>
>
>
>
Thanks again - I will take some measurements on the tranny.
```
```On 8/30/2021 2:55 PM, Bob Engelhardt wrote:
> On 8/30/2021 2:31 AM, Phil Allison wrote:
>> 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.
>>
>> **&nbsp;&nbsp; If your 12v tranny is really small, like say 100mA&nbsp; -&nbsp; it will
>> output about 15V to 16v&nbsp; AC off load.
>>
>> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; So would do at a pinch with a puny 10mA DC load .
>>
>>
>> .......&nbsp; Phil
>>
>>
>>
>>
>>
>  &nbsp;Thanks again - I will take some measurements on the tranny.

Thanks again - that's good to know.  The OC voltage is 15v & it doesn't
drop below 12v until loaded to 200ma.  That gives me a lot of working room!
```