Confirmation needed...

[Bob Engelhardt]

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]

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

 

[Bob Engelhardt]

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.

 

[Phil Allison]

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]

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.

 

[Bob Engelhardt]

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.

**   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.

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!