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Confirmation needed

Started by Bob Engelhardt 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. >> >> **   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!