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

# Confirmation needed

Started by ●August 29, 2021

Reply by ●August 29, 20212021-08-29

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

Reply by ●August 29, 20212021-08-29

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.

Reply by ●August 30, 20212021-08-30

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

Reply by ●August 30, 20212021-08-30

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.

Reply by ●August 31, 20212021-08-31

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!