Need schematic for AC to DC converter (12V AC in to 12-18V DC out) - diyAudio
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Old 5th January 2012, 02:03 PM   #1
maurycy is offline maurycy  United States
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Default Need schematic for AC to DC converter (12V AC in to 12-18V DC out)

I am working on a project that needs between 12V and 18V 2A DC. I have the following transformer available:

in: 120V 65W
out: 12V AC 4170mA

Does anybody know where could I find a schematic for the AC to DC converter that would give me DC output between 12-18V at 2A? I can't design it myself as I have very little knowledge of electronics.

Thanks.
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Old 5th January 2012, 11:55 PM   #2
! is offline !  United States
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It would help if you were more specific about what you need it for, and any critical parameters it must meet.

For example, if you merely connect that transformer to a typical bridge rectifier with a filter capacitor after it, you will have a DC voltage between 12V-18V at 2A already, there is no boost circuit needed for that. See "Full-wave Rectifier with Smoothing Capacitor" on this page: Full Wave Rectifier and Bridge Rectifier
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Old 6th January 2012, 01:25 PM   #3
maurycy is offline maurycy  United States
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Thanks for replying. The power supply is needed for small guitar amplifier I am building. The amp is 18W max at 18V. I have a switching power supply but thought that reusing one of the existing torroids I have would be better. The full wave rectifier you linked to is simple to build and should suit me well. I am using something similar for my gainclone.
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Old 6th January 2012, 03:43 PM   #4
! is offline !  United States
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Whether the circuit I linked will be all you need depends on the noise rejection level of the amp circuit. If it causes buzzing you could then add an LM1085 regulator (reference circuit in its datasheet) to drop the minimum voltage, about 1.2V at 2A. At that power level it will need a modest sized heatsink.
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Old 6th January 2012, 04:01 PM   #5
maurycy is offline maurycy  United States
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I have looked at the specs for the amp and it calls for filtered power supply to reduce hum.
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Old 6th January 2012, 06:35 PM   #6
! is offline !  United States
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Well, that is subject to interpretation. How filtered a power supply is or needs to be has been a subject of debate for decades and will continue to be one for decades longer.

The capacitor in the first schematic linked is the most important filter, it greatly reduces (doubled 60Hz mains AC frequency) 120Hz ripple, but there will still be some remaining. On low level amps like headphone amps that is usually a problem. On power amps like gainclones using chipamp ICs, it's not nearly as significant.

However the low dropout regulator I mentioned previously should provide sufficient filtering for this application unless the guitar amp is entirely discrete without feedback integral. It is the simplest way to add enough filtering that it would require methodical testing/equipment to see a difference rather than hear it - but again this is an area of debate that has been ongoing for decades. Some people will hear a difference in an amp even if all you do is turn it off and on again but tell them something more has changed. Other times people with good hearing can distinguish differences from minor noise reduction if they listen carefully.

You have to take it on a case by case basis and decide for yourself if the result is acceptable. More filtering can be added later by just putting another filter stage inbetween the PSU you start with and the amp board.

While I haven't built any guitar amps I will speculate that the first schematic with a linear regulator after it is going to result in a better filtered power supply than most guitar amps use. At that point, tweaking the guitar amp itself is likely to make more of a difference than tweaking the power supply further.

Last edited by !; 6th January 2012 at 06:39 PM.
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Old 6th January 2012, 09:27 PM   #7
maurycy is offline maurycy  United States
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Thanks for detailed reply. I will try the simple circuit first as I already have the capacitor and rectifier bridge is available at Radio Shack and see how that goes. Considering that I haven't heard a completely silent guitar amplifier yet, this may be all I need.
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