'Project:' Power Supply 15vdc @ 3 Amps

Thread Starter

kbyrne

Joined Dec 10, 2011
92
:D Question: I have a schematic and need assistance to turn into a reality.
Problem: 1. Origional schematic listed 47,000Uf @ 35 vdc for capacitors
C1, C2, & C3 of attached schematic. What is proper parts count. Am I right
in suspecting something amis due to too large of capacitance. Another book states 2000 Uf per amp. Transformer going into this power supply to be a
24vac @ 3 Amp.
 

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Thread Starter

kbyrne

Joined Dec 10, 2011
92
:D Due to pdf file I also uploaded a gif that is easier to view. How do I delete the pdf and keep the gif if need be. It should be easier to view. I have a Eagle 5.11 freeware copper clad board If that is allowed to be upladed. Thankyou and please advise.:eek:
 

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Adjuster

Joined Dec 26, 2010
2,148
I'm sorry to have to say that this circuit looks very wrong - where did it come from? Here are a few comments - not necessarily all that needs to be said.

  • The input is shown as 32.5VAC, which when rectified could give about 44V - far too much for a 15V supply.
  • An AC input is shown with no rectifier apparent. That must be wrong.
  • Three capacitors of unequal value are shown connected in series across the input. Capacitors are not normally used in series.
  • The regulators feed very large capacitances - protection diodes may be needed to prevent damage on switch-off
  • The current sharing between two regulators used in this way may be poor.
 

Sparky49

Joined Jul 16, 2011
833
Perhaps something like this?

(I couldn't find all the symbols I like to use, but I think you get the jist)

Sparky

I've removed the schematic because it was absolute rubbish
 
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Audioguru

Joined Dec 20, 2007
11,251
Perhaps something like this?
The input of a 7815 MUST be positive but you have one with a negative input voltage which will blow it up.

A negative 15V regulator is a 7915.

Usually a 78xx or 79xx regulator has a little 0.1uF to 0.33uF film or ceramic output capacitor, not a huge electrolytic.

EDIT: Your bridge rectifier is connected completely wrong.

EDIT again: added corrected schematic.
 

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Thread Starter

kbyrne

Joined Dec 10, 2011
92
Sorry for the confusion. The schematic came from a post on instructables.com. What I need is not the schematic you uploaded. Due to my needs for two positive 15vdc outputs and two grounds. The power supply has to have approx 3 amps to opperate two audio boards.
Two small amplifiers with tone control for each. One LED VU meter board also. I am trying for a 15vdc Linear power supply which has to handle 3 Amps. Thankyou for for feed back. The transformer is 24vac and after rectification shoud be
32.5volts at the entrance to the top line which goes into 10,000UF @ 50vdc. Is two regulators hooked up together
to produce three amps at the end positive only and ground possible??
 
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Sparky49

Joined Jul 16, 2011
833
The input of a 7815 MUST be positive but you have one with a negative input voltage which will blow it up.

A negative 15V regulator is a 7915.

Usually a 78xx or 79xx regulator has a little 0.1uF to 0.33uF film or ceramic output capacitor, not a huge electrolytic.

EDIT: Your bridge rectifier is connected completely wrong.

EDIT again: added corrected schematic.
Jeeze Louise! That'll learn me for not double checking!:eek:

It's interesting that the rectifier is wrong - that's the only way Eagle draws it...

I hope this is better?

(I've removed the 1st schematic so it doesn't confuse people)

Sparky
 

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PepeSmooth

Joined Jan 7, 2012
3
Kbyrne,

15V @ 3A load capacity is a huge power for a 7815 and 7915 regulators. You will be having problems in power dissipation especially for a 32.5V raw DC. All excess voltage will be dissipated on the regulators --> thermal runaway and it cannot be sustained by the heatsink.
A relatively large transformer will also be required on this kind of linear DC requirements.

Perhaps you would want to use the SMPS. Try to look buck regulators.
 

Audioguru

Joined Dec 20, 2007
11,251
Your new schematic has the bridge rectifier connected correctly but the negative input filter capacitor polarity is backwards and it will blow up.

Do not ever copy anything from Instructables. Many if not all circuits are designed by 10 years old little kids who know nothing about electronics.
 

Sparky49

Joined Jul 16, 2011
833
Do not ever copy anything from Instructables. Many if not all circuits are designed by 10 years old little kids who know nothing about electronics.
I agree with this - the problem is that many circuits aren't reviewed properly before they are posted. Imagine if I'd submitted my first (and second) circuit... Lots of magic smoke!:rolleyes:

Sparky
 

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Thread Starter

kbyrne

Joined Dec 10, 2011
92
Thanks Guys. Upon checking all of these messages I think I will try a different circuit
by a Knol author involving stacking a transistor Tip series with a Voltage regulator. I am very unsure about the huge ammount of capacitance in the origional circuit by the instructables author.
 

Thread Starter

kbyrne

Joined Dec 10, 2011
92
Message to You. I will end this thread but don't know how. Thankyou for the help thou.
My problem is inexperience and I am too unsure of the origional author and dont know how to modify to acheive 15vdc @ 3 amps.
 

Thread Starter

kbyrne

Joined Dec 10, 2011
92
18 Vac @ 3 Amps. Needs 15vdc @ 2.5 Amps - 3.0 Amps. Tanks That is circuit estimated over all requirements.
 

Audioguru

Joined Dec 20, 2007
11,251
Your transformer is rated for 18V x 3A= 54VA. But your circuits use 25.5V x 3A= 76.5W. If the amplifiers play continuously at full blast (unlikely) then the transformer will be overloaded.

The datasheet of National Semi's LM78xx has a high current circuit using one power transistor as a current booster and another transistor to limit the current if the circuit is shorted:
 

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Thread Starter

kbyrne

Joined Dec 10, 2011
92
Attn: Guys sorry about that but this power supply is for a second project. Power supply
for amplifiers as attached to 12 Watt thread as a thumbnail. All I need is help to see if this power supply could ever work at all due to ammount of capacitance. I will only post one thread at a time to avoid confusion.
 

Audioguru

Joined Dec 20, 2007
11,251
The power supply on the first post is wrong because it has an AC input instead of a DC input and it has the filter capacitors in Series instead of in Parallel. It does not say 47000uF, instead it says 4700uF, 4700uF and 680uF.

Post a video of the capacitors blowing up. I bet the 680uF capacitor will blow up first.

I would use a single 10000uF capacitor but a single 4700uF capacitor will work fairly well.
 

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