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 Forum index » DIY Hardware and Software
2A power supply
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LetterBeacon



Joined: Mar 18, 2008
Posts: 454
Location: London, UK

PostPosted: Sat Jul 28, 2012 12:53 pm    Post subject: 2A power supply Reply with quote  Mark this post and the followings unread

I'm coming to the upper limits of my 1A power supply and, because I don't have space in my cabinet for two power supplies, I'd like to build one that supplies 2A @ +15V/-15V.

I'm going to use a 18V-0-18V 80VA toroid transformer, which can supply 2.22A.

I was going to use the LD1085 for a positive regulator and the LT1033CT as the negative regulator. They are both capable of an output of 3A.

I was just going to follow the typical application notes on the regulators' datasheets to design the PSU (while adding a bridge rectifier and smoothing caps). This should work in theory, is there anything I'm missing, or is it really that simple?

Thanks!
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stewpye



Joined: Apr 30, 2009
Posts: 49
Location: Brisbane, Australia

PostPosted: Sat Jul 28, 2012 11:25 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi,

I'm in the process of layout out a PCB for a similar PSU design. I'll be using the LT1085 or LD1085 for the positive and negative regulators. If you use a dual 15V or 18V secondary transformer you basically have two regulator sections the same, but on one of them you connect the +15V output to ground (0V), and what would have been the 0V output becomes the -15V output.

I chose to do it this way because the positive regulators are cheaper and available in low dropout varieties. As they are low dropout regulators I'm hoping to get away with 15Vac secondaries to keep power dissipation to a minimum.

Check the current rating of the transformer. If it is rated at 2.2Aac, that doesn't mean it can supply 2.2A DC after rectification and filtering.

Regards,
Stewart.
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stewpye



Joined: Apr 30, 2009
Posts: 49
Location: Brisbane, Australia

PostPosted: Sat Jul 28, 2012 11:45 pm    Post subject: Re: 2A power supply Reply with quote  Mark this post and the followings unread

LetterBeacon wrote:
is there anything I'm missing, or is it really that simple?


The LT1033 is listed as "Not recommended for new designs", and not available from Digikey or Mouser. The LT1185 is suggested as a replacement. It is a 5 lead device and low dropout, though a bit pricey compared to the LD1085.

Regards,
Stewart.
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LetterBeacon



Joined: Mar 18, 2008
Posts: 454
Location: London, UK

PostPosted: Sun Jul 29, 2012 12:59 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for the reply Stewart.

stewpye wrote:

Check the current rating of the transformer. If it is rated at 2.2Aac, that doesn't mean it can supply 2.2A DC after rectification and filtering.


Do you think I should go one bigger then? That way I'll be able to get to 2A and keep the transformer cooler?

Quote:
The LT1033 is listed as "Not recommended for new designs", and not available from Digikey or Mouser. The LT1185 is suggested as a replacement. It is a 5 lead device and low dropout, though a bit pricey compared to the LD1085.


I seem to be able to get the LT1033 fine from RS and Farnell (my suppliers of choice). Why do you think it wouldn't be recommended for new designs?

Thanks!
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stewpye



Joined: Apr 30, 2009
Posts: 49
Location: Brisbane, Australia

PostPosted: Sun Jul 29, 2012 3:35 am    Post subject: Reply with quote  Mark this post and the followings unread

LetterBeacon wrote:

Do you think I should go one bigger then? That way I'll be able to get to 2A and keep the transformer cooler?


For a bridge rectifier/filter cap circuit Idc = 0.62 I secondary.

See this thread...
http://electro-music.com/forum/topic-51694.html


Quote:

I seem to be able to get the LT1033 fine from RS and Farnell (my suppliers of choice). Why do you think it wouldn't be recommended for new designs?
Thanks!


Because they are not planning on producing it for much longer.
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Osal



Joined: Aug 16, 2011
Posts: 145
Location: Here

PostPosted: Mon Aug 06, 2012 8:35 am    Post subject: Reply with quote  Mark this post and the followings unread

stewpye wrote:
LetterBeacon wrote:

Do you think I should go one bigger then? That way I'll be able to get to 2A and keep the transformer cooler?


For a bridge rectifier/filter cap circuit Idc = 0.62 I secondary.

See this thread...
http://electro-music.com/forum/topic-51694.html


Hello, thanks to point to that, notice that if we say that each rail can provide X current, it is the same as to say that X current is flowing from the positive rail to the negative rail.
Then, for a 80VA 36VCT transformer the maximum advised current per rail is 80/36/1.8 which is 1.23A

EDIT: Sorry, I think I misunderstood your post. However the minimum ratio advised is Isec/Iout=1.8 so Idc = 0.555... * I secondary

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Last edited by Osal on Mon Aug 06, 2012 3:21 pm; edited 1 time in total
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Osal



Joined: Aug 16, 2011
Posts: 145
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PostPosted: Mon Aug 06, 2012 9:07 am    Post subject: Re: 2A power supply Reply with quote  Mark this post and the followings unread

LetterBeacon wrote:
I'm coming to the upper limits of my 1A power supply and, because I don't have space in my cabinet for two power supplies, I'd like to build one that supplies 2A @ +15V/-15V.

I'm going to use a 18V-0-18V 80VA toroid transformer, which can supply 2.22A.

I was going to use the LD1085 for a positive regulator and the LT1033CT as the negative regulator. They are both capable of an output of 3A.

I was just going to follow the typical application notes on the regulators' datasheets to design the PSU (while adding a bridge rectifier and smoothing caps). This should work in theory, is there anything I'm missing, or is it really that simple?

Thanks!


Hi notice that, although a regulator can be rated at 3A, the maximum output current depends mainly of the power dissipation. If the regulator goes over its temperature rating it will not work correctly and/or it will short its life.

If you are using low dropout regulators you could use a 30VCT transformer as stewpye said reducing a lot the power to dissipate. Also modern regulators could have less junction to case thermal resistance making more easy the power dissipation.

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