+3.3v and +12v dc output linear psu

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slmnklyc

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Joined 2014
Hi guys.
I need your help.
I ordered 2*LT1083 adjustable regulator board and one 2*12v 65vac transformator.

2*12 65vac transformer.And I need 12v and 3.3v.
32vac for each board.
32/(12*1,4)=1,9a max current for 12v dc output
32/(3.3*1,4)=6,9a max current for 3.3v dc output.
1,4 is for transformer losses.

Is this calculation right?

I am trying to obtain at least 4a for 3.3 v and about 2a for 12v outputs.

Any suggestion?
 

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slmnklyc

Member
Joined 2014
peranders said:
For 12 VDC it may work with using the 12 VAC winding. The raw voltage should land at 16-18 VDC and that's enough for 12 V DC. For 3.3 V 12 VAC is definitely enough.
Click to expand...

About Lt1083.
Datasheet says max dropout 1.5v for max current output.Will it problem to obtain linear 3.3v with 4a high current?
Also says voltage difference limits the output current.
But it is difficult for me read from datasheet graphs..(using 12vac transformer may not be good for this regulator?)
 
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BZed

Member
Joined 2009
The transformer you are using will be OK for the 12 volt supply but can't supply the current needed for the 3.3 volt one. At 32 VA per transformer winding you can only get 2.66 amps AC from each winding. By the time you rectify the AC and filter it you will have 12 x 1.414 = 16.9VDC but you now calculate the peak current pulse for charging the filter cap you will be down to less that 2 amps by that time.

The other part is now calculate the power loss in the 3.3 volt regulator. At 2 amps 27.2 watts of the 32 the transformer can deliver turn into heat in the regulator and can't be delivered to the load.

This design will fail. The transformer is the wrong choice. For the 3.3 volt supply you will need a transformer the has a 5 or 6 volt output and a rated current of more than 10 amps. Other wise the transformer will overheat and fail.
 
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slmnklyc

Member
Joined 2014
BZed said:
The transformer you are using will be OK for the 12 volt supply but can't supply the current needed for the 3.3 volt one. At 32 VA per transformer winding you can only get 2.66 amps AC from each winding. By the time you rectify the AC and filter it you will have 12 x 1.414 = 16.9VDC but you now calculate the peak current pulse for charging the filter cap you will be down to less that 2 amps by that time.

.
Click to expand...
I didn't understand this part.
I calculated
32/(3.3*1,4)=6,9a current for 3.3v out
Do I think wrong?
 
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BZed

Member
Joined 2009
You need to look up Ohms law. The 32 VA is voltage x the current available from the transformer. So 32VA / 12VAC = 2.666 amps current Max you can get from that transformer. The peak current pulses to charge a cap will depend on the cap value and the load on the supply. When I build power supplies I like to size the transformer at 1.5 times the load on the supply. That would be a 6 amp transformer for a 4 amp power supply. You will never have a problem that way.
 
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