Dual Power Supply

Thread Starter

Diego Bonilla

Joined Sep 18, 2014
35
Hi i have a 12V switching power supply, but i need 12V, 5V and 3.3V.
Based in this premise i design this circuit...i need opinions

thanks for the help

upload_2014-9-19_9-44-56.png

somebody can help me?
 
Last edited by a moderator:

bertus

Joined Apr 5, 2008
22,270
Hello,

How much current do you need on each output?
Keep in mind that the current taken from the 3.3 Volts regulator also goes through the 5 Volts regulator.

Bertus

PS No bumping within 24 hours please
 

wayneh

Joined Sep 9, 2010
17,496
D3 may not really be needed unless reverse polarity hookup is a possibility. It does help drop the voltage a bit.
As Bertus has noted, you may want to consider running the 3.3V regulator directly off the supply so that its current is not added to the 5V regulator. The decision depends in part on the current demand from each.
 

Thread Starter

Diego Bonilla

Joined Sep 18, 2014
35
Excuse my english isnt very good.

trying to say: put the 3.3V Regulator Direct at 12V; did not generate much heat dissipation in this IC(3.3vRegulator)?
 

wayneh

Joined Sep 9, 2010
17,496
Yes, whatever the current demand is for the 3.3V regulator, driving it from 12V will increase its heat dissipation compared to driving it from 5V. However moving it to the 12V source will cause the same amount of extra heat to be removed from the amount that must be dissipated by the 5V regulator. So the question is, which chip do you want to take the heat load? If the 5V regulator is already warm because of the current demanded at 5V, adding the 3.3V heat might overdo it.
 

wayneh

Joined Sep 9, 2010
17,496
Option 1
Heat dissipation in 5V regulator: (12-5) x 0.5 = 3.5W
Heat dissipation in 3.3V regulator: (12-3.3) x 0.25 = 2.175W

Option 2
Heat dissipation in 5V regulator: (12-5) x 0.75 = 5.25W
Heat dissipation in 3.3V regulator: (5-3.3) x 0.25 = 1.45W

Personally, I would choose Option 1, your last schematic. Option 2 comes too close to the max rating of the IC, 750mA versus 800mA.
 

ericgibbs

Joined Jan 29, 2010
18,766
hi Diego,
Looking on the web using your part no: AML1117 there a many different ready built AML regulator PCB's, some have heat sinks, others appear to have minimal AML1117 cooling methods.
Which version of these ready built regulators are you buying.?

E
 

ScottWang

Joined Aug 23, 2012
7,397
For a light load I will in series all the regulators, as this.
In your case, if you want to connecting all the inputs from 12V, for the heat dissipation that I will using power resistor to bear it.

Vmin_U1 = Vo+Vdrop = 5V + 4V = 9V
Calculating the current limiting resistor.
V_R-limit1 = 12V - 9V = 3V
R-limit1 = 3V/500mA = 6 Ω

Vmin_U2 = Vo+Vdrop = 3.3 + 3V = 6.6V
V_R-limit2 = 12V - 6.6V = 5.4V
R-limit2 = 5.4V/250mA = 21.6 Ω, you could choose 20 Ω to get a little more current.
 
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