Getting 2 different power sources from a single source

[lukesmith123]

Hi,

For my project I want to have 1 single power source from a power adapter and I need to power 2 circuits.

one uses 12v at 3a

and the other uses 5v at 1a

Do I need a 17v, 4a power supply? How do I divide it in the way that I want?

 

[Harald Kapp]

You do not need a 17 V supply. The voltages don't add up.
You need a 12 V power supply with at least a 3.5 A rating. This will power your 12 V circuit.
You need a 12 V to 5V step down converter with a 1 A rating to generate 5V from the
12 V. This is but one example of such a converter.

You should add fuses in each leg (12 V and 5V) rated to the max. expected current to protect the power supply and the converter.

 

[BobK]

I hate to disagree with Harald, since he is usually spot on, but you will need more than 4A from the power supply. I would recommend 5A.

Even a step down DC-DC converter will draw the full power of the load from the power supply during a part of the cycle, and a linear regulator will draw the full current of the load all the time.

Bob

 

[Harald Kapp]

Bob,
o.k., it's not that simple as I made it look.

The beauty of a switch mode step down regulator is that the average current at the input is lower than at the output.
Assuming 80% efficiency, 5V*1A=5W -> 5W/0.8=6.25W at the input Since Vin=12V, this makes for I=6.25W/12V=0.52A.
To these 0.52A add 3A from the 12V leg and the average current is 3.52A. O.K., maybe going for a 4A supply puts you on the safe side.

I agree that the switched mode supply will draw pulsed current higher than the average 0.52A. Typically I'd expect Ipeak <=1A in this case. You can easily deliver this current from a good buffer capacitor. A rule of thumb is 1000µF per 1A, so placing at least 1000µF (again: to be on the safe side use 2200µF) low ESR capacitor between the 12V output and the step-down regulator's input will help. You can even place a series inductor from the 12V output to the capacitor and switch mode regulator to further filter and block the current spikes from the 12V rail - and to make the 12V supply see more an average current of 0.52A instead of the pulsed current.

 

[BobK]

A perfect 100% efficient buck converter from 12V supplying 5V 1A would draw 1 A 42% of the time and 0 A 58% of the time. One of less efficiency would draw more.

I see your point about the capacitor, and if the switching frequency is high, the smoothing capacitor will certainly be able to handle it. But I would certainly not go below total current rating.

Switch mode supplies also tend to draw a surge at startup, which could be problematic.

Bob

 

[Harald Kapp]

Agreed.
We need to know more about the application and detailed requirements for the two power supplies.