Two +5 power supplies in one circuit? Some other questions as well.

Thread Starter

drx

Joined Jun 16, 2007
3
Hello,

I am a newbie when it comes to electronics, and I searched the internet for a circuit like that and couldn't find it. It might be because I lack the proper terminology or something.

I am trying to build a circuit that can be powered by either of the two +5V supplies (one being an USB connector, the second being a custom connector, also 5V). What would be the best way to implement something like that?

I figure it would be smart to implement some sort of signal to know which of the power supplies is currently supplying power to the circuit. However, I don't know how to pull that off, since logic gates need a power supply as well, so I can't just do A & ~B or something like that.

On an unrelated note, does anyone have any tips for assembling PCBs? I will need to solder an IC with fine pins, spacing 1mm from each other. Any advice soldering them manually?

Also, what is the best equipment for soldering PCBs? Soldering iron-wise, that is. I have heard that using eutectic alloys helps, how true is that?

Thanks in advance, hope I don't ask too many questions :)

Luke
 

thingmaker3

Joined May 16, 2005
5,073
You can isolate the power inputs with diodes:

View attachment 2x5v.bmp

Eutectic not only melts at a lower temperature, but solidifies without precipitating constituent metals. (English translation: It won't booger up when solidifying.)
 

Thread Starter

drx

Joined Jun 16, 2007
3
The problem with diodes is that they have a huge forward voltage, 0.6V is too much for my circuits (some of the ICs I will be using can't be used with voltage lower than +4.7v, so even Schottky diodes are out of question...)

Is there any other component that acts in a similar manner but doesn't have such a large forward voltage?
 

n9352527

Joined Oct 14, 2005
1,198
If the absolute acceptable minimum voltage is 4.7V, then you are setting yourself for failure. USB Vbus could vary from 4.4V minimum during enumeration to 5.25V maximum. Even under one full unit load, the minimum is 4.75V, which doesn't leave you with enough safety margin.

How much current does the circuit draw?
 

Tube Tech

Joined Jan 11, 2007
46
"I figure it would be smart to implement some sort of signal to know which of the power supplies is currently supplying power to the circuit. "

http://en.wikipedia.org/wiki/Comparator

"The problem with diodes is that they have a huge forward voltage, 0.6V is too much for my circuits "

So use a 6V power supply...
 
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