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### Topic: Sources and Sinks (Read 1 time)previous topic - next topic

#### Verdris

##### Jan 19, 2012, 04:10 am
Hi All,

I have a circuit on a powered breadboard for testing, and it has the usual sources: 1A @ 5V, 500mA @ 12V (variable) and 500mA at -12V (variable).  I need to run approx 600mA across 12V.  Can I set my variable voltages to +-6V and have that act as a 12V drop providing 1A?

#### ajofscott

#1
##### Jan 19, 2012, 04:23 am
Series supplies add voltages not currents. The Adjustable 12 volt supplies can still only handle 500mA not to mention your arduino would possibly see inputs ranging from -6V to +6V. All of your outputs are Ground referenced, shifting your operational ground to the -6V output makes the PS ground in effect +6V, the 5V output +11V, and the +6V effectively +12V. To add currents supplies must be paralleled.

#### Verdris

#2
##### Jan 19, 2012, 04:25 am

Series supplies add voltages not currents. The Adjustable 12 volt supplies can still only handle 500mA not to mention your arduino would possibly see inputs ranging from -6V to +6V. All of your outputs are Ground referenced, shifting your operational ground to the -6V output makes the PS ground in effect +6V, the 5V output +11V, and the +6V effectively +12V. To add currents supplies must be paralleled.

Even if I tied the -6V to the ground rail of the breadboard?  The Arduino isn't part of this circuit, so I'm not worried about that.

#### cmiyc

#3
##### Jan 19, 2012, 04:35 am

Can I set my variable voltages to +-6V and have that act as a 12V drop providing 1A?

What is a +-6V supply?

As ajofscott said, supplies in series don't add current.  So a 500mA supply in series with a 1A supply can still only provide 500mA.

Even if I tied the -6V to the ground rail of the breadboard?  The Arduino isn't part of this circuit, so I'm not worried about that.

What would that accomplish?
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

#### Verdris

#4
##### Jan 19, 2012, 05:33 pmLast Edit: Jan 19, 2012, 05:48 pm by Verdris Reason: 1

Can I set my variable voltages to +-6V and have that act as a 12V drop providing 1A?

What is a +-6V supply?

One supply is +6V, and another is -6V, both providing 500mA.

Quote

As ajofscott said, supplies in series don't add current.  So a 500mA supply in series with a 1A supply can still only provide 500mA.

This makes sense, thank you both.

Quote

Even if I tied the -6V to the ground rail of the breadboard?  The Arduino isn't part of this circuit, so I'm not worried about that.

What would that accomplish?

I don't know, that's why I'm asking.  I had it in my mind that I could use the -6V as ground, and have it "pull" 500mA while the +6V acts as the voltage source, "pushing" 500mA.  Total voltage drop is 12V and if current sources added (which obviously they don't) I'd get 1A.

#### cmiyc

#5
##### Jan 19, 2012, 06:40 pm

I don't know, that's why I'm asking.  I had it in my mind that I could use the -6V as ground, and have it "pull" 500mA while the +6V acts as the voltage source, "pushing" 500mA.  Total voltage drop is 12V and if current sources added (which obviously they don't) I'd get 1A.

All voltages are relative to something.  The idea of "ground" in digital circuits is made up.  It represents a common reference which we conveniently call 0V.  When we say "+12V", we are actually saying "12V relative to 0V."  If all devices use the same "0V" reference, then that 12V is relative to those devices.
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

#### MarkT

#6
##### Jan 19, 2012, 07:14 pm
I think it is a little more involved than that - 0V may or may not be tied to a local ground (earth wire) but a "+100V" supply for instance would never be - that would be a lethal boobytrap.  You could view 0V as meaning "this supply can be earthed/grounded without burning something out" (you might form a ground loop that is a problem for small signals, but you shouldn't blow a fuse or damage the equipment).

When connecting some low voltage battery powered circuits none of this really matters of course, but I think it is always wise to label voltage rails logically w.r.t. local ground potentials and when high voltages (or currents) are involved its a safety requirement to label them correctly.
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