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### Topic: Current path for different voltages common ground question (Read 10496 times)previous topic - next topic

#### JPgs

##### Jan 31, 2012, 12:32 pmLast Edit: Jan 31, 2012, 12:35 pm by JPgs Reason: 1
*remark* all in DC (direct current)

Circuit A

• My question is the circuit for 5v part and 12V part consider isolated if they share a common ground?

Circuit B
Arduino(Atmega328) ground pin can sink a total 400mA.
12V 500mA is a power adapter.
I'm wondering and confusing with the path of the current flows after they share a common ground. For the 5V part, current will flow from Arduino and back to Arduino. For the 12V part, current will flow from power adapter and back to power adapter to complete a loop. Please corrent me if I'm wrong.

• is the 12V 500mA current will flow to Arduino ground pin or flow back to the power adapter itself to complete a loop?

• if the current 500mA from 12V is flow into Arduino board through Arduino ground pin, can it withstand a 500mA current while the current sinking limit is only 400mA for Arduino ground pin? is that a current sinking means?

• Is it possible the current 500mA flow from power adapter(12V) and goes to Arduino(5V)?

• Means the Arduino need to sink the 500mA current after sharing a common ground?

#1
##### Jan 31, 2012, 02:53 pmLast Edit: Jan 31, 2012, 03:00 pm by Graynomad Reason: 1
Quote
consider isolated if they share a common ground?

No. Remove the common GND and they are.

You're over thinking this I think. Unless you have large currents you shouldn't have to worry much about the return path. In B all the currents (540mA) return to GND and to the power source because it's a perfect world in a schematic. In the real world you would have to look at currents and wire/trace sizes.

So if the GND plug on the Ardiuno is not up to the total current you should run separate wires to the 12v device from your plug pack to take the load off the Arduino plug.

BTW you have no base resistor or LED current limiting resistor  and you should not draw 40mA from a single Arduino pin.
BTW2, that opto will not sink 500mA I think, you will need a darlington opto or another transistor.

_____
Rob
Rob Gray aka the GRAYnomad www.robgray.com

#### MarkT

#2
##### Jan 31, 2012, 03:43 pm
All (DC) current has to flow in a loop.  If two circuits only connect via ground then the current paths are separate.  However "isolation" means no connection at all (one circuit might be at +1000V for instance).  Opto isolators are used either when such large voltage differences exist, or to prevent electrical noise from the load affecting the driving circuit - since you are commoning up the grounds you have no need of an opto-isolator at all.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### JPgs

#3
##### Jan 31, 2012, 05:24 pmLast Edit: Jan 31, 2012, 05:31 pm by JPgs Reason: 1
so sorry for providing insufficient information for my question. Perhaps some of the parameter in the circuit is wrong or not suitable, but lets ignore it first and ignore the circuit A as well first.

for circuit B
what i'm confuse is

• For the 5V part, current 40mA will flow from Arduino and back to Arduino to complete a loop even they share a common ground.
For the 12V part, current 500mA will flow from power adapter and back to power adapter to complete a loop even they share a common ground.
current from power adapter will never flow to Arduino to complete a loop even they share a common ground.
current from Arduino will never flow to Power Adapter to complete a loop even they share a common ground.
is this correct?

• whether the 12V 500mA current will flow to Arduino's ground or flow back to the power adapter's ground itself to complete a loop?

• Is it possible the current 500mA flow from power adapter(12V) and goes to Arduino(5V) to complete a loop?

from the quote
Quote
If two circuits only connect via ground then the current paths are separate.

• means the Arduino will never need to sink the 500mA current from the power adapter even they share a common ground?

• means i can drive even 10 loads which draw 500mA each using different pin on Arduino at the same time as in circuit B?

• this is because no matter how much the current draws on the load side, they will never flow in Arduino even they share a common ground. Am i right?

So happy to hear that  "If two circuits only connect via ground then the current paths are separate"

Quote
In B all the currents (540mA) return to GND and to the power source

sorry, the power source u mean is? 40mA to Arduino and 500mA to power Adapter?

#### Grumpy_Mike

#4
##### Jan 31, 2012, 05:40 pm
Quote
Arduino(Atmega328) ground pin can sink a total 400mA.

No it is 200mA according to the data sheet.

Quote
current from power adapter will never flow to Arduino to complete a loop even they share a common ground.
current from Arduino will never flow to Power Adapter to complete a loop even they share a common ground.
is this correct?

Yes.

Quote
means the Arduino will never need to sink the 500mA current from the power adapter even they share a common ground?

Yes.

See :-
http://www.thebox.myzen.co.uk/Tutorial/Power_Supplies.html

#### pbechard

#5
##### Jan 31, 2012, 05:58 pm

Quote
current from power adapter will never flow to Arduino to complete a loop even they share a common ground.
current from Arduino will never flow to Power Adapter to complete a loop even they share a common ground.
is this correct?

Yes.

Mike - a variation on this:  If there was no middle ground symbol in his Circuit B diagram above, and just the Arduino Gnd and 12V connected together, where would the circuit ground out at?  Would it divide or all go to one ground?

In my very limited expeirence, I would have kept the Grounds separate to avoid any confusion, complications or burnouts.  Is there an advantage to one strategy over another?
Peter

Arduino and electronics newbie... but enjoying what I'm learning

#### jwatte

#6
##### Jan 31, 2012, 06:26 pm
If there was no common ground in diagram B, then there would be a floating voltage across the transistor, and you wouldn't know whether it would conduct or not. It still wouldn't be galvanically isolated, because there would be a connection across the base of the BJT transistor.

Wondering whether the AVR ground can "sink" current isn't really all that interesting unless you actually conduct GND through the AVR microcontroller (through its different pins). If you connect all GND pins to the same external ground plane/circuit, then the AVR ground does not "sink" anything other than what goes into the +5V of the chip.

For a 12V load, I would tie all the grounds together (using an external ground plane/circuit) and use a MOSFET transistor as a low end switch. I'd hook a 220 Ohm resistor from the AVR output pin to the gate of the MOSFET, and then a 10 kOhm resistor to ground from the gate to drain it when 5V power is off. If the load is inductive (motor) then I'd also use a kickback diode across the load, and if it's a HF load, I'd tie the output to the load through a ferrite bead or other filter. I would expect that to perform well like that for most cases.

#### JPgs

#7
##### Jan 31, 2012, 06:52 pm
can i make a conclusion that,
1) is it no point to isolate this circuit for the 5V part and 12V part.
2) because no matter how much the current draws on the 12V side(assume 10 Ampere), they will never flow in Arduino even they share a common ground.
3) and i can drive as much current as i want without effecting Arduino (Assume the transistor has unlimited current rating) even they share a common ground.
4) and i can drive as many as loads(which draw high current) by used up all the IO pin on Arduino without effecting Arduino also.
5) and i don't need to isolate both part.

Correct ?

#### retrolefty

#8
##### Jan 31, 2012, 07:17 pm

can i make a conclusion that,
1) is it no point to isolate this circuit for the 5V part and 12V part.

Not normally if proper wiring methods are used.

2) because no matter how much the current draws on the 12V side(assume 10 Ampere), they will never flow in Arduino even they share a common ground.

Correct as long as you don't have ground wires that are shared for both high current loads and low level logic circuits. Research on 'star grounding methods' for a better explanation. Grounds need to be at the same reference potential but not necessarily sharing the same current path.

3) and i can drive as much current as i want without effecting Arduino (Assume the transistor has unlimited current rating) even they share a common ground.

Yes, but again read my prior comment. There are proper and improper ways to wire up common grounds.

4) and i can drive as many as loads(which draw high current) by used up all the IO pin on Arduino without effecting Arduino also.

Yes, if wired properly and suitable bypass capacitors are used where required.

5) and i don't need to isolate both part.

No real simple answer to that general question. There are certainly application cases where one would want total galvanic isolation for an arduino circuit and a circuit needing to be controlled or sensed. Any application where the arduino is going to control something powered from household AC power is a good candidate for isolation circuits, be they simple relays, SSRs, etc.

Correct ?

I'm almost always correct.
Lefty

#### JPgs

#9
##### Jan 31, 2012, 07:29 pm
Got it all... Thanks... thanks a lot... BTW what it mean by?
Quote
Correct as long as you don't have ground wires that are shared for both high current loads and low level logic circuits.

circuit above, they do share a common ground, ehm... ?? correct me if im wrong...

#### retrolefty

#10
##### Jan 31, 2012, 08:05 pm

Got it all... Thanks... thanks a lot... BTW what it mean by?
Quote
Correct as long as you don't have ground wires that are shared for both high current loads and low level logic circuits.

circuit above, they do share a common ground, ehm... ?? correct me if im wrong...

Yes they do share a common ground reference, and as long as you run two separate wires from the transistor emitter lead (one the arduino ground pin and one to 12v negative terminal) they will not 'see' each others current flow, which is good.

Lefty

#### JPgs

#11
##### Jan 31, 2012, 08:20 pm
Final conclusion is, i can drive as much current as i want without effecting Arduino in this type of circuit.
Thanks... really got it all... thanks for your time... appreciate it... XD

#### jwatte

#12
##### Jan 31, 2012, 10:01 pm

can i make a conclusion that,
1) is it no point to isolate this circuit for the 5V part and 12V part.
2) because no matter how much the current draws on the 12V side(assume 10 Ampere), they will never flow in Arduino even they share a common ground.
3) and i can drive as much current as i want without effecting Arduino (Assume the transistor has unlimited current rating) even they share a common ground.
4) and i can drive as many as loads(which draw high current) by used up all the IO pin on Arduino without effecting Arduino also.
5) and i don't need to isolate both part.

Correct ?

That sounds basically right to me. Current needs a positive and a negative potential to flow. There is no multi-ampere positive hooked through the Arduino in your sketch.

#### MarkT

#13
##### Feb 02, 2012, 06:10 pm
I'd just add that the current load can't be completely unlimited since the output current is related to the base current by the current-gain of the transistor.  For really large loads consider a Darlington pair or a MOSFET.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

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