# Can you share the same negative wire on two separate DC power supplies?

Discussion in 'General Electronics Chat' started by eddie500, May 5, 2016.

1. ### eddie500 Thread Starter Member

Nov 5, 2011
21
2
I need to send 8 volts and 12 volts power to some lights. I have wires run already for the 12 volts, so I was wondering if I can just tap into the negative wire also for the 8 volts.

The positive would be separate wires.

Do I need to run two separate wires, or can I run one single wire for the negative and connect them to the two separate power supplies.

This would mean some parts of the wire would have 12 volts and 8 volts flowing through it at the same time, but the wire would split and go to the separate power supplies and lights.

Is this even possible? It may be a very stupid question but just wondering what would happen.

2. ### WBahn Moderator

Mar 31, 2012
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4,920
You can do it provided the two DC supplies are not otherwise ground referenced.

Voltage does not "flow", so that is part of your problem in thinking about it. Voltage is a measure of potential difference between to points. In some respects, your question is like asking if you can build an eight story building on the same ground that you've already built a twelve story building.

Mashly likes this.
3. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,798
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Yes done all the time.. not a problem under most circumstances..
One must ensure that a single wire is suitable for the current requirements of all circuits..
I'm sure this isn't an issue for your project (probably not) but I've known some that don't think there is any current flow on the negative leads. Like "current" comes out of the battery and only goes to the lights and they use it all or something..

Combining the 2 gives an "equal/common" ground reference for both circuits too so one isn't floating with respect to the other..

4. ### shteii01 AAC Fanatic!

Feb 19, 2010
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Voltage is a field, it does not flow, it propagates.

5. ### AnalogKid Distinguished Member

Aug 1, 2013
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If the two power supplies share a common ground, or can be made to, then it is no problem. However, the return wire must be sized for the sum of the 8 V and 12 V currents. What are the 8 V and 12 V currents, power levels, etc.

ak

6. ### eddie500 Thread Starter Member

Nov 5, 2011
21
2
Thanks guys for the responses. Actually I mentioned two power supplies to make the question easier.

Actually I have an 82 volt battery and I want to tap into the negative wire of that battery. This is for an electric bike and weight savings are important.

I need to run an 8 volt power supply and it would be easy to tap into the battery wire. Actually the wire is for the charge port and is hardly ever used, but it is connected to the battery ground.

I have another question, I may add a 12 volt power supply in the future.

SO I assume I can share this same battery negative wire for my 12 volt and 8 volt DC/DC power supplies?

The 82 volt battery powers the 12 volt power supply, The 12 volt power supply powers the 8 volt power supply.

Power supplies are just those cheap Chinese power supplies.

7. ### WBahn Moderator

Mar 31, 2012
18,096
4,920
Whether you can do it or not depends on the connection/isolation between the inputs and the outputs of the supplies.

8. ### hp1729 Well-Known Member

Nov 23, 2015
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What is the difference between flow and propagate? I would say it does neither. It is a static condition between different points in a circuit. Yes, a field.

9. ### eddie500 Thread Starter Member

Nov 5, 2011
21
2
Is there a way to check my supplies?

10. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,519
515
Path. Current REQUIRES a path. Voltage does not.

11. ### WBahn Moderator

Mar 31, 2012
18,096
4,920
Check the voltage between the combinations of output pins.

So say you have your 82 V and your 12 V supplies with V82+ and V82-, and with V12+ and V12-.

Use a multimeter to check the voltage between V82- and V12- and also (separately) between V82- and V12+. If both read zero (or possibly fluctuate seemingly randomly) then your supplies are almost certainly isolated. If you second reading is 12 V greater than your first reading, then the supplies are definitely not isolated. But even then, if your first reading is a solid 0 V, then that is their common connection point and you can hard connect them (in fact, they pretty much already are).

12. ### AnalogKid Distinguished Member

Aug 1, 2013
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1,302
I think things will go better if you post a block diagram or schematic of how you envision interconnecting your various components. Be secure in the knowledge that:
Some people will misread your drawing and tell you things that won't work.
Some people will call you an idiot.
Some people will say it is too dangerous for someone of your experience (stupid + idiot).
Most people will critique the drawing, evaluate the design, point out possible issues, suggest better configurations, confirm the correct parts, correct the bad parts, warn against the dumb parts, praise the smart parts, and just plain help.

ak

13. ### eddie500 Thread Starter Member

Nov 5, 2011
21
2
Since my power supplies aren't powered yet I checked the continuity and the grounds on my 12 volt and 8 volt both give continuity like they were connected.

Actually since my 12 volt power supply powers my 8 volt power supply and are connected, I get continuity between the negative input on my 12 volt and the negative output on my 8 volt.

My 82 volt battery ground, my 12 volt ground and my 8 volt ground all share the same ground.