# Back feed (DC circuit)

Discussion in 'General Electronics Chat' started by Ringojames, May 9, 2015.

1. ### Ringojames Thread Starter New Member

Nov 27, 2013
13
0
Hi, I am trying to find out about back feed in DC circuits. What causes it. How it happens. Examples of it. Etc.
I cannot find anything on the Internet to help me on this and it's really frustrating not being able to understand the basic principles of it. I cannot seem to understand why a current would go back down a cable??? I didn't think this was possible???

Thanks for taking the time to help me

Jul 18, 2013
13,182
3,646
Best if you give an example of a circuit.
Max.

3. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,071
You question is puzzling. What is the context?

4. ### Ringojames Thread Starter New Member

Nov 27, 2013
13
0
I am talking more automotive really, but any help would be be greatful

Jul 18, 2013
13,182
3,646
Do you mean BEMF ? Back EMF.
Max.

6. ### Ringojames Thread Starter New Member

Nov 27, 2013
13
0
I don't really know! All I know (or I think I know) is current can back feed and you can use a diode to block it. (I understand how a diode works) what I don't get is WHY would a current decide to go the opposite way it should be travelling. What would cause this (if this is in fact what a back feed is)

Jul 18, 2013
13,182
3,646
That is still a little vague, I would start with studying BEMF say on the removal of power on an inductive device, relay, solenoid etc .
Max.

8. ### nsaspook AAC Fanatic!

Aug 27, 2009
3,638
3,767

You can think of it as several buckets ([A] higher, lower) of 'energy' each with different amounts of energy from zero to some higher level. If you connect those buckets with a wire forming a circuit the 'system of buckets' [AB] will tend to equalize to a common energy level in all buckets by moving energy using the wires charge of electrons (current) to each other in the travel direction that requires. The travel direction by convention is from 'positive' higher level to 'negative' lower level so if you insert a device (diode) in the wire that only allows current travel from A to B, if B is filled higher by some external source than A, B can't transfer energy to A by moving that charge because of the diode.

Last edited: May 9, 2015
9. ### crutschow Expert

Mar 14, 2008
16,575
4,478
Where have you seen the term "back feed" used?
I've never heard of the term before.

Aug 27, 2009
3,638
3,767
11. ### ian field AAC Fanatic!

Oct 27, 2012
5,563
985
Its sometimes happens in pubs whe someone tries to guzzle too much beer.

darrough, MaxHeadRoom and nsaspook like this.
12. ### nsaspook AAC Fanatic!

Aug 27, 2009
3,638
3,767
You owe me a keyboard.

13. ### ian field AAC Fanatic!

Oct 27, 2012
5,563
985
Its in the post...................

14. ### Ringojames Thread Starter New Member

Nov 27, 2013
13
0

I'll give an example of what I mean. If I fitted an alarm to a car, I would need to find a door switch feed. If both door switch feeds were separate and I needed to connect them to one source, I would need to diode both wires and connect these to the wire going to to the alarm control unit, to stop the current back feeding to the vehicle electrics.

So why would this back feed happen and what would it do?

I'm not trying to fit an alarm by the way I'm just using this as an example that I know is a real case scenario

Thanks

15. ### crutschow Expert

Mar 14, 2008
16,575
4,478
You need diode isolation if, for example, you are driving the same load from two sources and you don't want the voltage from one source to feed voltage back to the second source or vice versa.
What would happen without the diodes depends entirely on the nature of the sources and circuit. There's no general rule, the need for the diode(s) is determined on a case-by-case basis.

Your alarm example is not clear. If you draw a diagram of exactly how it would be connected, then we can show if or where the diodes may be needed.

16. ### ian field AAC Fanatic!

Oct 27, 2012
5,563
985
A useful search term could be; "diode-or" - or maybe take a look at the internal schematics of the old DTL logic family.

17. ### nsaspook AAC Fanatic!

Aug 27, 2009
3,638
3,767
In your example the car electronics monitors the condition of each switch to know which door is open (switch shorted to ground). For the alarm you need to know if any door is open usually. If you just connected each door switch feed to one point directly the car monitor would think all doors where open when any one door was actually open. To stop this a diode is used in each switch feed line to the alarm to stop the 'back feed' of current to that one switch closure from the others.

So it's like the bucket analogy, if any one bucket in the system has zero energy from a short they all want to have zero energy so current will flow in the direction needed to make that happen unless a device like a diode is used.

Last edited: May 9, 2015
18. ### Ringojames Thread Starter New Member

Nov 27, 2013
13
0
Also I am under the impression that bad grounds/earths can cause back feeds. Is this because the voltage tries to get back to earth via any path and will go through other circuits to get there, thus causing other circuits to behave strange?

AGain thanks for taking the time to reply.... And have patience with me!!

19. ### nsaspook AAC Fanatic!

Aug 27, 2009
3,638
3,767
The 'technical' term is 'Sneak Path' but yes it can and does happen in sometimes very strange ways.
http://en.wikipedia.org/wiki/Sneak_circuit_analysis

Last edited: May 9, 2015