Wiring help

[michaelmorris]

I need to put a switch in a 12v cable, which involves cutting in to an existing cable from a plug-in 230v to 12v transformer.  Does it matter which of the two 12v cables I put the switch in?

[Scott]

I'm not sure Michael but I've always put it on the positive lead before the appliance. look forward to hearing I'v been doing it wrong all these yrs

Edit:-

This drawing seems to say I am wrong

[Scott]

Thinking about it, the cigar plugs are tip positive and they are fused on the tip (+).

Confused yet ?

[Physopto]

Electrons moving in a wire constitute the current. So the number of electrons per second gives you the actual rate of current flow.

Current flows from (DC) negative to positive.     

Confused even more now? Sorry.

Decided to give you a bit real help there. Please see this it will explain a bit better.

http://www.allaboutcircuits.com/textbook/direct-current/chpt-1/conventional-versus-electron-flow/

[RuralBill]

To be honest, it doesn't matter, it can go in either wire, as long as the circuit is interrupted.

nice and simple, but I guess someone will,completely disagree

Bill

[RuralBill]

31 minutes ago, Scott said:

I'm not sure Michael but I've always put it on the positive lead before the appliance. look forward to hearing I'v been doing it wrong all these yrs

Edit:-

This drawing seems to say I am wrong

There are no switches in that diagram 

[Physopto]

Just now, RuralBill said:

To be honest, it doesn't matter, it can go in either wire, as long as the circuit is interrupted.

nice and simple, but I guess someone will,completely disagree

Bill

As far as I can tell it only really matters if the negative is tied to earth. In the old days of positive earth used in vehicles it was the other way around. The vehicle  body is conductor and still is, (Negative now). If the circuit is not connected in any way to either does not matter unless you get a short on the wrong side.

[Scott]

5 minutes ago, RuralBill said:

There are no switches in that diagram 

the dc supply is switched and the blocks shown are dimmers. I just power up with themin the off  position. It's only for powering dew strips

 

[mapstar]

A normally open switch as drawn in most electrical diagrams 

[steppenwolf]

Michael, it doesn't matter what side you switch. However, if I am controlling a circuit by computer, I normally switch the ground (negative) and if I am using a manual switch I switch the positive. The advantage of switching the positive is that most electrical items with a metal case have that case connected to negative so if two items should touch one another no circuit will be completed by that contact!

[Scott]

2 hours ago, RuralBill said:

There are no switches in that diagram 

I'm really sorry, I understand your comment regarding switch now. In my haste, I read (or at least thought I did) fuse, not switch. you are quite right to point it out. once again, apologies

[Scott]

1 hour ago, mapstar said:

A normally open switch as drawn in most electrical diagrams 

these are the switches I'm used to

single pole double throw etc

 

[michaelmorris]

5 hours ago, steppenwolf said:

Michael, it doesn't matter what side you switch.

Thanks Steve, that's what I thought, but I wanted to be sure before I started cutting hard wired cables!

[ajk]

10 minutes ago, michaelmorris said:

Michael, it doesn't matter what side you switch.

Just for the record, in a low voltage DC circuit (less than 50v) it doesn't matter which you use, +ve or 0v wire.

In higher DC voltage circuits it can do for safety. In this condition putting the switch on the 0v return wire leaves the equipment "live" when it's switched off so if you are working on it expect to get an electric shock at some point. You should put the switch on the live side really. But 12v isn't going to harm you.

In AC circuits however different rules apply entirely, use an isolator that breaks BOTH wires.

[MarkyD]

1 hour ago, ajk said:

Just for the record, in a low voltage DC circuit (less than 50v) it doesn't matter which you use, +ve or 0v wire.

In higher DC voltage circuits it can do for safety. In this condition putting the switch on the 0v return wire leaves the equipment "live" when it's switched off so if you are working on it expect to get an electric shock at some point. You should put the switch on the live side really. But 12v isn't going to harm you.

In AC circuits however different rules apply entirely, use an isolator that breaks BOTH wires.

Actually just to be picky below 120V is classed as Extra Low Voltage and 120V to 1500V is classed as Low Voltage for DC.  AC ELV is below 50V and LV is 50V to 1000V

[Icosahedron]

Since most systems these days are negatively grounded, I would put switches and fuses in the positive line.

Two things about this drawing are worrying: the lack of LED current limiting resistors and the use of phono sockets to distribute power.

[MarkyD]

8 minutes ago, Icosahedron said:

Since most systems these days are negatively grounded, I would put switches and fuses in the positive line.

Two things about this drawing are worrying: the lack of LED current limiting resistors and the use of phono sockets to distribute power.

 

Another worrying thing is the short leg is not the Anode (+).  In fact the whole schematic is wrong tbh

[Gina]

Yes indeed.  The only thing right about this is that they used red for positive and black for negative and wired the dimmer connections accordingly.  Phono sockets aren't two pin either!  They're pin and frame.

[StuartJPP]

For consistency it is better to switch positive in/out and have the fuse in the positive line as well. For circuits like this listed it doesn't really matter of course and either will work.

If you are interfacing a circuit to another system, or may do in the future like a PC or an Arduino for instance, it is easier to prevent back powering of interconnected systems if grounds are connected and things are switched off at the positive end (high-side driving). It also makes fault finding easier if you can connect your meter/oscilloscope to the ground and measure voltages throughout the system relative to that common ground.

Just my thoughts on it.

As mentioned a current limiting resistor should be used for the LED, and the short lead is usually (but not always) the Cathode. I have even had LEDs with the chamfered side being the Anode, probably moulded incorrectly so best is to test before committing. The LED will dim depending on the output of the PWM controller, without the current limiting resistor the worst case is 12V straight on to the LED which will release a very distinctive smell and sometimes even makes green LEDs glow orange for a brief period or sometimes the tops will blow off...YMMV.