Low voltage connection basics

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AC voltage safetyEdit

AC electricity with a voltage less than 50 V (RMS) is called Extra Low Voltage. This voltage is unlikely (but NOT guaranteed) to give a human being a dangerous electric shock.

What is low voltage?Edit

Other names for low voltage:

  • Shock free power or voltage

What is DC voltage?Edit

DC voltage is voltage that has a Plus and Minus terminal.

DC voltage symbol:  

Other names for DC voltage:

  • DC power or voltage
  • Battery type of power or voltage (the chemistry arrangements in batteries are necessarily DC)
  • Plus-minus power or voltage

Electric DC polarity: plus and minusEdit

Electricity basics: Marked plus wire

For most kinds of devices it is important to have the right voltage and polarity. Wrong voltage or polarity can destroy a device

How to mark the plus or to find the plus pole:

  • Red is always plus. (This is a long established convention, but is not always observed. Check with a voltmeter.)
  • Painted line on the plus wire, no painted line on the min.
  • Square wire is plus, round wire is minus.
  • Sharp edge is plus, round wire is minus.
  • Knot is plus, no knot is minus.
  • Mark on the plus wire, the plus wire is special.
  • and so on, the plus wire is special or not normal than a normal wire.

Connectors must be in good working order, and properly set up, to save your electric devices and reduce the chance of fire or other bad outcome:

  • The voltage. Connectors and wires have limits which should never be exceeded.
  • The amperage. Connectors and wires have limits which should be exceeded.
  • The plus and minus position. The term commonly used is polarity, for DC circuits; AC circuits have no polarity.
  • The type of power, AC or DC. Getting this wrong is likely to damage electric circuits.
  • The type of low voltage connector. Mismatched connectors almost never give satisfactory results, and often are physically incompatible.
  • Protect your plug and connector against mechanical pulling force. This is commonly called strain relief, which if done properly reduced wiring damage due to vibration or twisting or .... Unexpected separation of the connectors is generally done with a locking arrangement of some kind.
  • Take the time to check each of these issues. Failure to do so may cause fire or electrocution.

Side and middle electric connection

sign on power output or this sign sign on power consumption or this sign harmful for output ? harmful for power consumer ?
        no no
        no no
        no YES !
        no YES !
  DC   AC no YES !
  AC   DC no YES !

Electric current ampereEdit

amp. on

power output

or this amp. amp. on

power consumer

or this amp. harmful

for output ?

harmful for

power consumer ?

500 mA =0.5 A 500 mA =0.5 A no no
400 mA =0.4 A 500 mA =0.5 A YES!* no
500 mA =0.5 A 400 mA =0.4 A no no

* - Depending on how your output device is made, this condition may cause the output device (usually some kind of wall plug adapter) to burn out. The better made ones will have some sort of over-current protection built in, either in the form of a thermal fuse (self resetting), or a current limiter.

Electric voltageEdit

volt output volt consumer harmful for

output ?

harmful for

power consumer ?

12 V 12 V no no
9 V 12 V no does not work well
3 V 12 V no does not work
12 V 9 V no YES !

Adapter with different connectorsEdit

Protect your power supply.

On the left is the power supply output and on the right power receiving device.



Plus and minus


Adapter connectors





Shortcut Plug Type


6.5 1.0 no AA =
6.5 1.75 no AB =
2.5 2.5 YES! A (*) =
3.5 3.5 YES! B (*) =
3.0 1.0 no C =
5.0 2.1 no D =
5.5 2.1 no E =
5.0 2.5 no F =
5.5 2.5 no G =
3.5 1.35 no H =
4.0 1.7 no I =
4.75 1.75 no J =
5.0 1.5 no M =
5.5 1.5 no N =
5.5 5.8 no P =
6.3 3.0 no Q =
7.0 1.0 no R =
2.35 0.75 no S =

(*) Mono jack

Type A USBEdit

Add a picture with positive and negative labelled (in red (5+) and black) on a female Type A USB connector.  

What is an electrical short?Edit

An electrical short is when the plus and minus are in contact with each other.

This is harmful for the adapter or battery.

Electrical short with (dis)connecting deviceEdit

Some smaller connectors shortcut the power supply if you (dis)connect .

(dis)Connect the connector to the device if there is NO power on the plug.




Car cigarette lighter plugEdit

A car cigarette lighter plug must have the plus in the middle and the minus on the outside.

Some female car sockets have 24 volts.


See alsoEdit

Interwiki linksEdit

external links