Low voltage connection basics

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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.

• Electricity basics
• Electricity safety basics

Other names for low voltage:

• Shock free power or voltage

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

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.

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 !

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.

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 !

Protect your power supply.

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

.

Plus and minus

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Adapter connectors

OUTSIDE (mm)	INSIDE (mm)	Shortcut	Plug Type (code)	mark
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

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

• 
  Female, Type A,USB connector with pin diagram.

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

This is harmful for the adapter or battery.

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.

Right

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.

• Electricity basics
• Power and energy basics
• Electrical wiring and switching

• Wikipedia:DC Power Connectors