How can you get negative voltage?

Your question should read "How do you get a negative potential?", for the reasons explained below.

The term, 'voltage', is another Word for 'potential difference'. The terms 'positive' and 'negative', when applied to a potential difference or voltage indicates the sense (or direction) in which the potential difference or voltage is acting. For example, if a second voltage is acting in the same direction as another voltage then it is acting in the positive sense; if it is acting in the opposite direction then in is acting in the negative sense'.

It's very important to understand that you cannot apply 'positive' and 'negative' is the sense of charge or polarity to potential difference or voltage. You can, however, apply these terms to 'potential'. Potential is relative, and always measured with respect to a given point (often, the earth). So a point can be, for example, +5 V or -5 V with respect to another point (usually earth or ground, but not necessarily).

Never confuse 'potential' with 'potential difference' and remember that 'voltage' means potential difference notpotential.

So, to obtain a negative potential, earth (ground) the positive terminal of the battery or d.c. supply. All potentials measured relative to earth are now negative. To obtain a positive potential, earth the negative terminal of the battery or d.c. supply instead. All potentials measured relative to earth are now positive.

If you want to get a negative potential,

• Earth the positive terminal of the DC power source
• Connect a resistor in series with the negative terminal of the source
• At the other end of the resistor you get a negative potential (the value's less than the value of the power supply)