Question

You have an equipment to install that has two input pins for power: GND and -5VDC. You look around and the only spare power supply you could find is a +15VDC. What else should you check on this power supply? Can you use it somehow? Explain.

Answer 1

Provided data :

1.The equipment to install has two input pins which are to be connected to -5VDC supply( one pin to -5VDC terminal and the other pin to ground terminal).

2. The only spare power supply is +15VDC supply.

Requirement :

Need to generate -5VDC supply from the +15VDC power supply.

Solution :

Initially we need to verify/check whether the spare power supply is a fixed +15VDC power supply or is it a variable power supply i.e. A power supply which allows to vary the voltage (we can set to any voltage in the range, here it is 0VDC to +15VDC)..

Let's say it is a variable power supply which allows to vary the voltage from 0VDC to +15VDC.

Normally any power supply has three terminals as said below

1. Positive terminal

2. Negative terminal

3. Earth.

Normally, on DC power supplies the voltage is measured between Negative terminal and Positive terminal. And these two terminal are floating. The earth terminal is fixed at 0V potential voltage. The positive terminal potential voltage is higher than the negative terminal potential voltage, that is, with respect to negative terminal the positive terminal has positive voltage and with respect to positive terminal the negative terminal has negative voltage.

So, we understood the power supply positive and negative terminals are floating and the positive terminal is at higher potential than the negative terminal.

As per our requirement we need to generate -5VDC from +15VDC power supply.

Initially we need to adjust/set the +15VDC power supply to +5VDC as it is a variable power supply (our assumption).The +5VDC means, on the power supply, with respect to negative terminal the positive terminal is at +5VDC, and with respect to positive terminal the negative terminal is at -5VDC.  

The equipment to install has two inputs  

1. GND (ground)

2. -5VDC.

Now use the adjusted (to +5VDC) +15VDC power supply to connect to the equipment to install as explained below.

1. Connect positive terminal of the spare supply to the GND terminal of the equipment.

2. Connect negative terminal of the spare supply to the -5VDC terminal of the equipment.

The above solution explains how to use +15VDC power supply to connect to the -5VDC equipment.