6 Dual Power Supply
Disconnect the DSO from the circuit.

Set the current limit for CH1 to 0.2 A. 

Explain the results by drawing circuit diagrams of the measurements, showing the earthing connections.
Enable series mode on the power supply by pressing the ser key.

Using Dual Supplies
In this section we are going to demonstrate the various methods of taking ‘an output’ from the dual power supply. Each output of the power supply is ‘floating’ with respect to earth at the general power outlet (GPO), and thus is similar to a battery.
Set the power supply in series mode, with a current limit on each channel of 0.2 A.

A dual power supply is really just two independent power supplies, either with or without a ‘common’ connection. If the power supplies are truly independent, the output can be connected in series for additional voltage, or they can be connected in parallel for additional current capacity. This section will explore the operation and connections of a dual power supply.
Thursday, 12 July 2018
Setting up the Supply
9:44 AM
Conceptually, the dual power supply looks like this under default settings:

We are going to set up the power supply for a 5 volt output on one pair of output connectors.
Now confirm your connections by wiring the power supply and measuring the voltages with the digital multimeter.
Consider the blue interconnection terminals as a ‘common’ voltage reference, measure the voltage between this common and each of the other two terminals. You should get +10 V and -10 V.


Voltage between ‘plus’ (red) and common =

Voltage between ‘minus’ (blue) and common =

Programmable DC Power Supply
If you need an accurate output voltage, always use a digital multimeter connected to the output voltage at the load, not at the supply, since there may be a voltage drop in the leads due to the lead resistance, if the current is large.
Current, I =


Voltage across the 4.7kΩ resistor, V1 = 


Voltage across the 4.7kΩ resistor, V1 = 


Voltage across the 4.7kΩ resistor, V1 = 

Voltage across the 10kΩ resistor, V2 = 
Voltage across the 10kΩ resistor, V2 = 

Voltage across the 10kΩ resistor, V2 = 
Connect 4mm leads from the left-hand power supply to the multimeter and adjust the output voltage until the CH1 voltage display on the Power Supply reads as close to 5 V as possible. 
Adjust both variable outputs to 10 V using the CH1 and CH2 voltage displays.
Set the digital multimeter to read DC Voltage on the Auto range by pressing the DCV key.
2.	Turn the power on. With the multimeter, measure the voltage across the   resistor, and then across the   resistor. Calculate the current in the circuit. Note the polarity of the voltages and current in the circuit above. Record your values for later reference:
The reason is that when you connect the DSO earth to the point in between the   and   resistors, you are earthing that point, and hence, shorting out the   resistor. You must always earth your circuit for your own safety and to avoid damage to the lab equipment and / or your circuit. When you do so, be careful when using a DSO or other earthed equipment. A DSO always measures voltages with respect to EARTH.
•	Measure one voltage, then the other, and subtract them.
Using Triple Supplies
4.	Disconnect the DSO from the circuit.
2.	Now measure the voltage between the ‘-’ terminal on the left-hand side and the ‘+’ terminal on the right-hand side of the power supply. You should get 20 V, because you have connected the supplies in series, as shown below:
1.	Draw the connections you would use to create a triple power supply that provided +5 V, +10 V, and – 10V, with all voltages measured with respect to earth. Label the outputs, and show the various voltages.
Turn on the output for CH1.
Earthing the Supply
The Mini-Lab also has a fixed 5 V supply to facilitate the powering of digital integrated circuits (ICs). This fixed 5 V supply will occasionally be used, so it is important to note that its output is with respect to earth, and not the common of the dual power supply, as shown below:
Explain the results by drawing circuit diagrams of the measurements, showing the earthing connections.
Voltage across the   resistor,    =
Voltage across the   resistor,    =
This is the way we get a ‘plus and minus supply’ for analog circuits.
Earthing lab power supplies is very important. The output of the supply is electrically ‘floating’, even if you connect two sides in series and use the interconnection point as a common reference. To make this common reference equal to the earth voltage, you must connect this point to the earth (green) terminal. This makes the circuit safe and allows you to use other test and measurement equipment on your circuit (e.g. a DSO), reducing the risk of damage. Let’s see an example of this concept.
To measure voltages across components in a circuit using a DSO, do one of the following:
3.	Now using the DSO, measure the same voltages in the circuit:
1.	Set the output voltage of the right-hand supply to 10.0 volts. With the power off, construct the circuit shown below using the resistors and breadboard from your lab kit. Do NOT connect an earth lead to the circuit just yet.
•	Use a DSO with a maths function to subtract the two channels.
Record the multimeter reading:
Voltage across the   resistor ,    =
Voltage between ‘minus’ (blue) and common =
3.	Considering the white power supply terminal as a ‘common’ voltage reference, measure the voltage between this common and each of the other two terminals. You should get +10 V and -10 V.
Voltage across the   resistor,    =
Voltage across the   resistor ,   =
Current,   =
5.	Now earth the supply by connecting the common terminal (white) to earth (green). Measure the voltages in the circuit using the DSO:
Voltage between ‘plus’ (red) and common =
Voltage across the   resistor,    =
2.	Now confirm your connections by wiring the Mini-Lab and measuring the voltages with the digital multimeter.
Current talk!