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Bench power supplies are a valuable tool for every lab. They are used to generate a constant voltage, selectable by the user. They are stable and reliable tools for electronic experiments.
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What are the key factors when reviewing a lab supply?
The most important factors of bench top power supplies are:
- AC vs. DC
- Adjustable Output Voltage Range
- Coarse and fine adjustment
- Constant current / Current limiting
- Programmable Power supply
- Digital Interfaces
- Number of channels
- Displays for voltage and current
- Maximum electrical power
- Accuracy / Regulated Stability
- Galvanic Isolation and Grounding
- Switched vs. Linear
- Portability
- Package Content
AC vs. DC
Lab power supplies are usually used for supplying a circuit with DC voltage. This constant DC voltage is adjustable by the user. AC signals are in most cases not supported by variable power supplies. For AC voltage have a look at universal switching power supplies.
Adjustable Output Voltage and Range
A lab power supply allows to set a variable but constant output voltage. The output voltage range is a key factor when buying a variable power supply. Common voltage ranges are between 0-30 Volts. Although 30 Volts is not common for most circuits it is beneficial to buy a power supply for 30 Volts. With a dual output lab supply or two devices you can generate +15 V and -15 Volts instead of 30 V by connecting two outputs. Positive and negativ voltages are commonly used for operational amplifiers.
Coarse and fine adjustment
Good variable DC power supplies have a knob for coarse and fine adjustment. Coarse adjustment is used for roughly controlling the voltage output. Fine adjustment allows us to precisely set the output voltage of the bench supply. Usually a lab bench power supply has two rotary knobs to controll coarse and fine adjustment. Some devices have only one knob which can be pressed or pulled for fine adjustment.
Current limiting / Constant current
Another important feature is current limiting. Current limiting imposes an upper limit on the DC current. This feature helps us to test electronic circuits efficiently. The user sets a current limit and the bench power supply guarantees that the output current does not exceed this limit. A power supply should withstand a short circuit of the output channel!
This feature is commonly used to generate a constant current. If you set the current limit to a fixed value, e.g. 1 A and plug a high load to the input (e.g. short circuit or a circuit with low input resistance), the output current will be constantly at 1A. In this way you simulate a constant current source.
Programmable Power Supply
In some cases you might not want to adjust the output voltage manually. For this purpose there are special lab power supplies which are programmable via USB. You have to install a USB driver for your computer and chose a programming language to talk to the bench power supply. Usually the driver plugs in as virtual com port and is easy to program in most programming languages.
Digital Interfaces
Common digital interfaces for programmable sources are USB connectors and / or RS232 serial interfaces. If your lab supply is not programmable you won't find any interface.
Number of output channels The number of channels usually scales with the product price. Cheap power supply have only one output port. This means you can connect one circuit with variable voltage. If you need two output channels you could either buy two power supplies and plug them together or use a dual output supply. Dual output bench supplies combine two output channels, voltage control and current adjustment in one unit.
Display for Voltage and Current Very cheap power supplies come without display, which means you have to check the output voltage via multimeter. However, normal supplies show the voltage output and current limit on a display. Both values are very useful to monitor to avoid wrong settings.
Maximum electrical power Not only the output voltage range is important, the maximum output current also matters! If your DC current delivers 10V with a maximum current of 10A you have 10*10=100 Watts of output power. The higher the value the better the device. Buy at least 150 Watts.
Stability / Accuracy
The stability of lab power supplies is shown in the datasheet. As the output voltage is regulated, you will find values for line regulation like e.g. 0.01%+3mV. The percentage value shows the stability for a given output voltage. If your output voltage is e.g. 5V, 0.01% means the value is stable up to 0.5mV+3mV=3.5 mV. Thus the output value will fluctuate around 5V+-3.5mV.
Accuracy means how precisely the selected output voltage is hit. Again the accuracy is given as percentage value plus a common offset in mV. If your datasheet describes accuracy with e.g. 0.5%+20mV, this means for 5V a value of 25mV+20mV=40mV. You may miss the 5V level by 40mV.
All of these values are influenced by temperatur and humidity. Smaller percentage values are better for stability and accuracy. However for most experiments these values are not of high importance.
Galvanic Isolation and Grounding
All bench supplies should have galvanic isolation, which means that neither output channel is connected to mains ground. Thus, a good lab supply has a floating voltage output like a battery. You can connect two galvanic isolated lab power supplies to add voltage levels. However keep an eye to maximum voltage range and output power.
Switched vs Linear
There are two types of power supplies. Switched power supplies directly convert the AC power into a DC voltage. No transformer is required for this step which makes these devices cheaper. However switched power supplies have more output ripple, especially in the audio frequency regime. The might not be as precisely regulated as linear power supplies. Linear DC sources are based on transformers and can archive better accuary and stability. However, switching power supplies become better and better and are in most cases interchangable with transformer based devices.
Portability
In some cases you might want to bring your bench power supply somewhere else. In this case choose a portable device with a small footprint and a low weight. If you want to use it at home or in your lab only, portability should not be an issue.
Package Content
Have a look at the package content of your lab supply. Two cables for connecting a circuit would be nice. You will need banana connectors and crocodile clips anyway. Also have a look for a power cord.
Review of commercial lab bench power supplies
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