Choosing a Benchtop Power Supply What to Know | Tektronix

The problem is that different benchtop power supplies offer unique levels of functionality. With that in mind, you should look beyond your current needs and find a benchtop DC power supply that can support your intended use case in the long run.

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Direct current (DC) plays an important role in the charging of batteries and the powering of many electronic systems. It’s also useful in several testing and research applications. However, the standard wall outlet emits alternating current (AC), which means you’ll need a reliable benchtop DC power supply to facilitate your research and testing needs.

This guide explains everything you need to know about these testing tools so you can make an informed decision.

What Is a Benchtop Power Supply?

A benchtop power supply is an instrument designed to support testing, prototyping, and electronic product development tasks by providing a reliable source of DC electricity. These versatile tools offer adjustable voltage and current, making them indispensable for engineers, technicians, and hobbyists working on electronic projects.

Whether it’s for designing circuits, testing components, or bringing new ideas to life, a benchtop power supply ensures that users can apply precise power settings to meet the unique demands of each application. The importance of a reliable bench power supply can’t be overstated; it allows you to administer controlled, measurable inputs to create specific electrical conditions.

Moreover, in the prototyping phase, a benchtop power supply enables you to iterate quickly, adjusting power parameters like output voltage as needed to refine your designs. This flexibility accelerates the development process, helping to bring products from concept to market faster.

Programmable power supplies offer the most versatility, as you can use them to create custom sequences or implement built-in protection mechanisms. These instruments facilitate the simulation of complex power scenarios. Several types of benchtop power supplies are available, and each one offers unique benefits.

Understanding Benchtop Power Supplies

Some of the key industries that use benchtop power supplies include:

• Electronics
• Telecommunications
• Automotive
• Aerospace

Common alternatives to benchtop units include rack-mount and modular test equipment. Rack-mount supplies are intended for integrated systems or industrial applications. They are mounted in standard equipment racks, often prioritizing higher outputs and durability for continuous operations.

Modular power supplies offer flexibility via customizable configurations, allowing you to tailor components like outputs and interfaces to your unique needs. They are best suited for complex or evolving systems.

Benchtop supplies bring the best mix of accessibility and convenience, providing your team with immediate, hands-on control for a wide range of testing and development tasks. As such, these power sources are a staple of laboratories and workshops the world over.

Key Considerations When Choosing a Benchtop Power Supply

As you explore lab power supply options, you must consider a range of factors, including the following:

Voltage and Current Requirements

First, you should think about your output power needs. Evaluate the voltage and current limit of each DC bench power supply you are considering. This will allow you to quickly rule out options that can’t provide the output current or voltage you require.

You can begin to determine your voltage and current requirements by examining your intended applications and the electronic load characteristics of the equipment you’ll be testing. Don’t choose a lab bench supply that barely meets your output needs. Instead, select an output power supply that exceeds your target thresholds by at least 25% so you can perform stress testing and other advanced evaluations.

Features and Specifications

Next, identify which features you’ll need. Some specifications to consider include:

• Regulation: The unit’s ability to maintain a constant voltage
• Ripple: The small, undesirable fluctuations in the output voltage
• Transient Response: How quickly and effectively a supply can react to changes in load

The best systems can maintain a near-constant current and voltage, even while being subjected to fluctuating input voltages. High-precision output DC voltage allows you to conduct more accurate and reliable tests and avoid overvoltage situations that may compromise your results.

Other factors to consider include set voltage and current accuracy. All systems have a margin of error. However, you must ensure that these margins fall within acceptable thresholds based on the sensitivity of your tests.

Form Factor and Size

The form factor of switching or linear DC power supplies refers to their physical specifications, including their size and shape. Consider the space constraints of your workstations and whether technicians will need to move the instruments around or store them in between testing.

If you have large workbenches, you can accommodate bigger instruments. However, ensure that they are not so cumbersome that it becomes difficult for your staff to manipulate them. Generally speaking, the more powerful the unit, the greater its form factor. However, higher-end systems tend to be more compact than their less sophisticated counterparts.

User Interface and Controls

The most basic instruments have analog controls and gauges. If you need an adjustable power supply, it will have many more controls to manipulate, which is why you need an LED or LCD display. An LED display takes the guesswork out of making adjustments to the settings, as your technicians can verify output levels in much more precise increments.

Choosing an instrument with user-friendly controls also promotes more efficient testing. With that in mind, it’s probably a good idea to get your team involved when comparing equipment options. After all, they are the ones who will be using the units every day.

Safety Features

Most high-end power supplies have advanced safety features. However, older or less sophisticated units might have fewer protections. At a minimum, you want to ensure that your benchtop DC power supply includes:

• Overvoltage protection
• Overcurrent protection
• Short-circuit protection

Purchasing equipment with robust safety features isn’t enough. Ensure you teach your technicians how to use these features so that they can keep themselves and others safe while running diagnostics tests. Safety is especially important when dealing with high-voltage or high-amperage equipment.

Types of Benchtop Power Supplies

There are multiple ways to classify benchtop power supplies. The following takes a closer look at some of the most common types to see how they compare in terms of cost-effectiveness, efficiency, and size.

Linear vs. Switching Power Supplies

Linear power supplies operate by first transforming the input AC voltage to a lower AC voltage before rectifying and filtering it to produce a clean DC output. These machines are favored for their simplicity, low noise, and minimal ripple. If you are testing audio or analog circuits or other sensitive equipment, a linear system may be a good fit.

However, linear supplies are less efficient because they dissipate excess voltage as heat, leading to larger and heavier designs due to the need for heat sinks and often fans. Despite these drawbacks, their output quality and reliability keep them in demand.

Switching power supplies use more sophisticated techniques involving converting input AC voltage to DC by rapidly switching the AC signal on and off through a transformer to achieve the desired output voltage. This process is much more efficient, allowing for smaller, lighter designs since less heat is generated. However, these systems may introduce higher levels of ripple and noise, though advancements have mitigated these effects.

Single Output vs. Multiple Output

As the name suggests, a single output supply provides one DC output voltage. They are widely used for tasks that require a consistent power source for a single device or circuit. Their simplicity and lower cost make them a practical choice for many applications.

Multiple output power supplies offer two or more independent channels, allowing users to power several devices or circuits simultaneously. A dual output supply can emit different voltages and currents from each channel. Although these instruments tend to be more expensive and sophisticated, their ability to support diverse applications makes them extremely valuable in multifaceted testing environments.

Selecting the Right Benchtop DC Power Supply

With so many options out there, it’s important to carefully compare several options before placing an order. As you begin exploring power supply equipment, consider these factors:

• Your project’s specific voltage and current requirements
• High accuracy and low ripple thresholds
• Form factor to ensure the device fits your workstation
• The type of supply (e.g., linear or switching)

Additionally, determine whether you need advanced features or functionality to accommodate more complicated requirements. For instance, if you are conducting complex tests, choosing a supply with autoranging capabilities would be wise. Conversely, if you need a simple instrument and want to minimize your initial investment costs, consider a supply that offers one output and fewer features.

Explore The Selection of Benchtop DC Power Supply Solutions

Are you in the market for a bi-directional power supply? There are a wide range of sophisticated testing EA instruments, including autoranging DC power supplies and bi-directional systems. The instruments feature digital displays, variable power supply functionality, and high-performance measurement capabilities to facilitate precise testing.

AC-DC Conversion Basics

A power supply takes the AC from the wall outlet, converts it to unregulated DC, and reduces the voltage using an input power transformer, typically stepping it down to the voltage required by the load. For safety reasons, the transformer also separates the output power supply from the mains input.

Figure 1, Figure 2, and Figure 3 illustrate the general transformation from AC to DC.

Alternating current takes the form of a sinusoidal wave with the voltage alternating from positive to negative over time.

Figure 1: Alternating Current from Wall Outlet

In the first step of the process, the voltage is rectified using a set of diodes. Rectification transforms the sinusoidal AC. The rectifier converts the sine waves into a series of positive peaks.

Figure 2: Full Wave Rectified

Once the voltage has been rectified, there is still fluctuation in the waveform—the time between the peaks—that needs to be removed. The rectified AC voltage is then filtered or “smoothed” with a capacitor.

The capacitor is typically quite large and creates a reservoir of energy that is applied to the load when the rectified voltage drops. The incoming energy is stored in the capacitor on the rising edge and expended when the voltage falls. This significantly reduces the amount of voltage droop and smooths out the voltage. Increasing the storage capacity of the capacitor generally produces a higher quality power supply.

Figure 3 shows the rectified voltage and how the capacitor smooths the droop.

Figure 3: Full Wave Rectified + Capacitor

Once the voltage conversion is complete, there is still some variation in output, called ripple. In a regulated power supply, the voltage is then passed through a regulator to create a fixed DC output with less ripple.

Power Supply Comparisons

AC power supplies come in two varieties, unregulated and regulated. Unregulated is the most basic type of power supply and does not have the ability to supply consistent voltage to a load, while regulated power supplies do and have many different design options.

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Linear converters are the least complex but also create the most heat, while switched converters are more intricate and cooler but create more noise. Batteries are typically switched converters. Each has advantages and drawbacks but which to use will be based mostly on the type of application and the conditions under which it will be run.

Table 1 illustrates how the types of power supplies are categorized and summarizes many of the pros and cons of each type.

AC-DC Fundamentals

Alternating current is generated by electrons flowing in alternating directions. Direct current is generated by electrons flowing in only one direction.

AC is also called mains electricity, household current, domestic power, line power, or wall power because it is the voltage supplied by a wall outlet. Worldwide, AC voltages range from 100 to 240 V. The rate of direction change is typically 50 to 60 times per second and is designated as Hertz (Hz). The two most common frequencies are 50 Hz and 60 Hz.

The equipment designed to use AC tends to require large amounts of voltage, so the voltage is not stepped down as frequently as equipment using DC.

DC allows for a constant flow of current to a device. Because AC is delivered initially, power must first be converted from AC to DC.

Most smaller electronic devices (such as computers) require direct current to operate with an AC-to-DC converter from wall power.  Alternating current is used for equipment that have motors (refrigerators are an example). Which type of current to use depends on the load being powered.

CAUTION: WHILE BOTH AC AND DC CAN SHOCK YOU, AC IS BY FAR THE GREATER RISK BECAUSE THE CURRENT CAPACITY IS SO MUCH GREATER. BE SURE THAT ALL ELECTRICAL EQUIPMENT THAT ACCESSES THE AC MAINS HAS A GROUND FAULT CIRCUIT INTERRUPTER (GFCI) IN USE.

Unregulated Power Supply Theory

Because unregulated power supplies do not have voltage regulators built into them, they typically are designed to produce a specific voltage at a specific maximum output load current. These are typically the block wall chargers that turn AC into a small trickle of DC and are often used to power devices such as household electronics. They are the most common power adapters and are nicknamed a “wall wart”.

The DC voltage output is dependent on an internal voltage reduction transformer and should be matched as closely as possible to the current required by the load. Typically the output voltage will decrease as the current output to the load increases.

With an unregulated DC power supply, the voltage output varies with the size of the load. It typically consists of a rectifier and capacitor smoothing, but no regulation to steady the voltage. It may have safety circuits and would be best for applications that do not require precision.

Figure 4: Block Diagram — Unregulated Linear Supply

The advantages of unregulated power supplies are that they are durable and can be inexpensive. They are best used, however, when precision is not a requirement. They have a residual ripple similar to that shown in Figure 3.

NOTE: Wavelength does not recommend using unregulated power supplies with any of our products.

Regulated Power Supply Theory

A regulated DC power supply is essentially an unregulated power supply with the addition of a voltage regulator. This allows the voltage to stay stable regardless of the amount of current consumed by the load, provided the predefined limits are not exceeded.

Figure 5: Block Diagram — Regulated Supply

In regulated power supplies, a circuit continually samples a portion of the output voltage and adjusts the system to keep the output voltage at the required value. In many cases, additional circuitry is included to provide current or voltage limits, noise filtering, and output adjustments.

Linear, Switched, or Battery-based?

There are three subsets of regulated power supplies: linear, switched, and battery-based. Of the three basic regulated power supply designs, linear is the least complicated system, but switched and battery power have their advantages.

Linear Power Supply
Linear power supplies are used when precise regulation and the removal of noise is most important. While they are not the most efficient power source, they provide the best performance. The name is derived from the fact that they do not use a switch to regulate the voltage output.

Linear power supplies have been available for years and their use is widespread and reliable. They are also relatively noise-free and commercially available. The disadvantage to linear power supplies is that they require larger components, hence are larger and dissipate more heat than switched power supplies. Compared to switched power supplies and batteries, they are also less efficient, sometimes exhibiting only 50% efficiency.

Switched Power Supply
Switched mode power supplies (SMPS) are more complicated to construct but have greater versatility in polarity and, if designed properly, can have an efficiency of 80% or more. Although they have more components, they are smaller and less expensive than linear power supplies.

Figure 6: Block Diagram — Regulated Switching Supply

One of the advantages of switched mode is that there is a smaller loss across the switch. Because SMPS operate at higher frequencies, they can radiate noise and interfere with other circuits. Interference suppression measures, such as shielding and following layout protocols, must be taken.

The advantages of a switched power supply is that they are typically small and lightweight, have a wide input voltage range and a higher output range, and are much more efficient than a linear supply. However, a SMPS has complex circuitry, can pollute the AC mains, is noisier, and operates at high frequencies requiring interference mitigation.

Battery-based
Battery-based power is a third type of power supply and is essentially a mobile energy storage unit. Battery-based power produces negligible noise to interfere with electronics, but loses capacity and does not provide constant voltage as the batteries drain.  In most applications using laser diodes, batteries are the least efficient method of powering the equipment. Most batteries are difficult to match the correct voltage to the load. Using a battery that can exceed the internal power dissipation of the driver or controller can damage your device.

Selecting a Power Supply

• When choosing a power supply, there are several requirements that need to be considered.
• The power requirements of the load or circuit, including
  • voltage
  • current
• Safety features such as voltage and current limits to protect the load.
• Physical size and efficiency.
• Noise immunity of the system.

Important Specifications

While all power supply specifications are valuable, some are more critical than others. A few specifications of note are:
Output Current: The maximum current that can be supplied to the load.

Load Regulation: The load regulation is how well the regulator can maintain its output with a load current change, and usually is measured in millivolts (mV) or as a maximum output voltage.

Noise & Ripple: Noise is any added and unwanted electronic interference, and ripple is the small variation in voltage when AC is transformed into DC. These are typically combined into one measurement. In switching power supplies, the measurement is given in peak-to-peak, showing the extent of the noise spikes that arise from the switching.

Overvoltage Protection: Sometimes output voltages can exceed their nominal values and can damage the load. Overvoltage protection is a circuit that shuts down the power supply should the voltage limits be exceeded.

Overload Protection: Overload protection is a safety measure used to prevent damage in the event of a short circuit or overcurrent event. Much like the circuit breaker in a house, the overload protection shuts off the power supply so the load will not be damaged.

Efficiency: Efficiency is the ratio of power being pulled from the power grid that is effectively being converted to DC power. A good SMPS power supply will operate with at least 80% efficiency and, with a proper system design, can operate at even higher rates. An efficient system will reduce heat generation and can save energy.

Noise & Ripple

Noise and ripple are artifacts of the transformation of AC to DC and are the byproduct of rectification and switching.  During conversion, the alternating sine wave cannot be completely suppressed.  These artifacts are typically combined into one specification, given in peak-to-peak voltage, showing the extent of the noise spikes that arise from switching, which can negatively affect sensitive instrumentation.

The small voltage variations are called ripple.  Many times, the amount of fluctuation depends on how well the power supply is matched to the load.

Noise is the unwanted additions that occur outside of the normal ripple. It comes from many other sources, including switching and electronic noise generated outside of the power supply, such as from nearby electronics. Noise usually occurs in conjunction with ripple and is much more variable and unpredictable. Switching noise typically occurs at very high frequencies.

Figure 7 is an example of noise (created by switching) and ripple in a medium quality power supply.

Figure 7: Ripple & Noise for Regulated, Switched Power, Representative of a Mid-Quality Supply

Figure 8 illustrates the noise potential in a regulated linear power supply. While much less than the ripple of a regulated switched supply, it can still be significant enough to mask data.  If the noise and ripple are very high, small signals can be overwhelmed or the life of the hardware can be significantly shortened. However, with a high quality power supply it can be virtually eliminated.

Figure 8: Ripple & Noise for Regulated, Linear Power, Representative of a Mid-Quality Supply

Frequently Asked Questions

• How much current does my power supply need?
  A good rule of thumb is to combine the amount of drive current needed to meet the needs of the load and the current needed to power the controllers, and then add 10% as a buffer.
• Can I use the same power supply to drive my temperature and laser controllers?
  Yes, as long as the voltage selected meets the needs of the load and is within the Safe Operating Area (SOA) of the controller, and the laser, thermoelectric, and temperature sensor are not electrically tied through ground.
  For more information on SOA, see Application Note AN-LDTC01: The Principle of the Safe Operating Area.
  For more information on grounding, see Application Note AN-LD16: Grounding with Special Laser Diode Configurations.
• Can I use the same power supply to drive several temperature controllers at the same time?
  Yes, but they should be connected using star (individual) connections rather than serial (linked) connections to ensure that the voltage to each unit is the same.
• Can I use a 30 V power supply with a temperature controller whose specifications are for 5 V – 30 V?
  Yes, if the load voltage is high enough. Use the Safe Operating Area (SOA) calculator for the controller to determine if the load and the power supply voltage are compatible.
• Can I use a 4.5 V power supply?
  Review the minimum specifications for the controller in the “Absolute Maximums” section of the datasheet. If allowed, subtract the compliance voltage drop across the controller from 4.5 V to see if there will be enough voltage to the load.
• How long should my cables be?
  Longer cables introduce more noise and allow for more voltage drop between the power source and the load. To minimize the affects of noise, twist the cables together or use shielded cables. To help minimize voltage drop, keep them as short as possible.
• What gauge wire should I use?
  The gauge determines the current carrying capacity and needs to exceed the specification of the load. For example, using Table 2, which shows some standard gauges and carrying capacities for solid wire, a 20 gauge wire, not a 22 gauge wire, would be required for a load of 2 A. This is because the 20 gauge wire allows a 3.2 A current, which exceeds the 2 A requirement.

A Little History

At the advent of electrical distribution, the standard current in Europe was alternating current (AC) and in the US was direct current (DC). The main load of electrical usage was the light bulb, which was designed by Thomas A. Edison to use direct current. The early competition between which electrical distribution system would dominate the market in the US was called The War of Currents and was typically personified as a conflict between inventor Edison (Con Edison/General Electric) and entrepreneur George Westinghouse (Westinghouse Electric), who invested in the AC technology as a power distribution method. The conflict was actually much larger, with American and European companies having a vested interest in the decline of one type or the other.

The drawback of alternating current at the time was that load on the system was affected when appliances were turned on and off, affecting others using the line. Direct current had the advantage of using only the current needed and not affecting the rest of the load on the line.

Unfortunately for the DC proponents, the voltage drop across the wires from source to output was significant and different voltages could not be sent across the same wires. This meant that DC generation required generating plants to be located within about a few miles of their destination and multiple wires strung for each voltage needed, both of which were prohibitively expensive for rural communities. A further disadvantage was that DC was really only feasible for small appliances, as the low voltages were too inefficient to be scaled up for technology that required large voltages.

In contrast, high voltages of alternating current could be transmitted very long distances on a single line, for hundreds of miles and therefore required fewer generating plants. Transmission of AC resulted in very little voltage drop, and the voltage could then be reduced through the use of transformers to the desired amount needed at its destination. Additionally, with increasing industrialization, the high voltages were desirable for large loads, such as railways and industrial motors.

Competition remained between AC and DC proponents until a series of events, contract awards, and field trials proved that AC transmission lines were the most economical, although DC lines remained in use well into the 20th Century. Since most appliances, developed by Edison, were DC devices, the need for direct current was strong. Since AC could be stepped down to any DC voltage, DC systems are still utilized.

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