What is a Variable Power Supply?
A variable power supply, also known as an adjustable power supply, is an electronic device that provides a controllable DC voltage source for powering and testing electronic circuits and devices. Unlike fixed power supplies that only output one constant voltage level, variable power supplies allow the user to adjust the output voltage to a desired value within the supply’s voltage range. This makes them incredibly versatile tools for working with a wide variety of electronic projects.
Variable power supplies typically use a built-in potentiometer or digital controls to adjust the output voltage. They also usually have built-in current limiting to protect the connected device from drawing too much current and damaging itself. Most bench-top variable DC power supplies used for electronics work output an adjustable voltage anywhere from 0V up to about 30V DC and can supply currents up to around 5A, although higher voltage and current models are available for specialized applications.
How Does a Variable Power Supply Work?
The basic design of a linear bench power supply is relatively simple. Here are the key components and how they work together:
Transformer
The first stage is a transformer that steps down the AC line voltage (120V AC in the US) to a lower AC voltage. This is needed because the final DC output will be less than the input AC voltage. The transformer also provides isolation from the AC line for safety.
Rectifier
The stepped down AC voltage is then sent to a rectifier circuit that converts it to a rough DC voltage. This is typically done using a full-bridge rectifier made up of four diodes. The rectifier output is a pulsing DC voltage at twice the AC line frequency.
Filter
To smooth out the pulsing DC into a fairly constant DC voltage, it is sent through a filter circuit, usually consisting of large electrolytic capacitors. The capacitors charge up during the voltage peaks and discharge during the gaps to even out the voltage.
Regulator
The filtered DC voltage is then sent to an adjustable voltage regulator circuit. This is the heart of a variable power supply. The regulator circuit, along with the adjustment potentiometer or digital setting, allows the output voltage to be set to a desired level. Linear regulators work by dropping the excess voltage to maintain the set output voltage.
Adjustable Current Limit
Most bench power supplies also incorporate an adjustable current limiting circuit. This works to limit the maximum current that can be drawn by the connected load to prevent damage. If the current draw exceeds the set limit, the supply will drop the voltage to keep the current within the limit.
Meters and Output Terminals
Analog or digital meters on the front panel display the output voltage and current levels. The positive (red) and negative (black) output terminals allow convenient connection of the power supply to the device being powered using banana plugs or alligator clips.
Types of Variable Power Supplies
Variable power supplies come in a few different common types and form factors:
Bench Power Supply
This is the most common type for electronics work. Bench power supplies are designed to sit on a workbench and provide variable DC power for powering circuits, charging batteries, and testing devices. They provide controls to adjust voltage and current limit and have meters to display the output.
Laboratory Power Supply
Lab grade power supplies are high precision instruments designed for critical applications in science and engineering labs. They offer very precise voltage and current regulation and advanced features like programmable outputs and remote control capability. Lab power supplies are much more expensive than standard bench supplies.
Programmable Power Supply
Programmable power supplies can have their voltage and current settings controlled digitally through a computer interface. This allows automated testing under software control. Some programmable supplies can be programmed with a sequence of voltages and currents to test a device under dynamic conditions.
Battery Eliminator
A battery eliminator is a compact variable power supply designed to replace batteries for testing battery-powered devices. They allow easy testing with different voltages to simulate various battery conditions. Battery eliminators are commonly used for testing consumer electronics.
Form Factors
In addition to the bench-top form factor, variable power supplies are also available in rack-mount, modular, and wall adapter styles for integration into test systems or end products.
Key Specifications of Variable Power Supplies
When selecting a variable power supply, there are several key specifications to consider:
Voltage Range
This specifies the minimum and maximum output voltage that the supply can provide. Make sure the range covers the needs of your application. Common voltage ranges for bench supplies are 0-20V, 0-30V, 0-40V, and 0-60V.
Current Range
The current specification is the maximum current the supply can provide at full voltage. Typical current ratings for bench supplies are from 0-1A up to 0-5A or more. Higher currents may be available at lower voltages. Always choose a current rating that exceeds your needs.
Load Regulation
Load regulation specifies how much the output voltage changes with load current. This is a measure of the quality of the power supply’s voltage regulator. Lower percentage is better. For example, a 0.01% load regulation means the voltage will change by only 0.01% over the full current range.
Line Regulation
Line regulation specifies the change in output voltage with changing input AC line voltage, another measure of regulation quality. Lower is better.
Ripple and Noise
This indicates the amount of AC voltage that is superimposed on the DC output. It is a function of the quality of the supply’s rectifier/filter circuit and voltage regulator. Lower ripple and noise is desirable, especially for powering sensitive analog or RF circuits. Ripple is typically expressed in millivolts RMS.
Stability
Stability is how constant the output voltage stays over time with a fixed load. This can be affected by temperature changes. Good bench supplies have stability better than 0.1%.
Protection Features
Variable power supplies should have built-in protection against short circuit, overload, over-voltage, and over-temperature conditions to prevent damage to the supply and the connected equipment.
Metering
Look for a supply with clear voltage and current meters. Some have analog moving-coil meters while others use digital LCD or LED readouts. Digital meters are more precise, but some users prefer the responsiveness of analog meters.
How to Use a Variable Power Supply
Using a variable bench power supply is quite straightforward, but it’s important to follow some basic procedures to ensure safe and effective operation. Always double check your connections and settings before turning on the power.
1. Set the Current Limit
Before connecting your circuit, it’s a good practice to set the current limit to a safe value that won’t damage your device under test if there is a fault. Consult your circuit’s documentation and set the current limit to a value above the expected maximum current draw, but below a damaging level.
2. Set the Voltage
Many power supplies have a separate voltage adjustment knob or buttons. Set this to the voltage required by your device. Again, consult your device’s documentation and ensure the power supply voltage is compatible.
3. Connect the Device
With the power supply still off, connect the positive (red) and negative (black) terminals to the appropriate points on your device. Double check your connections to make sure the polarity is correct.
4. Turn On Power and Check Operation
Turn on the power supply and check that the voltage and current readings are what you expect. If something looks wrong, turn off the power immediately and recheck your connections and settings.
5. Adjust Voltage if Needed
If you need to change the voltage, you can adjust it while the device is powered on. Keep an eye on the current meter to ensure you aren’t drawing too much current.
6. Power Off and Disconnect
When you’re finished, turn off the power supply before disconnecting your device. It’s generally a good idea to turn the voltage and current limit settings down before powering off.
Safety Tips for Using Variable Power Supplies
Working with electricity always carries some risk, so it’s important to follow some basic safety guidelines:
- Always double check your connections and settings before applying power
- Never exceed the voltage and current ratings of your device or the power supply
- Use the appropriate gauge wires for the current levels
- Ensure your circuit has proper fusing or other overcurrent protection
- Never work on a powered circuit
- Use caution when working with high voltages as they can deliver a dangerous or lethal shock
- Don’t defeat the safety ground on AC powered equipment
- Wear safety glasses to protect from potential sparks or debris
- Work in a well-ventilated area when working with batteries or power components that can get hot
Applications of Variable Power Supplies
Variable DC power supplies are essential tools for anyone working with electronics. Here are just a few common applications:
Circuit Design and Prototyping
When designing and testing new electronic circuits, a variable power supply allows easy testing with different voltage levels to optimize the design.
Equipment Repair
Variable power supplies are invaluable for troubleshooting and repairing electronic equipment by providing a controlled power source to isolate problems.
Battery Charging
The adjustable voltage and current limiting features of a variable power supply make it effective for safely charging various battery types and testing battery performance.
Product Testing
Variable power supplies are used in product testing to verify the proper operation of a device under different voltage and load conditions and to test its protection circuitry.
Lab Experiments
In science and engineering labs, precision variable power supplies are used to provide controlled power for experiments and measurements.
Frequently Asked Questions (FAQ)
Q: Can a variable DC power supply be used as a battery charger?
A: Yes, the adjustable voltage and current limiting capability make variable DC supplies suitable for charging many types of rechargeable batteries, as long as the voltage and current are set correctly for the specific battery chemistry.
Q: What happens if I short the output of a variable power supply?
A: Most bench power supplies have short circuit protection that will limit the current to a safe level and shut down the output voltage until the short is removed. However, it’s never a good idea to intentionally short a power supply.
Q: Can I connect multiple variable power supplies in series or parallel?
A: In general, it’s not recommended to connect multiple supplies in series or parallel unless they are specifically designed for it. Connecting in series can create dangerous high voltages, and connecting in parallel can cause uneven current sharing and potentially damage the supplies.
Q: What’s the difference between a regulated and unregulated power supply?
A: A regulated power supply maintains a constant output voltage under varying load conditions, while an unregulated supply’s voltage will drop as more current is drawn. Most bench-type variable power supplies are regulated for more precise and stable voltages.
Q: How accurate are the voltage and current meters on a typical bench power supply?
A: The accuracy of the meters can vary depending on the quality of the supply, but most decent bench supplies have an accuracy of within ±1% for voltage and ±2% for current. Higher-end lab grade supplies can have much better accuracy, on the order of ±0.01%.
Conclusion
A variable DC power supply is an indispensable tool for anyone working with electronics, from hobbyists to engineers. The ability to precisely control voltage and limit current makes them incredibly versatile for powering, testing, and repairing a wide range of electronic devices. When selecting a variable power supply, it’s important to choose one with the right specifications for your needs, including voltage and current ranges, regulation, and protection features. By following proper usage and safety procedures, a good bench power supply will provide years of reliable service in your electronics projects.
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