To size a DC power supply, determine the total current required by all connected devices and select a power supply with a suitable voltage and sufficient wattage capacity (voltage multiplied by current).
Here's a breakdown of the steps involved in properly sizing a DC power supply:
1. Determine the Voltage Requirement
- Identify the voltage needed: All devices you intend to power have specific voltage requirements (e.g., 5V, 12V, 24V). This information is usually printed on the device itself or in its documentation. The power supply you choose must match this voltage.
2. Calculate the Total Current Draw
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List all devices: Make a comprehensive list of all devices that will be powered by the DC power supply.
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Determine the current draw for each device: The current draw (measured in Amperes, or Amps) of each device can be found on its label or in its specifications. Be sure to note whether the current listed is the typical current or the maximum current. It's best to use the maximum current for safer calculations.
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Add up the current draws: Sum the current requirements of all the devices on your list to find the total current the power supply needs to deliver.
Example:
Device Voltage (V) Current (A) Device 1 12 0.5 Device 2 12 1.0 Device 3 12 0.2 Total: 12 1.7
3. Calculate the Required Wattage
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Multiply voltage and total current: Calculate the minimum wattage required by multiplying the required voltage by the total current draw.
Wattage (Watts) = Voltage (Volts) x Current (Amps)
In the example above: 12V * 1.7A = 20.4 Watts.
4. Add a Safety Margin
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Account for overhead: It's crucial to add a safety margin (or "headroom") of at least 20% to your wattage calculation. This prevents the power supply from operating at its maximum capacity, which can reduce its lifespan and reliability. Operating at or near maximum capacity can also cause the power supply to overheat. A larger safety margin might be necessary for devices that have significant inrush current on startup.
In the example above: 20.4 Watts * 1.20 = 24.48 Watts.
5. Select a Power Supply
- Choose a power supply: Select a power supply with a voltage that matches your devices' voltage requirements and a wattage rating higher than your calculated value including the safety margin. In the example above, a 12V power supply rated for at least 25 Watts would be a suitable choice. It is always better to go slightly higher than right on the number.
Considerations:
- Inrush Current: Some devices, especially those with motors or large capacitors, draw a very high current for a brief period when first turned on. This is called inrush current. Ensure your power supply can handle the inrush current of your devices, or use a soft-start circuit to limit the inrush.
- Power Supply Efficiency: Real-world power supplies are not 100% efficient. A less efficient power supply will draw more current from the AC mains to deliver the required DC power, generating more heat in the process.
- Regulation: A well-regulated power supply will maintain a stable output voltage even with changes in input voltage or output current.
- Ripple and Noise: The output of a DC power supply is not perfectly smooth; it contains small amounts of ripple and noise. Choose a power supply with ripple and noise specifications that are acceptable for your application.
- Operating temperature: Be sure to consider the environment your power supply will be operating in. Heat can affect the output of your power supply.
By carefully considering these factors, you can select a DC power supply that will reliably power your devices.
