How to Size a DC Power Supply

To choose a DC power supply, you need to know how much current you'll need.

If you're looking to purchase or build a DC power supply, give main consideration to your application's maximum current requirements. Any voltage up to 30 volts is simple to specify. However, the size of components, and therefore the cost of the supply, is directly related to current. Supplies fall into three rough categories: to 1 amp of current, to 5 amps and over 5 amps. You can address the first category with an off-the-shelf AC adapter-style wall plug. The next two sizes are handled by small and large linear supplies, respectively. The last can also be covered by a switching power supply.

Things You'll Need

• Circuit schematic

Instructions

1. Determine the maximum current demands your circuit will have on a power supply. Its schematic and related documentation will tell you how much current it will need at various voltages, and how tightly the current needs to be regulated. Most off-the-shelf power supplies are well-regulated.

2. Determine the voltages your circuit needs. Simple circuits need only a single, positive voltage and ground, but more complex systems, like personal computers, may need several positive and negative voltages with different current requirements. You can also get variable-voltage power supplies that can meet uncommon voltage requirements. The most common power supplies go up to about 30 volts. This satisfies the vast majority of electronic circuits. Specialty or research-grade power supplies can go to much higher voltages, though at greater cost.

3. Select a power supply category using the previously determined current and voltage requirements. This will give you a general range of the power supply's size and cost. Some supplies can deliver two voltages at the same time, but if you need more, you'll have to invest in more than one supply. You can use several supplies by connecting the ground terminals of each one together and using the voltage outputs as you normally would.

4. Narrow down your choice of power supply by determining if you need extra degrees of control. Some applications require controlling or monitoring voltage and current. Better power supplies have a universal serial bus (USB) port and software that let you run them from a computer.

References

• Sparkfun Electronics: 9V DC Power Supply
• National Semiconductor: Introduction to Power Supplies
• Electronic Product News: 1700 Watt Power Supply