Finding the Full/Minimum
Load of a Power Supply

Troubleshooting tips from our Technical Team

Here at Jameco, we get a lot of calls and emails from customers asking for troubleshooting tips as well as tips on how to maximize the performance of their products. This article provides tips on how to troubleshoot some of the most common questions we receive. If you would like us to solve a technical problem or have a solution you think is worthy, send a message to: [email protected].

Question: The datasheet of my power supply mentions something about applying a full load and a minimum load. What is a full load, a minimum load, and how do I know how much it is?

Each power supply is designed to operate in a certain range of conditions, and each has maximum operating conditions in which the supply should not exceed.

The full load of a power supply refers to the maximum operation ratings of the power supply. If it is supplying the rated current (same as maximum current) at the rated voltage, then the load that is connected is the full load. There is no set value for a full load because each power supply is rated differently.

The more important value that many people should be concerned with is the minimum load requirement. This is the value needed for many switch mode power supplies as well as many unregulated power supplies to function correctly.

When the proper minimum load is not applied, the power supply usually flickers, and seems to go on and off rapidly. Leaving an output without a load on it will cause this to happen. This is because for most switch mode and unregulated power supplies, the outputs need to be stabilized.

Using Ohm's Law: V = IR, you can calculate the minimum load if you know the voltage and minimum current ratings.

I = Current in units of amperes (A)
V = Voltage in units of volts (V)
R = Resistance in units of ohms (Ω)

Manipulating this formula yields a resistive load of R = V/I. From here, just plug in the values for V and I, and that is your minimum load resistive value. Important: Keep in mind the wattage rating of your power supply. It must match up with the wattage rating of the minimum resistive load. It is a good rule of thumb to use a load with a wattage rating at least 1.5 times larger than the supply's wattage rating.

For Switch Mode and Unregulated Power Supplies:
1) Find the voltage and minimum current ratings of each output.
2) Use Ohm's Law: R = V/I to calculate each output load.

Example: You have Triple Output AC/DC Power Supply, which has the following ratings:

+5 Volts @ 0.6 Amps (Channel 1)
+12 Volts @ 0.2 Amps (Channel 2)
-12 Volts @ 0.1 Amps (Channel 3)
Using Ohm's Law, we calculate the minimum resistive load for each channel:
Channel 1: R = V/I = 5V/0.6A = 8.3Ω
Channel 2: R = V/I = 12V/0.2A = 60Ω
Channel 3: R = V/I = 12V/0.1A = 120Ω

Notice that Channel 3 is rated at -12V, but it is not included as a negative value in our calculations. We cannot apply a negative resistive load. Once again, the wattage ratings of the loads should be at least 1.5 times the rated output of the supply. Use the formula for power: Power = Voltage x Current, or P = VI.

If you are trying to calculate the minimum load and you happen to only know the power and voltage ratings of your power supply, you can use the formula P = V2/R, which can become R = V2/P.

If for any reason you only have the current and power ratings of your power supply, you can use P = I2R, which can be changed to R = P/I2.

As you can see, calculating the minimum required load of your power supply is a very simple process. Just look up a few ratings from the datasheet and you can apply the correct size load in no time.

Note: Remember not to apply a load over the full load value for a sufficient length of time because it may result in damage or overheating of your power supply.

For more information about power supplies and accessories please visit the Jameco Power Resource Center.