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A good, low-noise power supply is an essential piece of test equipment whether you’re testing analog circuits, like audio amplifiers, digital circuits, like a new Arduino board, or RF circuits, like a new SDR radio. These vital pieces of test equipment supply clean voltages to your devices under test while having the option to limit things like current draw. There are many other protections built into the best DC power supplies, but these are often listed on spec sheets as confusing acronyms. In this post we will run through 10 common acronyms associated with power supplies and explain each of them in layman’s terms.
Let’s jump right in!
10 Common DC Power Supply Acronyms
CC – Constant Current mode
Constant Current Mode (CC) is an operational mode of a power supply such that it outputs constant current. Constant current mode is typically engaged automatically when the power supply hits its current limit. When a power supply is operating in constant current mode, you’ll likely see the output voltage of your power supply drip lower than the set value.
This may be a bit disorienting at first and may take you a second to figure out why your power supply is showing a lower voltage than you want, even if you try to adjust the voltage higher. When the current limit is reached, the device under test is trying to draw more current than beyond the limit. In order to limit the current, the power supply must reduce the voltage output to make everything balance… think Ohm’s Law.
If you unexpectedly see your DC power supply switch to CC mode, it’s best to power down and use a good multimeter to check for shorts or poor connections. Once you’re satisfied, you can report your device and slowly increase the current limit to see if you can get the DC power supply to switch back to constant voltage mode. Just be careful not to let the magic smoke out! ;)
CV – Constant Voltage mode
Constant Voltage mode (CV) is the ‘normal’ operating mode of a DC power supply. When operating in this mode, the power supply will produce a constant voltage output based on the voltage output setting. A power supply will operate in CV mode as long as the current limit is not violated. In this way, the power supply behaves like a voltage source by holding a constant voltage across the output terminals and the current will vary based on the draw from the circuit under test.
PSRR – Power Supply Rejection Ratio
Power Supply Rejection Ratio (PSRR) is a representation of a power supply’s ability to block or suppress noise from reaching the output terminals. For better noise rejection, the PSRR should be negative and the more negative, the more noise suppression. PSRR is important because it is a measure of how much noise injected into the input of the supply (from dirty power, or nearby equipment) can impact the stability of the output signal. Keysight has a great article detailing some of the unwanted signals that can cause noise in power supplies.
OCP – Over Current Protection
Overcurrent protection (OCP) is a mechanism for ensuring sensitive devices are not overloaded. Overcurrent protection is not independent of current limiting and actually works hand and hand with current limiting. Current limiting is engaged when the power supply is operating in constant current mode. Overcurrent protection is additional safeguard. When enabled, OCP will actually turn off the power supply completely after a specified time delay. This implementation ensures that temporary current spikes don’t cause the power supply to shut down completely. If OCP engages, typically it must be reset before the power supply output will be re-enabled. OCP is usually able to be turned off and on through a setting of the power supply.
OPP – Over Power (Overload) Protection
Over Power Protection (OPP) is a safeguard that protects the power supply from burnout caused by a short circuit or overload condition. While OCP and OVP are strictly guarding against over current and over voltage, OPP is a holistic protection that will shut the power supply output off if the power draw surpasses the maximum rated wattage.
OTP – Over Temperature Protection
Over Temperature Protection (OTP) is protection mechanism that monitors the internal temperature of the power supply and disables the output of the power supply if the internal temperature exceeds the limit. Unlike other protections like OCP, OTP cannot be disabled.
OVP – Over Voltage Protection
Over Voltage Protection (OVP) is a safeguard that is meant to protect sensitive devices. OVP works by monitoring the positive and negative terminals of the power supplies output. If the voltage exceeds a set limit, the output is either clamped or shut down entirely.
SCP – Short Circuit Protection
Short Circuit Protection (SCP) is similar to over current protection. When a short circuit is detected the power supply will ‘break’ the circuit and disable the output of the power supply.
SMPS - Switched Mode Power Supply
A Switched Mode Power Supply (SMPS) is a type of DC power supply that uses switching regulators to convert AC signals to DC outputs. Switched mode power supplies are typically more efficient than their linear power supply counterparts. The major drawback of these types of power supplies is that output noise that can appear as a result of the switching circuitry.
UVP – Under Voltage Protection
Under voltage protection (UVP) is a specialized type of power supply protection and therefore may not be found on every model. The primary purpose of UVP is to prevent the output voltage from falling below a user defined low-voltage limit. This is useful in certain applications such as discharging batteries. When the output voltage is lower than the limit, the power supply output will be disabled.
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