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Prevent an Inductive Load from Damaging Your Power Supply

Posted by Grady Keeton on Sep 27, 2019 9:55:43 AM

Resistive loads are easy on a DC power supply. When you turn off the power, the current quickly drops to zero and no damage is done.

Inductive loads are another matter. If you're using a DC power supply to power DC motors, solenoids, fans, relays and other inductive loads, you need some kind of circuit protection. Without this protection, your power supply can be damaged by high voltage spikes from these devices.

Figure 1 shows an inductive load connected to a DC power supply. When the power supply is turned on, current flows through the coil and a magnetic field builds up around the inductor. That magnetic field is a source of potential energy.

 

Graphic

 

When the power supply is turned off and no longer is supplying current, the magnetic field collapses, and this collapsing field induces a current that will flow in the opposite direction. It's as if you connected a battery (shown in blue in Figure 1) of opposite polarity to the power supply input. This voltage is called the fly-back voltage or back EMF.

The fly-back voltage can be much higher than the power supply voltage originally supplied to the inductive load. Even if you were powering the inductive load with only 12 V or 24 V, the fly-back voltage could be several hundred to several thousand volts. This voltage is equal to L, the inductance of the load times di/dt, which is the rate of change of current with respect to time. The more quickly the current changes, the higher the voltage.

One way to protect your power supply from high fly-back voltages when driving inductive loads or loads having stored energy that could be circulated back to the power supply is to use a protective diode network at the output of the power supply. This is also shown in Figure 1.

To prevent damage to the power supply from inductive voltage kickback, connect an anti-parallel diode (rated at greater than the supply’s output voltage and current) across the output. Connect the cathode to the positive output and the anode to return. Where positive load transients, such as back EMF from a motor might occur, or stored energy is present such as a battery, a second blocking diode in series with the output is recommended to protect the power supply.

Ensure that the chosen components are suitably rated for the inductance and energy to be dissipated. The peak reverse voltage ratings should be a minimum of two times the power supply maximum output voltage. The continuous forward current ratings should be a minimum of 1.5 times the power supply maximum output current. In some cases, a heatsink may be required to dissipate the power caused by the flow of current.

For more information on AMETEK Programmable Power solutions, contact one of our sales representatives by visiting powerandtest.com/sales. You can also email us at sales.ppd@ametek.com or call 800-733-5427 or 858-450-0085.

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