Frequently asked questions
What switching technology do DC output power controllers utilize?
Most DC output Power Controllers utilize Crydom MOSFET solid-state relays, which typically have a very low on-state impedance (Rds) and dissipate a minimal amount of power compared to AC output solid-state relays in applications below 100A of load current. Unlike AC output SSRs, DC controllers utilizing MOSFET solid-state relays can be wired in parallel to reduce power dissipation and effectively increase the maximum load-current rating.
Can DC output power controllers be used to control AC loads?
MOSFET power controllers and solid state relays can be wired in inverse-series with the load to switch AC load current. This is an effective method for significantly reducing conducted emissions or minimizing power dissipation. However, since two relays are required the cost of doing so is often prohibitive. Consideration must also be given to the maximum off-state voltage rating of the controller, which can be damaged if exceeded by the peak AC voltage.
How can I protect DC output power controllers from Back EMF?
Back EMF is a phenomenon that occurs when turning off DC loads, such as a DC motor or coil. When power is first removed from an inductive load, the magnetic flux creates an electro-magnetic force with a polarity that tries to maintain the pre-existing current flow. If there isn’t an electrical path for the inductive load current to flow, the collapsing field will generate a voltage high enough to damage the DC solid-state relay portion of the power controller.
The simplest method for protecting the solid-state relay is to place a reverse-biased fast-recovery diode directly across the load. This will allow the field to discharge through the diode and prevent the solid-state relay from being damaged.
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Can DC output power controllers be configured to control the direction of a DC motor?
Yes, controlling the direction of a permanent magnet DC motor with MOSFET output solid-state power controllers is fairly common and easy to accomplish. Essentially, all you need to do is to reverse the polarity of the supply voltage to the motor. To do so, four DC power controllers are wired in an “H Bridge” configuration, a name given because the wiring diagram forms a crude letter “H”, with the motor being the horizontal bar and the two vertical bars comprising two DC power controllers each. To rotate the motor in one direction, two of the four DC output power controllers are turned-on simultaneously; one between the “+” terminals of the DC motor and power source, and the other between the “-” terminals of the DC motor and power source. To reverse the motor, the “forward” DC output power controllers are turned off and the two “reverse” DC power controllers are then turned on; one between the “+” terminal of the power source and “-” terminal of the motor, and the other between the “-” terminal of the power source and “+” terminal of the motor.
Caution must be taken to ensure that the two “forward” and two “reverse” DC output power controllers are not turned on simultaneously. Doing so could result in damage to the power controllers and/or the DC power supply.
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