NI-9478 Getting Started

• The NI-9478 has sinking outputs. Sinking outputs drive current from DO to COM when the channel is on.
• You must connect the Vsup pin to the power supply to enable a weak cable clamping diode that protects the module from cable inductance flyback.
• This power supply provides the current for the devices you connect to the module.

The inductance of the cabling stores energy when a channel is on and driving current. When a channel turns off, energy in the cabling is released as flyback voltage, which dissipates as heat in the NI-9478. The heat dissipated increases with higher switching frequencies, higher currents, and longer cables.

The effective cable length is the total distance from DO to COM.

NI recommends adding a capacitor across the power supply leads and a diode from the DO lead to the Vsup lead. Connecting a capacitor and a diode reduces the effective cable length and dissipated heat.

The NI-9478 provides protection against overcurrent and short-circuit conditions.

The NI-9478 provides two configurable current limit thresholds, Limit A and Limit B. The device monitors each output channel. You can review the output status in software to determine if the current has exceeded the limit threshold.

You can configure each channel on the NI-9478 to disable the output that exceeds the current limit threshold. Some applications require a large inrush current that may exceed the current limit threshold. You can disable overcurrent protection by setting no current limit for that channel.

The NI-9478 provides an overcurrent refresh setting. This setting enables the channel to recover within a specified refresh period. If the refresh setting is disabled, the channel remains off until it receives a command from the software.

The total amount of current that you can switch with a channel depends on the duty cycle of the channel, the ambient temperature, the switching frequency, and the amount of current switched by other channels within the same module at the same time.

NI recommends using 2 meters of cable for safe operating conditions. Higher currents, higher frequencies, and longer cables contribute to increased heat in the NI-9478.

You can determine safe operating specifications for the NI-9478 by using the following equation.

where

• P is the percentage of total allowable power the channel uses
• I_out is the output current in amperes while the channel is on
• length is the total cable length in meters
• freq is the frequency rate in kHz at which the output channel goes through an on-off cycle

Calculate the percentage of total allowable power each channel uses then add the percentages for all the channels. For ambient temperatures from 55 °C to 70 °C, the total power for all channels must remain under 100%. For temperatures below 55 °C, the total power of all channels can run up to 150%.

The NI-9478 passes 1.25 A current through 2.0 meters of cable. The channel cycles on for 40 μs and off for 10 μs for a total output period of 50 μs. This yields an output frequency of 20 kHz, using 14.8% of the total power budget.

The NI-9478 is on and passing 3 A of current through 2.0 meters of cable for 0.25 seconds. The channel passes 1.25 A of current at 20 kHz for 1.25 seconds. The channel then drives the load off for 4 seconds.

While the channel is on, the output frequency is 0 kHz and the channel uses 36% of the total power budget. The channel uses 14.8% of the total power budget driving at 20 kHz and 0% when the channel is off. The total percentage of allowable power over the complete 5.5 second cycle is 5%.

The NI-9478 is available with conformal coating for additional protection in corrosive and condensing environments, including environments with molds and dust.

In addition to the environmental specifications listed in the NI-9478 Safety, Environmental, and Regulatory Information, the NI-9478 with conformal coating meets the following specification for the device temperature range. To meet this specification, you must follow the appropriate setup requirements for condensing environments. Refer to Conformal Coating and NI RIO Products for more information about conformal coating and the setup requirements for condensing environments.