Concept
Power Supply Selection for MNB Configuration 1, 2 and 3
The power required from the 5 V DC supply for the bias circuit is extremely small (approximately 5 mA) so just about any small isolated 5 V DC supply will have more than necessary power rating. A low noise power supply with an output isolated from local ground is recommended to minimize the injection of differential noise onto the bus.
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Characteristics |
Recommendations |
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Type |
5 V DC output isolated from local ground or equipment connections |
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Output Voltage |
5.00 V DC +/- 5% (or better) |
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Maximum Output Current |
0.1 A to 1.5 A (0.5 W to 7.5 W) Any model in this popular range |
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Minimum Output Current |
Operates/regulates down to 0 current (no load required) |
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Maximum Output Ripple/Noise |
150 mV pp (or less) |
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Safety/EMC Agency Approvals |
Applicable approvals for country of application |
Frequently, the most convenient power source for the 5 V DC bias supply will be the 24 V AC typically powering the various RS-485 device products. A couple options for the 24 V AC to Isolated 5 V DC power supply would include the models PS-200-3-A-3-L and PS-200-3-A-3-N from Mamac Systems and the model DCP-524 from Kele. Another isolated 5 V supply option with a smaller package/footprint and lower cost would be the combination of the Altronix model VR1TM5 regulator and the small (20 VA) Veris Industries X020ADA 24 V AC to 24 V AC isolation transformer. The figure below shows the Veris/Altronix configuration.
The 1000 ohm resistors shown in the figure above represent the application in Configuration 1 or Configuration 2 showing dual end-point bias. For more information, see MNB Configuration 1: Dual End-point Bias (MNB Source) .For more information, see MNB Configuration 2: Dual End-point Bias (External Supply Source) . For single mid-point biasing such as configuration 3, the values would be 510 ohm. For more information, see MNB Configuration 3: Mid-point Bias .
Other manufacturers/models of 24VAC/24VAC isolation transformers can be substituted to accommodate preferred package, size, local availability, and approvals.
If the use of 115/230 V AC line voltage for the + 5 V DC supply is preferred, the Veris X020ADA transformer can be replaced with a common 115 V/24 V or 230 V/24 V transformer. This can be a separate transformer of the same type used to power the 24 V AC devices. However, the transformer size used for this application can be as small as you have available. In this application, the VR1TM5 presents a load of less than 1 VA. The transformer output used to power the VR1TM5 should not be connected to any other device.
MNB BACnet Devices
MNB Configurations
MNB Configuration 1: Dual End-point Bias (MNB Source)
MNB Configuration 2: Dual End-point Bias (External Supply Source)
MNB Configuration 3: Mid-point Bias