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Minimum Load Requirements for LP and NT Power Supplies

This document discusses minimum load requirements for some LP and NT series power supplies from Astec Power. It explains that these supplies require a minimum load on some or all outputs to ensure proper regulation. If the system load does not meet the minimum, resistors can be added. As an example, it shows that an LPT45 supply requires a 0.4A minimum load on its 5V output, which can be achieved with a 12 ohm, 4 watt resistor. The minimum load specifications are provided on the individual data sheets for each power supply model.

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115 views1 page

Minimum Load Requirements for LP and NT Power Supplies

This document discusses minimum load requirements for some LP and NT series power supplies from Astec Power. It explains that these supplies require a minimum load on some or all outputs to ensure proper regulation. If the system load does not meet the minimum, resistors can be added. As an example, it shows that an LPT45 supply requires a 0.4A minimum load on its 5V output, which can be achieved with a 12 ohm, 4 watt resistor. The minimum load specifications are provided on the individual data sheets for each power supply model.

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Embedded Power for

Business Critical Continuity

Application Note
From Astec Power Rev. 00

APPLICATION OVERVIEW: Power Supply Minimum Load Requirements

PRODUCT: LP and NT Products with Minimum Load Requirements

ORIGINATOR: Tom Tillman, Product Line Manager


MOLEX
09-50-8061
-RS +RS

Pin 2&3 = +5V

Pin 1 = +15V
Pin 4&5 = Common
Pin 6 = -15V

12 ohm
100 ohm
100 ohm
Figure 1

DESCRIPTION
There are LP and NT power supplies that require a minimum load on some or all of the outputs.
For proper operation minimum loading is required on these outputs to ensure regulation. Without
minimum load the power supply may turn on and off repeatedly or have output voltages that are
out of regulation. If the system load does not draw the minimum current then resistors can be
added to meet the minimum load. As an example, figure one shows a typical minimum loading
requirement for the LPT45 model supply. The 5v output requires a 0.4A minimum load. The value
of the resistor can be determined by dividing the required minimum load current into the respective
output voltage, R = V/A. For the 5V output the minimum load is 0.4A, so 5V/0.4A = 12.5, a 12 ohm
resistor will do. To determine minimum power rating, square the current requirement and multiply
that value times the resistor value determined above, W = A2*R. The above requirements for the 5
volt output would be 0.4 X 0.4 = 0.16A. Then, 0.16A X 12 ohms = 1.92 Watts. It is recommended
to double this value to 4 watts. Repeat this procedure for the other 2 outputs. This method can be
used for any power supply requiring a minimum load. The minimum load is specified on the data
sheets. System fans, LED’s, lights and other devices can be included towards the minimum load.

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