Renard64 Power Supply Calculations: Difference between revisions

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These margins are not great, but they should be satisfactory (especially for locales with 60Hz power line frequencies).  The calculations largely assume worst case conditions for all of the factors at the same time (which is how they should be made), but in real life all of the worst case conditions will seldom be encountered at the same time.  In addition, none of the components should stop working if the power supply voltage dips below 5.0V, so the small margin should not be of great concern.
These margins are not great, but they should be satisfactory (especially for locales with 60Hz power line frequencies).  The calculations largely assume worst case conditions for all of the factors at the same time (which is how they should be made), but in real life all of the worst case conditions will seldom be encountered at the same time.  In addition, none of the components should stop working if the power supply voltage dips below 5.0V, so the small margin should not be of great concern.


===Methods of Improving the Margins====
===Methods of Improving the Margins===


1) Use a higher-current transformer.  The output of an over-rated transformer will be higher because the current is less than the rated value.
1) Use a higher-current transformer.  The output of an over-rated transformer will be higher because the current is less than the rated value.

Latest revision as of 19:53, 2 June 2011

Power Supply Calculations for Renard64

This will be divided into two parts. The first part is the power draw calculation, the second part is the power supply margin calculations for that load.

Power Draw

Power Draw Calculations
Item Calc Value Qty Total
PIC16F688 3mA (assumed) 3mA 8 24ma
Power LED 3V/300Ω 10 mA 1 10mA
RS485 RX chip 0.5mA 1mA 1 1mA
RS485 TX chip 0.9mA 1mA 1 1mA
RS485 Terminator 3.6V/120Ω 30 mA 1 30mA
Channel LEDs 3V/650 = 4.6mA 4.6mA 64 295mA
SSR Optos 4V/650 = 6.2mA 6.2mA 64 394mA
SSR Power LEDs 3V/650 = 4.6mA 4.6mA 8 37mA
Total 792mA

Notes:

1)680Ω Resistors are assumed to be at about -5% (650Ω)

2)PIC16F688 maximum current is not specified, but assumed to be 3mA.

Power Supply Margin Calculations

The method here is to start with the peak output voltage from the 6.3VAC transformer, and subtract the various voltage drops (including the regulator dropout voltage and the regulator output voltage). The resulting number will be the margin at the input to the regulator.

"Power Supply Margin"
Item Calc Value Delta Result
Transformer Nominal Output 6.3v * 1.4 8.8V 8.8V
Low Power Line Offset (-5%) 8.8V * 0.05 0.44V -0.5V 8.3V
1N5817 Diode Drop (x2) 0.45V * 2 0.9V -0.9V 7.4V
Capacitor Droop (@ 50Hz) ΔV = IΔT/C 0.8A * 0.01S / .0054 -1.5V 5.9V
Regulator Dropout Voltage 0.6V (extrapolated) 0.6V -0.6V 5.3V
Regulator Output Voltage 5.0V 5.0V -5.0V 0.3V
Margin 0.3V

Notes: 1) the 6800μF Capacitor is assume to the 20% low (tolerance + low-temp derating).

Discussion

These margins are not great, but they should be satisfactory (especially for locales with 60Hz power line frequencies). The calculations largely assume worst case conditions for all of the factors at the same time (which is how they should be made), but in real life all of the worst case conditions will seldom be encountered at the same time. In addition, none of the components should stop working if the power supply voltage dips below 5.0V, so the small margin should not be of great concern.

Methods of Improving the Margins

1) Use a higher-current transformer. The output of an over-rated transformer will be higher because the current is less than the rated value.

2) Use a higher-value capacitor in place of the 6800 uF cap. This will decrease the capacitor droop, and thus increase the margin.

3) Remove the Channel LEDs. This will substantially lower the current drawn by the board, and thus greatly increase the voltage margin.