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
| 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.
| 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.