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I'm an electrician going back to school online to get my electrical engineering degree. I am having a hard time with some of the homework, as the book is extremely confusing, and does not always go into detail as to how the math is done. I need to calculate the minimum value of the surge limiting resistor required for a full bridge rectifier with a peak output of 50 volts. The rectifier is using 1N4933 diodes. I'm not asking for the answer-just the math formula to calculate. I would appreciate any and all help. Thanks!

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    \$\begingroup\$ can you post a drawing of the circuit? \$\endgroup\$ May 3, 2015 at 2:21
  • \$\begingroup\$ Id love to-however, there was no picture-strictly a verbal question \$\endgroup\$ May 3, 2015 at 12:40

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The simplest calculation is to assume the worst: that at the instant of switch-on the cap on the far side of the bridge is not charged, and therefore presents almost a short circuit at the instant when power is first applied, for that first half-cycle of 50/60Hz, that's the worse case peak current you have to limit, the "inrush current" (to charge up the cap). Every cycle of the AC after that point, the cap will become more & more charged, drawing less & less current per cycle, until it settles to whatever power is being drawn by the circuit powered.

In that case, the max surge capability for that diode is 30 Amps for 8.3mS (i.e. 1 half-wave of the 60Hz mains frequency). This is straight off the datasheet.

In that case, R=V/I = 50 / 30 = 1.66 ohms. Let's say 1.8 or even 2.2 ohms just to be safe, and to choose a standard resistor value (E12-series).

The only problem with this approach is that resistor is always there, dissipating heat (diode is rated for 1.0A continuous, so presumably this is the design max current for the power supply, so Power = IIR = 1.0 * 1.0 * 1.8 = 1.8 Watts, so it won't be a small resistor! Other real-world solutions would use an NTC resistor whose resistance drops as it heats up, so it limits that inrush current, but then after a few seconds dissipates somewhat less heat (wasted)compared to the 'dumb' resistor. Or, a relay powered by the power-supply turns on after a short delay that shorts out the resistor.

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The formula is ultimately going to be Ohm's law, which I hope they have shown you. E=IR. What I imagine you have to find out is, what is the value of the current it's supposed to limit to? Hopefully they told you, otherwise you may have to consult a data sheet for that particular rectifier to get that information.

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  • \$\begingroup\$ thank you... i will check data sheets, since it does not tell me what the current should be limited to. \$\endgroup\$ May 3, 2015 at 12:41
  • \$\begingroup\$ consulting the datasheet is fundamental to designing with virtually all components. \$\endgroup\$ May 4, 2015 at 3:45

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