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I have a rectified DC Voltage and I will switch in a dump load resistor. I have a 25PV045 Schottky diode connected across it as a flyback to deal with the inductive properties of the load when the switch is turned off.

I also have an LED which goes on when the switch is on and this has a protective 1N4148 diode.

My question is when the Schottky is in flyback mode will any of the current go through the small diode and perhaps damage it or will resistor R68 prevent this?

enter image description here

Thanks.

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    \$\begingroup\$ D28 will protect it. \$\endgroup\$ Jul 4, 2018 at 13:55
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    \$\begingroup\$ Is the R68 rated for 600 V and 1.8 W? You should use several resistors in series. The maximum reverse voltage of a 1N4148 is only 75 V, it is protected by the LED only. \$\endgroup\$
    – Uwe
    Jul 4, 2018 at 14:31
  • \$\begingroup\$ @Uwe Yes its a 2W rated resistor. The LED surely cannot protect it, is it best to use a higher rated diode? \$\endgroup\$
    – MXG123
    Jul 4, 2018 at 15:29
  • \$\begingroup\$ I would not use a 2 W rated resistor for 1.8 W load in this circuit. It should be rated for 3 or 5 W, but it should be rated for more than 600 V too. \$\endgroup\$
    – Uwe
    Jul 4, 2018 at 18:33
  • \$\begingroup\$ @Uwe - Why the large headroom for a 1.8W resistor? \$\endgroup\$
    – MXG123
    Jul 5, 2018 at 13:54

2 Answers 2

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The 25PV045 Schottky diode will die as soon as you apply an input voltage greater than 45 volts and the relay/MOSFET switches on. This is because its maximum voltage rating is only 45 volts and according to your schematic you may have 600 volts at the input.

My question is when the Schottky is in flyback mode will any of the current go through the small diode and perhaps damage it or will resistor R68 prevent this?

Given that the main diode is likely dead, any inductive kick-back current will force its way through the 1N4148 and the 200 k resistor and probably damage the resistor. Now your LED circuit is fairly functionally dead.

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  • \$\begingroup\$ You are right about the shottky being only rated for 45V. So assuming I replace this with a higher rated one does the second part of you reply still apply? \$\endgroup\$
    – MXG123
    Jul 4, 2018 at 15:32
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    \$\begingroup\$ If the schottky is rated correctly then it will safely transport the kick-back current with only the odd micro amp passing through the 1N4148. Make sure your resistor is rated at 600 volts or higher. The standard for quarter watt resistors is only 200 volts from memory. \$\endgroup\$
    – Andy aka
    Jul 4, 2018 at 15:37
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    \$\begingroup\$ @MXG123 are we done with this Q and A now? \$\endgroup\$
    – Andy aka
    Jan 20, 2023 at 16:04
  • \$\begingroup\$ Yes it is done. \$\endgroup\$
    – MXG123
    Jan 21, 2023 at 17:14
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No, there will be no significant current going through D32, as it will be protected by both the resistor and the lower forward voltage of the schottky.

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