Help picking a voltage rectifier diode

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Anthony Reynolds

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Hi,
Im designing a Power interrupt (Emergency Machine Off) as a project and having a bit of a problem with picking diodes for the design.
Im using a 24Vdc power supply as the control loop and am looking to put rectifier diodes across the relay coils, only problem is i do not have a lot of experience with diodes.

The power supply i have is a 24Vdc up to 6A.
I was going to buy (BAT42, Schottky Diode, 0.2A 30V 5ns, 2-Pin, DO-35) to do the job but thought id ask here first.
Any info would be helpful.

Thanks
 

When the relay is turned off then its coil current goes through the diode for a moment. We do not know how much current is used by the relay coil.
You do not need a Schottky diode, an ordinary 1N4148 diode will work fine and will have a higher voltage rating of 75V if the coil current is 0.3A or less.
 

The diodes reverse voltage rating can be just above the + volt line, as the diode is normally reversed biased. As the current in the coil is switched off, the magnetic field collapses and tries to send the energising current out again, this results in the "earthy" end of the coil going more positive then the + volt line. The protection diode is then forward biassed and reduces the voltage to about .8 V above the +line. The actual current it conducts during this period depends on many variables and I do not know of a quick way to approximate it, because as the current flows it flows through the relay coil again, but in the opposite direction, so is limited by the coils resistance and inductance.
Frank
 

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