The MOSFETs drive the coils of a model railway turnout. I'm using 12V and the coil resistance is 4.5 Ohm. NE556 configured as oneshots with a pulse of approximately 500 ms.
It's my understanding that I should use a diode rated for roughly double the coil current; this would include a safety margin). And the higher the voltage the better.
Looking at the datasheet of e.g. the 1N4001 (e.g. https://www.vishay.com/docs/88503/1n4001.pdf) I see three currents mentioned (IF and 2x IFSM) and I'm not sure which one is relevant. Please advise; if the 1N400x is not suitable, which one would be?
Rectifier diodes can have slow turn-on times, resulting in high voltage spikes. In your application the single (non repetitive) pulse peak current applies, as @awneil pointed out. Otherwise, do you apply the same current safety factor for the switching MOSFET?
The reverse voltage can not be higher than the supply voltage, no need for "the higher the better" - again see turn-on times.
The 1N400x series is quite robust. Use the model which you have at hand or use frequently.
Thanks people. I have none at hand so I am free from that perspective.
It brings the next question
IFSM is specified for 3 different pulse widths; the longer the pulse, the lower the maximum current. The pulse width of the circuit is approximately 500 ms; "interpolating" would possibly result in a IFSM close to 1A. So maybe the pulse width is not the pulse width that I apply but the pulse width when the FET goes from on to off (though I think that that would not be a nice square wave).
It's rated 30A; safety margin 10x But those I have in stock and that is why I picked it.
I learned on this forum that the "slow" rectifier diode 1N400x can perfectly be used as a fly-back diode. It is not suitable for high frequency applications but it does not allow that the forward voltage gets high. It is not possible to give it a peak of 20V and that 20V lingers sometime before it starts to conducting in forward direction, luckily it does not work like that.
Both statements are wrong.
Max reverse voltage across the diode is no more than the power supply voltage.
Turn-on time of different diodes is about the same, and usually not listed in the datasheet.
Turn-off time can be very different.
It could be wise to use a faster (Schottky) diode if PWM is involved.
The 1N4148 , 1N4004, 1N5819 are my general purpose diodes for low voltage.
Leo..
There was definitely a bug in the schematic in the opening post; the reset pins of the NE556 should have been connected to 5V an not to the trigger pins.
I think that I've found a purpose for the two remaining gates of the 74HC05. In the below design they will (or are supposed to) prevent both FET outputs being active at the same time; this will make it a little more idiot-proof.
I think it's time to buy the components and start building a prototype.
But those I have in stock and that is why I picked it.
