Relay Flyback Diode Hack

[Bo Deadly]

I accidentally made the relay flyback diode on a pcb too small. The footprint is for a signal diode. But I'm wondering if I could get away with using a signal diode in this particular case because the coil is powered through a 2n7000 mosfet which is turned off by disconnecting the gate from the control voltage which means the capacitance of the gate will need to drain through a 10K pull-down resistor (although this value could probably be increased to 100k without a problem). So this will slow down the turn-off action ever so slightly. Do you think that will reduce the back emf of the relay coil enough to get away with using a signal diode instead of a proper high voltage rectifier?

Does anyone want to take a guess as to the inductance of a TQ2-12 relay coil? If I knew the inductance of the relay coil I could test this in LTSpice.

 

[sahib]

Two ways to go about it.

1). Try on the bench and see.

2). Place the rectifier vertically.

 

[JohnRoberts]

Probably OK try it...

JR

 

[Samuel Groner]

Almost 100% sure a small-signal diode is perfectly fine; I've never used or seen anything else for such a small relay. Also MOSFETs are protected by their internal body diode, so you don't actually need an external clamp diode.

For a 5 V small-signal relay, the coil is probably in the 100 mH range.

Samuel

 

[Bo Deadly]

I have the coil in series with the mosfet so I don't think the mosfet protection diode is going to help. I just tried to simulate this but it peaks at 60V which is the 2N7000 max Vds so I guess the simulation is just showing me the mosfet reverse breakdown.

The signal diode footprint is wide enough (0.3"/7.62mm) for a rectifier diode.  The real problem is the holes are too small. Those rectifier diodes use really thick bus wire.

I will just try a signal diode. If I fry the transistor I'll try 2 diodes in series maybe or drill out the holes and solder both sides to compensate for breaking the through plating.

 

[sahib]

...... or drill out the holes and solder both sides to compensate for breaking the through plating.

Why? Just solder from the top and the thru plating will link to the bottom.