Flyback diodes on boats

[Lake-Effect]

Great. Thanks for posting your measurements and the followup.

[rbthorson]

Originally Posted by YANDINA

So much advise from experts. My turn .
Current is unrelated to motor load current, the diode is never in series with the motor and only carries the inductive spike and only for milliseconds. Any 5 amp diode would be ample.
Voltage is unimportant, when blocking voltage the maximum it gets is battery voltage, any 50 volt or more diode will do. It never sees the inductive spike voltage, it is conducting it away.

I would recommend a minimum reverse diode voltage of twice the operating voltage of the motor so it can survive any positive transient voltages which may be on the line.

[rbthorson]

Originally Posted by team karst

This magic diode continues to conduct the full coil current, which reduces as L x I sq energy dissipates in the R of that loop. Notably, if its a relay coil, the release time is significantly lengthened.

The diode is in forward conduction almost immediately after the relay voltage is removed (counter-EMF) causing it to dissipate the energy of the coil to ground so there is no delay.

[rbthorson]

Originally Posted by EngNate

1. The flyback peak current will be equal to the current flowing in the coil when it is switched off. Your suppression diode should have a peak forward current rating (IFpk) of twice this value. A diode's peak current will be much higher than its average current, you will have to consult the data sheet for this.

2. Flyback is of the opposite polarity to the applied voltage. The diode's voltage rating (peak reverse voltage) is relevant to the supply voltage, not the flyback voltage. The flyback will be applied to the diode in the forward direction and so will be clamped to the diode's forward voltage drop value. Choose a diode with a PRV/PIV at least twice the supply voltage. (We always double the minimums)

3. Suppression diodes are only needed to protect components that remain connected to the coil when it is switched off, typically a transistor that drives a relay or solenoid. This is usually within the circuitry of a piece of equipment. The only instances I've seen where one was needed elsewhere is run-indicator lights connected to, for instance, an electric clutch coil.

That said, suppressing the spike at its source should work, but other things I would try are: connect a 0.1mfd capacitor across the connections of the sensor; Run the power for your counter back to the main distribution, don't have it share with any other circuit, and use filter capacitors at the power input - 470mfd and 0.1mfd in parallel.

The best place to suppress this issue is at the source. you probably will solve all your problems. Motors generally do not generate much, if any "flyback". A motor big enough operate a windlass is most certainly relay actuated. A relay coil can easily generate a flyback pulse of several hundred volts on a 12V system. A diode can shunt the flyback current to ground immediately thereby preventing a large pulse.

[markxengineerin]

Do most 12V relays (like automotive style) have these built in?

[team karst]

I tend to doubt it. The drive ckt on the pcb usually has the diode; this way it is not dependent on the replaceable relay for it.

[YANDINA]

Relays with built in diodes are very common. When ordering thousands from China we had to specifically request non-diode version. One shipment that came with them we hit with high forward current to fuse them out.

[rbthorson]

Originally Posted by markxengineerin

Do most 12V relays (like automotive style) have these built in?

I am not aware that automotive relays have diodes built in but with all the equipment being electronically controlled it is likely that some of the relays may have some kind of protection built in. Resistors or capacitors are sometimes used instead.

[danno7]

Originally Posted by rbthorson

I am not aware that automotive relays have diodes built in but with all the equipment being electronically controlled it is likely that some of the relays may have some kind of protection built in. Resistors or capacitors are sometimes used instead.

I agree, almost all relays do not have flyback diodes. The person designing the use of the relay should know this. Design it into the circuit if you think it is needed. Any relay with a flyback diode that is corrected backwards will blow the diode instantly.

[danno7]

Originally Posted by YANDINA

Relays with built in diodes are very common. When ordering thousands from China we had to specifically request non-diode version. One shipment that came with them we hit with high forward current to fuse them out.

I guess your experience with flyback diodes installed in relays is different than mine. I never observed this in the products I used relays in. You could be in a different industry application.