Flyback diodes on boats

[hjohnson]

Originally Posted by markxengineerin

It might be possible to add a sensor to the holding tank, but I have no space on top (v berth mattress sitting directly on top). I do have a SCAD TM2 tank monitor that does it wirelessly from one of the side walls, but it can be flaky at times when it's humid out or when the wall it's mounted to gets wet (condensation, from cleaning, whatever). So far, the repeatability of this stroke counter thing is +/- 5 pumps on 460, pretty good! I've been using it for 3 or 4 months now.

Ahh, we had about an inch and a half, so just drilled a hole in the cleanout hatch and added a cable gland. Cable routes from there back to the electronics (our tank is also under the vee berth)

[Lake-Effect]

An oscilloscope would be very useful as you troubleshoot and try mods to the sensor interface.

[markxengineerin]

I do have an oscilloscope, so I'll do some before/after when I start experimenting

[danno7]

I must add a couple of statements to consider considering flyback diodes

1. A lot of motors are operated in two directions. Generally a windless is used to bring in the anchor and let it out. The voltage to the motor is reversed to go the other way. So you put a flyback diode on the motor, and it will last about a microsecond when you send the motor the other way. Flybacks diodes can work on motors in both direction, but you need a bidirectional setup with TVS style diode. That gets complicated. Not worth solving because of discussion item 2.

2. Most all large motors are turned off and on by a relay. The relay turns on, and there is a very large current to the coils in the motor. When you turn it off, the relay contacts open up and the current does convert to a rather large voltage spike on the motor coils. But you must consider that the relay contacts are OPEN when the spike occurs, and the large voltage spike self dissipates in the motor. It does not hurt a thing; there is nothing connected to hurt.

3. Relay bounce....This is a complication with discussion item 2 above. Relay connections are often not simply on or off. Relays can bounce upon making or breaking contact. Suppose you have a cheap relay on happily driving your windless. Then you shut off the relay to stop the windless. After shutting off, the voltage on the motor will always spike up. Suppose in the next millisecond the relay contacts bounce from open to closed again. They close again just for a millisecond, but they do so when the voltage is spiking on the motor, and that voltage will then carry back through the 12 volt wires to other electronics. So when relay bounce is considered, it can happen.

Most large relays will not bounce, and some have mechanical dampers to stop it. The starter motors in a billion cars do not have flyback diodes. Maybe a few do, but most do not. It is not an issue with them because starter motor relays do not bounce.

Relay bounce can happen. In that case, a flyback diode is a good idea, but put it on the input side of the relay. Not on the motor side. That way if the voltage spike occurs when the relay contacts close again on the bounce, you are protected.

4. Where you do always need a flyback diode is on the relay coil that turns on your windless. Most electronics engineers know about the flyback diode needed on every relay ever made. They automatically include it in all relay control circuits.


That being said, you can get those voltage spikes coupling to other wires (if near) and there can be other minor complications. Electronic FET style relays can also creat complications, but that is a matter for a different thread.

[Wotname]

Originally Posted by danno7

I must add a couple of statements to consider considering flyback diodes

1. A lot of motors are operated in two directions. Generally a windless is used to bring in the anchor and let it out. The voltage to the motor is reversed to go the other way. So you put a flyback diode on the motor, and it will last about a microsecond when you send the motor the other way. Flybacks diodes can work on motors in both direction, but you need a bidirectional setup with TVS style diode. That gets complicated. Not worth solving because of discussion item 2.

2. Most all large motors are turned off and on by a relay. The relay turns on, and there is a very large current to the coils in the motor. When you turn it off, the relay contacts open up and the current does convert to a rather large voltage spike on the motor coils. But you must consider that the relay contacts are OPEN when the spike occurs, and the large voltage spike self dissipates in the motor. It does not hurt a thing; there is nothing connected to hurt.

3. Relay bounce....This is a complication with discussion item 2 above. Relay connections are often not simply on or off. Relays can bounce upon making or breaking contact. Suppose you have a cheap relay on happily driving your windless. Then you shut off the relay to stop the windless. After shutting off, the voltage on the motor will always spike up. Suppose in the next millisecond the relay contacts bounce from open to closed again. They close again just for a millisecond, but they do so when the voltage is spiking on the motor, and that voltage will then carry back through the 12 volt wires to other electronics. So when relay bounce is considered, it can happen.

Most large relays will not bounce, and some have mechanical dampers to stop it. The starter motors in a billion cars do not have flyback diodes. Maybe a few do, but most do not. It is not an issue with them because starter motor relays do not bounce.

Relay bounce can happen. In that case, a flyback diode is a good idea, but put it on the input side of the relay. Not on the motor side. That way if the voltage spike occurs when the relay contacts close again on the bounce, you are protected.

4. Where you do always need a flyback diode is on the relay coil that turns on your windless. Most electronics engineers know about the flyback diode needed on every relay ever made. They automatically include it in all relay control circuits.


That being said, you can get those voltage spikes coupling to other wires (if near) and there can be other minor complications. Electronic FET style relays can also creat complications, but that is a matter for a different thread.

Yes to the all of this plus a couple more points.

1. The magnitude of any spike is proportional to the current flowing at the time of disconnect. The current is proportional to the load (torque) on the motor and in the case of many windlass motors, inversely proportional to speed of the motor. Thus if the windlass is turned off when the load is minimal (speed maximum), the current will be low, thus the spike will be small.

2. EMI (i.e.an inductive voltage spike) is delivered as a voltage in the wiring near the source of the interference (as referenced in #2 by danno7) OR as RF source (with the wiring being the antenna etc). RFI is generated every time there is spark. A spark is exactly like a lightning bolt only much much smaller. It is possible that very sensitive poorly filtered electronics near the RFI source (i.e. a spark) could be affected. I have to say I have never heard of a windlass being a problem though.

3. All EMI (voltage or RF) is tamed (reduced) in the same manner. Get as close to the source as possible and add series inductance and/or parallel capacitance. Ferrite beads / clamps are usually the easiest way to add series inductance.

4. It is always better to remove or reduce the source EMI than to shield the affected circuit but sometimes the only solution is shielding.

[danno7]

Originally Posted by Wotname

Yes to the all of this plus a couple more points.

1. The magnitude of any spike is proportional to the current flowing at the time of disconnect. The current is proportional to the load (torque) on the motor and in the case of many windlass motors, inversely proportional to speed of the motor. Thus if the windlass is turned off when the load is minimal (speed maximum), the current will be low, thus the spike will be small.

2. EMI (i.e.an inductive voltage spike) is delivered as a voltage in the wiring near the source of the interference (as referenced in #2 by danno7) OR as RF source (with the wiring being the antenna etc). RFI is generated every time there is spark. A spark is exactly like a lightning bolt only much much smaller. It is possible that very sensitive poorly filtered electronics near the RFI source (i.e. a spark) could be affected. I have to say I have never heard of a windlass being a problem though.

3. All EMI (voltage or RF) is tamed (reduced) in the same manner. Get as close to the source as possible and add series inductance and/or parallel capacitance. Ferrite beads / clamps are usually the easiest way to add series inductance.

4. It is always better to remove or reduce the source EMI than to shield the affected circuit but sometimes the only solution is shielding.

I agree completely with wotname. He just gets into the technical details a bit more. The bottom line is you can probably put a huge flyback diode on the 12 volts near the windless relay, but it might not do too much. Again, think of all the starter motors in every car, and none have flyback diodes.

[rbthorson]

I have had similar problems caused by the power trim solenoids on two Mercruiser stern drives. If your motors are solenoid actuated, connect a 5A 1KV diode cathode to the solenoid positive terminal and anode to the solenoid negative terminal. If you have more than one solenoid do the same on every one.

[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.

[markxengineerin]

Thanks- Amazon has lost my first package of 9V power supplies, so more waiting for 1st test. Lots of good info here, some above my head but it will be enough to help me troubleshoot step by step.

[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.

[team karst]

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.

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.

[markxengineerin]

It turns out I was powering the prox sensor with 5V, same 5V used to power arduino, and coming from the house battery bank 12V through a buck converter. Here are some measurements as wired now:



(>3V peak above hard to see, but present)


Does this info help to narrow down what the best next step is? It seems the system is picking up noise spikes even with the digital input channel totally disconnected from anything and with the power wires cut.

In the meantime, I'll keep disconnecting other wires from the breadboard and try to isolate the issue.

[team karst]

Sounds like a job for a well placed capacitor.

[markxengineerin]

Here was one of the capacitor suggestions

1591hz lowpass filter I think

Others from EngNate
"connect a 0.1mfd capacitor across the connections of the sensor"- Which terminals? Is this the same as above?

"use filter capacitors at the power input - 470mfd and 0.1mfd in parallel." Easy to understand this one, but what does it do, for my own curiosity?

[markxengineerin]

Above circuit works beautifully.
(I haven't even switched the power supply over to 9V yet, but it's working on 5V despite the minimum datasheet spec being 6V for the prox sensor.)
Thanks to all who contributed, nice to solve this problem in such a simple way.

Noise free now no matter what I do with the windlass.