Alternator / Zener Diode

[4329]

Without a condition monitor for the surge protector like the Balmar posted has I don't find much peace of mind in installing one.

If you are not going to incorrectly reverse the battery connections or remove a battery terminal when the engine is running, then there should be no need for a protection diode.

Protection diodes where fitted many years ago when people used to disconnect the battery cable to see if the alternator was charging, this will damage the alternator and regulator due to voltage spikes.

The old generators were tested in this way by people with zero electrical knowledge and generators not being electronic in any way generally resulted in no damage.

 

[JimPghPA]

Any circuit where a switch or relay is driving a motor or electromagnet, a diode in parallel (with proper polarity aligned) is very helpful and extending the life of things.

This statement is an over-simplification, and therefore in error. The first word "Any" could be taken literally by persons without an understanding of electronics. It should read "DC circuits" instead of Any. Put that diode across an AC circuit and you had better leave plenty of room for smoke.

 

[JimPghPA]

I never bothered using a zener - used in the right alignment, I used those more for linear power supplies as they have a unique characteristics of a high forward voltage drop, good as a voltage reference. For this kind of “surge protection,” any standard 1A diode would do.

This statement ^ above is also in error. A one Amp diode would not work in this application.

In the event of a sudden disconnect of load the voltage regulator will very quickly shut-down the excitation drive current to the spinning armature. However, there will be residual magnetism in the metal of that spinning armature that will for a brief time continue to excite the coils in the stator windings, resulting in the output diodes still providing output of the same polarity that was normally produced when the load was still connected. However, without any load, the output voltage will suddenly become very high. A reverse diode acros the output would do nothing to stop this because the polarity will not flip in the other direction like it does when the coil of a DC relay is suddenly disconnected. Because the output voltage polarity remains the same when the output voltage spikes, from a sudden disconnect of load of an alternator, a zener is the correct snub device to use.

Note, some (the correctly rated maximum voltage before conduction, and response time, and power disipating ability) Varistor may also work as a snub in this application. And while MOV's are designed for use with AC they can be used with DC applications. Using a MOV in a DC circuit is using a more expensive than necessary device than the less expensive Varistor, but it will work.

And if the response time of a Varistor or MOV was too slow, a capacitor could be used in addition, to slow down the rise time of the spike until the device absorbed the spike.

 

[4329]

Any circuit where a switch or relay is driving a motor or electromagnet, a diode in parallel (with proper polarity aligned) is very helpful and extending the life of things.

You mean any dc circuit and not an ac circuit. In a dc circuit a diode will as stated be very usefull and is widely used .

 

[JimPghPA]

You mean any dc circuit and not an ac circuit. In a dc circuit a diode will as stated be very usefull and is widely used .

No. In this DC circuit a regular diode will NOT provide the proper snub. Read the long second paragraph of my above post.

There is an interesting side-bar to this. A regular diode may be damaged if used in this application, because when the output spikes it could excede the maximum reverse voltage of a regular diode and cause that diode to fail and conduct as it blew up, thus possibly providing a one time use snubbing device.

 

[4329]

No. In this DC circuit a regular diode will NOT provide the proper snub. Read the long second paragraph of my above post.

There is an interesting side-bar to this. A regular diode may be damaged if used in this application, because when the output spikes it could excede the maximum reverse voltage of a regular diode and cause that diode to fail and conduct as it blew up, thus possibly providing a one time use snubbing device.

Perhaps we ar talking about different things. Please read this.

Why are Protection Devices used in a Relay?​

Relays can produce a large voltage spike when they are switched off due to the coil de-energising. Resistors or diodes are sometimes fitted across the coil of the relay to stop/reduce these spikes travelling back into the control circuit and damaging sensitive components. Resistors are more durable than diodes, but not quite as efficient at eliminating voltage spikes. You need to assess the sensitivity of the components in the control circuit when deciding if / what type of protection is required.

 

[meep]

@JimPghPA - excellent clarification. Thank you for the expertise. I clearly punted to the edge of my understanding and you carried the ball the rest of the way down the field.

I am not formally trained in this and have been self taught when it comes to DC electronics. I've built audio preamps, control systems for small scale PV, and small scale power supply / power controls, but not advanced in any particular area. I'd literally never seen zeners applied there and usually started with something around the 1N00* family in most cases. But I was not dealing with Big Power here, either.

Great stuff!

 

[JimPghPA]

I was / am, aware that a resistor could be used to limit a spike, but using one like that is not efficient.

Somewhat changing the subject, an interesting device that I only worked with designing into a circuit one time in all the electronics I ever designed is a Varistor with a reverse voltage vs conduction curve. An industrial computer I worked on could do an incorrect boot if the power experienced a very short power disruption, resulting in the filter capacitor dropping low enough to not provide proper DC output from the power-supply, but for a brief time not low enough to trip the reset for a clean reboot start-up. The solution was to put a reverse slope Varistor of the proper voltage and power ratting across the main filter capacitor. When the voltage started to drop the reverse slope Varistor conducted a lot of current quickly discharging the filter capacitor, quickly bringing the 5 V supply low, resulting in tripping a circuit in the compute to cause a clean reboot when power came back on. It was a big deal, because otherwise the software was lost in sequence of running of that industrial computer that was controlling industrial processes.

A ups for only the computer would of been another fix, but insuring a clean reboot was good enough. Besides, the company using that industrial computer was not going to install a ups large enough to power everything it controlled.