Alternator / Zener Diode

[dark66eyes]

I have a Hatz 2L41C diesel engine on a Haulotte 3388 scissor lift that the alternator isn't charging due to an open diode in the exciter circuit. While diagnosing the problem I discovered that it has a zener diode between B+ and D+ at the alternator. Machine wasn't charging when I received it so I have no idea what was done to it prior in regards to the charge system. My question is why is there a zener diode between B+ and D+? Was someone trying to make it self exciting to remedy the exciter circuit break? Is the zener diode machine specific and installed by Haulotte? Hatz wiring diagram doesn't show a zener diode at the alternator. It's a 4 wire (W, B+, D+, D-) 14 volt 60 amp alternator (Hatz # 50504200) with an external voltage regulator. Zero tech support from Haulotte in the U.S. for this machine.

 

[midnightlaundry]

I think it's there to prevent flyback voltages but not sure. According to your description, I assume D+ is output..

 

[Lubener]

The zener will be rated for a specific voltage. The likely reason for its existance is to keep the voltage which is between B+ and D+ lines (difference between the two) at that specific voltage which the zener is rated for. It helps with regulation.

 

[dark66eyes]

That was my suspicion that it had something to do with voltage regulation but Hatz doesn't call for it. I spoke with Hatz service and they couldn't find any reference for it to be there. Hatz suggested having one of their techs look at the system which sounds like an awfully expensive shot in the dark if they don't have reference of the diode. I read that a zener should show high resistance in one direction but an ohm test didn't show any resistance in either direction.

 

[dark66eyes]

B+ is out put midnight and D+ is the exciter circuit and battery light.

 

[meep]

Are you sure that’s a zener?

 

[dark66eyes]

Number came back as a zener

 

[dark66eyes]

I was told it was used to protect from lighting and transient voltage (surge / spike protection) which should have it connected to D- rather than D+. If how I found it installed is correct it appears as Lubener suggested as a voltage regulator between B+ output and D+ exciter.

 

[JimPghPA]

It could be there to limit output if the output becomes totally suddenly unloaded (open). There can be a delay in reduction of magnetic drive flux of the rotating armature after the voltage regulator detects the output is too high and shuts down the excitation. Without a zener that output could shoot up to a very high voltage for a very brief time, possibly high enough to arc to something, and or bridge a small gap to + resulting in a high voltage spike that takes out electronics of the vehicle.

Zeners have a very limited power dissipation capacity when used to snub a spike, but can limit output until the voltage regular and magnetic reluctance of the armature no longe overdrive an output. Were talking very brief amounts of time here. Just a guess but probably a few 1/1000's of a second (miliseconds).

 

[dark66eyes]

Would it work as you suggest Jim if installed the way I found it? It appears based on my ohm test to be faulty. I haven't run the engine with the alternator working yet for fear of damage to some other component without it.

 

[JimPghPA]

An ohm meter is not a good way to test a zener.

Connect a dropping resistor (to limit current) to the + output of a DC power supply, and connect the other side of that resistor to the zener while measuring the voltage across the zener. See if the zener limits how high the voltage gets across the zener.

As to how you connect it to the alternator, I'm not sure I understand the connections you described.

 

[spackard]

The data sheet shows it turning on at 40.90 to 45.20V, so nothing to do with regulation. Kind of makes sense between D- and B+. I don’t know what it’s supposed to clamp; it’ll work between 10 and 1000 microseconds.

 

[4329]

Reverse polarity protection

 

[dark66eyes]

If it's polarity protection how should it be connected?

 

[dark66eyes]

If for polarity / surge protection here's an example wired B+ to Negative = D-

 

[dark66eyes]

Going to reach out to Balmar's tech support and see what they have to say about my diode.

 

[dark66eyes]

Do these surge protection diodes survive a surge or do they require replacement if a surge occurs?

 

[Brad_C]

Depends on the size of the "surge". It'd have to be a pretty short spike for a zener to do anything and not blow up.

Given a working battery has a very low internal resistance, I suspect they're relying on the zener not being useful or failing without leaving a physical indication (ie blowing in half). In the event of a failure (say a regulator shorts or battery goes O/C) the zener will "protect" against spikes for a few milliseconds before it pops. It's certainly not going to protect any devices in the vehicle (which should already be protected against load dump).

A bit like those capacitors you plug into your lighter that will double your fuel efficiency. It's most efficient task is separating you from some $$ in your wallet. Harmless, but useless.

 

[dark66eyes]

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

 

[meep]

Diodes are particularly useful in certain circumstances. It’s not so much a surge protector, though the name does work … a diode is a one way valve. A coil of wire with electricity through it begins to store something I could describe as “electrical momentum.” While doing so it builds up a magnetic field around it as the electricity flows through it. If you open the circuit, it’s kind of like water hammer - the magnetic field now presses current to continue flowing through the coil, but the circuit it open and it can’t. The voltage can go quite high in this instance as it stacks up looking for a place to go. Putting the diode in parallel with a coil can provide an easy, low current pathway to bleed off the spike immediately before it ever stacks up.

Think of this. Plug in an electric fan at home. Unplug it while it’s running. See the spark?

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.

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.