Mystifying bulbs-update I

[rlea]

Brill advice, and to one and all thankyou.
Re: William H's reply. It would appear the red LED is used as an alternator warning light. Once the Donkey is fired up, it takes a while before the rev counter kicks in and untill then there's no feed to the batteries.
I've sent an e-mail to all the sites mentioned and eagerly wait for a reply.
Once again thankyou, I should have just came here 1st.

 

[pampas]

I would not have a LED as a warning light for an alternator. change to a 2,5 watt bulb.

 

[VicS]

I would not have a LED as a warning light for an alternator. change to a 2,5 watt bulb.

Why not ?

 

[pappaecho]

I am very surprised by the fact that until the rev counter fires up there is no feed to the batteries. If the alternator is generating current and it is not connected to the batteries, then it will cook the diode pack.
Exactly the same scenario as running the engine with the ignition in the off position

 

[VicS]

I am very surprised by the fact that until the rev counter fires up there is no feed to the batteries. If the alternator is generating current and it is not connected to the batteries, then it will cook the diode pack.
Exactly the same scenario as running the engine with the ignition in the off position

The tachometer does not register because there is no signal to it from the W terminal until the alternator is generating. The tacho is dependent on the alternator, not the other way round.

There is no problem running an alternator disconnected from the battery. The diodes are likely to be blown if the battery is disconnected from an alternator which is already running ( and generating). Interrupting the current flow causes a voltage surge. It is this voltage surge which is likely to blow the diodes

 

[nigelmercier]

alternator is generating current and it is not connected to the batteries, then it will cook the diode pack.
Exactly the same scenario as running the engine with the ignition in the off position

No it won't, and no it isn't! In the latter case, the alternator isn't even switched on.

 

[lw395]

Why not ?

Because the current through the warning light goes through the field coil on start-up, so the alternator starts generating at lower RPM. Without this current, the alternator is relying on the residual magnetism of the rotor to start, which may not be enough, especially at low rpm.

 

[lw395]

....
There is no problem running an alternator disconnected from the battery......

Not necessarily true, if the rotor has a high residual magnetism (or if the warning light ispowered...), the alternator may generate a high voltage under zero load, particularly at high RPM.

 

[pampas]

as an auto lecky we did boats as well, most of our alternator problems arose after the owner fitted an LED either connected the wrong way, and in most cases the engine had to be rev`ed quite hard to get excitation, all complained this only happened after fitting the LED, some took our advice and let us fit bulbs.

Many are still on LEDs and still have the same problem, I smile every time I hear one starting up.

 

[VicS]

Because the current through the warning light goes through the field coil on start-up, so the alternator starts generating at lower RPM. Without this current, the alternator is relying on the residual magnetism of the rotor to start, which may not be enough, especially at low rpm.

You can overcome the problem by fitting a resistor in parallel with the LED. The resistor then supplies the initial field current.

In some alternators there is already a parallel circuit through a resistor

 

[VicS]

Not necessarily true, if the rotor has a high residual magnetism (or if the warning light ispowered...), the alternator may generate a high voltage under zero load, particularly at high RPM.

Except of course that the regulator will control the voltage. At least it will if the alternator is machine sensed. A potential problem maybe if it is battery sensed.

 

[srp]

In some alternators there is already a parallel circuit through a resistor

Which I think is why the led photographed in the original thread had 2 LEDs, one to protect the led (wired in series) and one wired in parallel to allow enough current to flow to extinguish the charge lamp.