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I have an alternator on an in-board motor in a boat, I have almost completed re-wiring the electrical system and am wondering if it is possible to replace the existing filament lamp.

I have researched and came across a forum discussion which, from my basic understanding, sounds like it is possible but wanted to check and confirm that it is do-able and was hoping that I could get some advice on the resistors/diodes to use.

In the above forum, the OP attached a schematic which has a bridging 5 ohm resistor, a 270 ohm resistor on the anode and a 5 ohm resistor going to the alternator, in this forum it was suggested to use high watt resistors around the 25W mark due to the heat.

Does all of this sound correct, is there a risk of harming the alternator or battery?

Can I use a 12V LED such as this one?

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  • \$\begingroup\$ What are the electrical specifications of your warning lamp? If you need a 25W resistor, something is clearly wrong. \$\endgroup\$ – Botnic Oct 22 '15 at 13:22
  • \$\begingroup\$ At this point I am unsure, I will need to dig up the old lamp. I thought the suggestion for the 25W resistor was a bit odd and overkill. \$\endgroup\$ – Lima Oct 22 '15 at 13:24
  • \$\begingroup\$ I'd use the 12V LED you have cited (chances are it has the right dropping resistor for the type of LED used), unless you want to get a cheaper solution or use a particular LED which is not available as 12V. \$\endgroup\$ – Dmitry Grigoryev Oct 22 '15 at 13:32
  • \$\begingroup\$ And still use the resistors mentioned, my basic understanding of the lamp is that it's draw is what enables the alternator to charge the battery and a LED doesn't have the same current draw, is this correct? \$\endgroup\$ – Lima Oct 22 '15 at 13:37
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    \$\begingroup\$ Connect your LED resistor in parallel with the existing lamp. The LED should have a negligible effect on alternator performance, and the existing lamp in essence replaces the ~5 ohm shunt. \$\endgroup\$ – Optionparty Oct 22 '15 at 16:12
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Often the current through the alternator warning lamp is necessary for the alternator to start producing power. An LED may not pass enough current to start that process without additional resistors.

An alternator uses its own power to energize the rotor and create a magnetic field that when rotating induces voltage in the stator.

When not rotating the iron in the rotor will retain some magnetism to start the process, however the amount of remnant magnetism may not be enough to generate adequate voltage until a fairly high RPM. In the case of car alternators I have had it require an engine speed of ~4000 RPM if the alternator light is not functioning.

When the alternator light is present the current from the battery through the light creates additional magnetism that allows the generating process to start at a much lower speed.

Boat engines typically run at much lower speeds than car engines so this is even more important. From the forum I see that the resistor values are fairly low, implying a large current when the ignition is on but the engine not running - that will be the worst case for power dissipation - when the engine is running and the light off the dissipation will be low.

With 5 ohm resistors I would expect about 1/2 amp to flow in this case, this will result in about 1.5 Watts of dissipation in each so a 25 W resistor is overkill, 5W would be adequate. The 270 ohm resistor only needs 1/2W.

There is low risk of damaging anything but obviously you should check your wiring very carefully as mis-wiring could cause damage to the alternator, wiring or even cause a fire.

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  • \$\begingroup\$ Awesome answer, touched all the required points. The indicator light current is used to supply the bootstrap current for the alternator rotor winding before the alternator provides the power directly when it starts generating. \$\endgroup\$ – KalleMP Oct 22 '15 at 18:39
  • \$\begingroup\$ Your answer is great, it ties in with what I have read and is well explained. Thanks heaps for your answer. \$\endgroup\$ – Lima Oct 22 '15 at 23:26
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An LED has very low forward resistance compared to an incandescent bulb. Adding resistors to an LED will lower the current, not increase it. But ultimately, this is what you want: a sufficient load.

Also, LEDs only pass current in only one direction. The lamp 'indicates' by balancing 2 voltages: 1 from the battery and 1 from the alternator. By only allowing current in 1 direction, you have to be sure the current direction is correct, or it will not illuminate(or extra circuitry to ensure it illuminates in both direction.

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