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?

  • 1
    \$\begingroup\$ What are the electrical specifications of your warning lamp? If you need a 25W resistor, something is clearly wrong. \$\endgroup\$
    – Botnic
    Oct 22, 2015 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, 2015 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\$ Oct 22, 2015 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, 2015 at 13:37
  • 3
    \$\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\$ Oct 22, 2015 at 16:12

3 Answers 3


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.

  • \$\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, 2015 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, 2015 at 23:26

Don't do it!!!!!!!! LED will stay on,and the alternator won't S T A R T charging(once it starts charging-(try 3000 or 4000 RPM!!!)) all will be ok; the problem is starting charging!!!

I replace dash lights with LEDs on lots of cars,but N O T the alternator light,too much trouble wiring a resistor--or the stock bulb,no PLACE safe for it,it gets H O T!!-- in parallel with the LED;

It's a 2 WATT bulb (max),so a R= (V x V)/2; R=144/2 R=72 ohms(60 to 100 would do); Power= do N O T use a 2 watts resistor!! it will get hot,and you do not want that!!!! try like a 10 watt,leave key on for 5 minutes(in hot weather!) and make sure it stays cool,and will not get your car on fire;

Like i said,too much trouble,L E A V E the original INCANDESCENT bulb in there; it gets used so little,it lasts for ever...


An LED has very low forward resistance compared to an incandescent bulb. Adding resistors in series with an LED will lower the current, not increase it.

In simple analog systems, the alternator or generator lamp 'indicates' by balancing 2 voltages: 1 from the battery and 1 from the alternator. If one puts out more than the other, current flows through the lamp and illuminates it. There is some hysteresis since the alternator generally puts out more voltage than the battery. Sometimes this is handled via the current necessary to illuminate the lamp being significantly above 0. Sometimes there is extra circuitry.

But ultimately you need series resistance with and LED to limit the current and prevent the LED from burning out.

Also, since the LED only allows current in 1 direction, you have to be sure the current direction is correct, or it will not illuminate at all(or employ extra circuitry to ensure it illuminates in both direction).


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.