simulate this circuit – Schematic created using CircuitLab

I've got a fairly odd LED setup whereby a vehicle's alternator is being used to generate the power (rectified to DC) required to run said LEDs, which will be used for it's headlights. I would like to use PWM to control the brightness of the LEDs and bypass that pesky LED voltage-to-brightness curve. However I'm not sure how to handle the inductive kickback when the inductor is the thing generating all my power!

edit I have included a rough diagram. There will be voltage control circuitry to regulate the alternator (3 inductors) to the 36V i need, and the part numbers & values are not what I plan to use at all. But you can see now where I intend to apply the PWM much more clearly. I apologize for my previous ambiguity.

Do i treat it like any other inductor circuit or are there special concerns that i don't know about?

  • 2
    \$\begingroup\$ "I've got a fairly odd LED setup" - I'm a level 5 mage and I've got mind reading so I know exactly how it's set up. Could you please share the setup in the form of a circuit so the other peasants on this site can see it as well? The peasants has shown me that there is an easy tool to set up a circuit under the edit section. \$\endgroup\$ – Harry Svensson Oct 8 '17 at 18:19
  • \$\begingroup\$ Ah, yet you cannot answer my question. Are you sure you're level 5? \$\endgroup\$ – Daniel Oct 8 '17 at 21:22

You won't have inductive "kickback". This occurs when you suddenly disconnect an inductive load. This is not going to happen in your case.

The three-phase supply from the alternator will be reasonably good DC without any stabilisation.

enter image description here

Figure 1. Rectified three-phase alternator output.

Your schematic shows the micro-controller driving an NPN transistor in emitter follower configuration. The most you can get out of this is the GPIO output voltage (probably 5 V) - 0.7 V drop across the base-emitter junction of the transistor giving you 4.3 V on your LEDs. This is probably not what you want.

enter image description here

Figure 2. An NPN transistor can be used to act as a switch controlled by a low-voltage micro-controller switching a high voltage or high-current load. Source.

The linked article explains how to calculate resistor values.


Presumably your alternator supply is rectified and filtered by some capacitance. If it's not, put some capacitance on the output.

There will be some small perturbations in the supply voltage from the PWM supplying the LEDs, but just because your alternator is inductive doesn't mean you are going to get a large inductive kick. The current in the inductance has a large capacitance to flow into on load release.

The inductive kick when switching a relay coil (for example) comes from the fact that if you don't have a place for the inductive current to flow (your anti-kickback diode) then interrupting the current in an inductor quickly causes a large voltage spike. V=L*di/dt or the voltage is the inductance times the rate of change of the current through it.

You state that you have an odd setup, but without additional information or a schematic it's hard to answer any better than this.

  • \$\begingroup\$ Thank you very much. I do apologize for the vagueness. I should have said that the setup electronically is fairly standard past the alternator, but the application (headlamps) is odd. Entirely my mistake. I will slap together a schematic and post it posthaste. \$\endgroup\$ – Daniel Oct 8 '17 at 21:23
  • \$\begingroup\$ OK, thanks for the diagram. The answer still stands, as does the excellent response by @Transistor \$\endgroup\$ – John D Oct 9 '17 at 3:15

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