Very Stale EE here (30 years maybe?), so be gentle ;) Now I own a horse farm and do lots of equipment maintenance (and other things). Replacing an engine in a mower with a kit from another manufacturer.

Problem is that kit does not provide a way to drive alternator charge light. I think the only way I can sense charge is to compare battery output (12v nominally, but of course +/- 1.5v depending state of charge) to rectifier output (battery voltage plus junction voltage of diode).

Lamp should work as follows:

1) Key off, lamp off off

2) Key on, engine not running lamp on

3) Key on, engine running, normal rectifier function - lamp off

4) Key on, engine running, rectifier malfunction (i.e. not charging) - lamp on

Net is anytime battery voltage is higher than rectifier output, lamp should be on.

Lamp is incandescent, 2 - 3 watts I think, max 5w (about 420mA)

Explored transistor switching circuits, but I think the small junction voltage will not be enough to turn the transistor on.

An opamp comparitor is not really a good option - a bit complicated, and I would still need a drive transistor for the lamp, along with several resistors - creates a large parts count.

This design and physical package needs to be small, simple and low parts count - it is in a commercial grass mower. Planning on soldering parts into a ball and dipping them in plastic to protect.

Thanks in advance!

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You could use a comparator circuit like this:


simulate this circuit – Schematic created using CircuitLab

It uses an inverting comparator to check if the alternator voltage has fallen ~ 1/25 below the battery voltage and turns the fight on if that is the case.

  • \$\begingroup\$ This is exactly what I came up with (except I would run the existing lamp instead of an LED - no dash mods- so I would need a little beefier transistor I think), and was looking for fewer parts. \$\endgroup\$ – KY Horse Geek Mar 18 '16 at 7:23
  • \$\begingroup\$ This may be as simple as it gets! \$\endgroup\$ – KY Horse Geek Mar 18 '16 at 7:32
  • \$\begingroup\$ If there's no external AVR (automatic voltage regulator) then the AVR must be built in. The alternator voltage will be set internally and the 0.7 to 1.0 V drop across the diode will reduce the battery charge. (Other notes: you forgot to remove the default 1N4148 reference which is only good to 100 mA. You have no series resistance in with the LED.) \$\endgroup\$ – Transistor Mar 18 '16 at 9:13
  • \$\begingroup\$ I just could not find a lamp component in this online schematic editor :) \$\endgroup\$ – KilowattLaser Mar 18 '16 at 13:28
  • \$\begingroup\$ It's down at the bottom. \$\endgroup\$ – Transistor Mar 19 '16 at 16:58

All the alternator charging light circuits I've looked at use the auxiliary output to the AVR (automatic voltage regulator) to feed the lamp. Without more details we can't answer your question.


simulate this circuit – Schematic created using CircuitLab

Figure 1. Simple ammeter circuit.

Probably the simplest solution would be to add a bidirectional, ammeter as shown in Figure 1.


simulate this circuit

Figure 2. Reed-relay current detection.

In this circuit a reed relay is created by winding several turns of wire around a normally closed or changeover magnetic reed-switch. On turning on the ignition the warning lamp will light. When DC current flows through the turns of the coil the reed switch changes over, disconnecting the lamp.

The number of turns would have to be determined by experiment. The reed switch would have to be rated for your lamp. Note that in the unlikely event that your battery starts discharging into the alternator the lamp will turn off.

  • \$\begingroup\$ So I don't have access to an "Auxillary" output. They call it a rectifier, and it it really only has 1 output - the charging output. Per the schematic in the owners manual, all else is input: 2 for charge coil (windings), 1 connection that goes to fuel cut off solenoid and ignition switch, and ground. This is a Honda GX690 btw \$\endgroup\$ – KY Horse Geek Mar 18 '16 at 7:26
  • \$\begingroup\$ See the update. \$\endgroup\$ – Transistor Mar 18 '16 at 9:37
  • \$\begingroup\$ I like the reed switch idea - \$\endgroup\$ – KY Horse Geek Mar 19 '16 at 15:31
  • \$\begingroup\$ Remember there is a diode (it would be between the "x turns" item and the + side of the battery - see my original diagram). The effect would be that when current flows from the rectifier, it would activate the reed. If there is no current flowing (engine not turning or a malfunction in the rectifier), the reed would be inactivated. Trick would then be the wiring - an inverting reed switch ;) \$\endgroup\$ – KY Horse Geek Mar 19 '16 at 15:38
  • \$\begingroup\$ No, the diodes would be in the alternator. No diodes between the alternator connection and the battery. I think I've covered the logic inversion by using a normally-closed contact of the reed. \$\endgroup\$ – Transistor Mar 19 '16 at 17:01

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