Alternator: D+ How Many Amps?

Hi,

I have trouble installing an automatic relay system for the headlights where the 1st relay is activated by the 12V originating from the D+ wire which is connected to the battery warning light. When the engine is off, there is 0 V on the D+ and when the engine is running there is 12V on the D+. I fear that D+ is not supplying enough current to power my 12V relay (coil resistance 63 Ohm). Does anyone have information on how much current is max supplied in the D+ of a standard alternator?

Lucas

Yes, there is no standard.

A D+ or L output only needs to sink (GND) the current thru the charge lamp and any tested bulbs on the circuit.
There is no need for D+ or L to supply substantial +12V current, it merely needs to be "not grounded".

And there are so many D+/L circuit possibilities - eg, SPST or SPDT relays; semiconductors that might be capable of supplying several Amps or merely micro-Amps, or be Open Collector etc.


The simplest solution may be to use an NPN transistor or N-ch MOSFET that grounds the relay coil (it's other end 86 to IGN +12V).

I presume your present relay is 85 to GND with 86 to D+ ?

Tomorrow I will try to upload a wiring diagram of my system. In the meantime I can clarify the problem:

According to the official wiring diagrams of my car, the battery warning light is a LED with a load resistor. Additionally, there is an extra diode between the load resistor and the alternator terminal itself. From reading the internet it makes sense that the LED has a voltage drop of 3V and is supplied by 20 mA (this should be the most common setup). This requires the load resistor to be approx 470 Ohm with the remaining 9V voltage drop across. A total of 3+9 = 12 Volt. So my car does not have a normal 2W (or similar) bulb and nothing more.

The 12V relay (with 63 Ohm coil resistance) is connected to the same D+ wire which means, its connected right between the alternator terminal and the extra diode mentioned above - so effectively it is connected between the LED and the alternator terminal.

Now I make a test where I plug in my relays:
When turning the Ignition on, the warning light turns on as usual. However, when starting the engine, the warning light stays ON and is not turned off as usual.

Thus, the question is, if my old school relay system is confused by the modern LED technology (or vice versa)?

My immediate conclusion is: The 63 Ohm coil resistor draws so much current that the LED does not "see" 12 V on the alternator terminal anymore.

Lucas

That's the wrong wiring. If lucky, your relay might energise when the IGN/meters are on but the alternator is NOT charging.

It should be as I said - relay 86 to D+ and 85 to GND. The relay then energises when IGN is on AND the alternator is charging (ie, D+ = 12V).
I'd suggest a reverse biased diode between 85 & 86 with the line end towards 86 (D+). That's to ensure relay spikes do not wreck D+ circuit electronics. Use a 1N4004 or 1N4007 diode.


FYI - Your "charging" LED is a 0.25W load, about 10x less than 2W.

And if your D+ is only designed to handle a LED and hence sink aka ground 20mA, then it is quite possible it won't handle be able to source aka supply the ~225mA for the relay coil.
However most alternators (regulators) are designed to handle at least a few 2W-3W bulbs (ie, 1A - 2A total) - not that that means they can supply 12V at 1A etc.


A wiring diagram will be useless. You require the current sourcing ability of the alternator's D+ terminal - ie, can it supply ~250mA at 12V?