Rob asks.
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Does anyone know how much current an LR-3 voltage regulator draws to drive a 40 amp alternator in a 12 VDC system?
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Rob, That is a little like asking "How long is a piece of string?" because a voltage regulator's job is to provide a variable current to the field winding. You can get an idea of the maximum current draw by measuring the field winding 's resistance to ground using an ohm meter and then dividing 14 volts by the resistance. Field resistance will be on the order of 3 to 6 ohms. This will only be the current draw when the alternator is putting out maximum current. Average field current will be significantly less (1/4 to 1/8).

For more information go here:
http://www.sacskyranch.com/altfram.htm

Assuming you have a 40 amp alternator with a 5 ohm field and your aircraft nominally draws 10 amps after the battery is charged. You could estimate your current draw as 14 volts / 5 ohms = 2.8 amps maximum so typical would then be 2.8 x 10amps/40 amps = 0.7 amps. Add about 20% for regulator losses and you get about an amp.

I hope this helps.

Regards
Brent Regan