Date: Sat, 10 Oct 2009 17:45:51 -0700
Reply-To: Rocket J Squirrel <camping.elliott@GMAIL.COM>
Sender: Vanagon Mailing List <firstname.lastname@example.org>
From: Rocket J Squirrel <camping.elliott@GMAIL.COM>
Subject: Brake light circuit current with trailer, WAS: Re: No High Beams
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A continuation of a side thread.
On 9/29/09 I opened my big fat yap to comment on a thread in which a list
member said that he was blowing high beam fuses after going to higher
wattage lamps (see "No High Beams").
I helpfully added that on my van, fuse #8 (fusebox, 1984) popped moments
after applying the brakes when my little trailer was hooked up. Changing
the 8A fuse to a 16A fuse* fixed the problem.
I thought that contributing this anecdote to the thread would help
illustrate how adding more or bigger bulbs to a lighting circuit might
exceed the current rating of the fuse.
Ohm's Law says one 21W bulb will draw 1.75A at 12V, and around 1.95A at
13.4V (alternator booted). With four 21W bulbs that's nearly 8A, which
would pretty quickly pop the fuse, although it should have taken a few
seconds, and not the brief moment that I witnessed. My guess was that the
bulbs' inrush current** was responsible for the fast blow.
In the thread, a couple list members took me to task for not taking
current measurements, I was speaking through my backside. So I measured
With stock-type brake lights, no trailer: 3.9 amps @ 13.4V (1.95A/bulb
which is in agreement with the calculated 2A/bulb @ 13.5V.)
With trailer: Exactly the same. I had forgotten that I had wired in a
"powered" 3-to-2 tail light converter****
So. Why the heck did the 8A fuse pop the first time I pressed on the brake
pedal once the trailer was connected. The converter is designed to make
sure that no additional loading is presented to the circuit. And my
measurements confirm it. I think it's fair to say that if there was a
momentary short or glitch in the trailer pigtail or wiring, the converter
would have had to deal with it, not the dash fuse.
I should point out that this scenario played out exactly the same way back
in August 2008 when I first hooked up the trailer for my Escape From SoCal.
Science is baffled.
* These GBC "ceramic torpedo" fuses only come in three flavors: 8A, 16A, 25A
** Inrush current, I felt, was the reason why the fuse was popping so
quickly since light bulbs will draw 5 to 10 times as much current when
power is first applied than when they are hot.
*** My reader will recall that I can't be trusted with an ammeter: none of
my ammeters have intact fuses in them. I've blown them all. To make these
measurements I made a 0.01 ohm (+/-) "resistor" using 19'' of 40-mil
diameter solid-core copper wire soldered across a blown fuse. By measuring
the voltage drop across this current-sampling resistance and applying I =
E/R I had my ammeter. Easy as cake.
**** The trailer has two bulbs, one on right, one on left, Vanagon uses
four bulbs: right turn, left turn, and two brake lights wired together,
three circuits total. A converter is needed to drive the two-circuit
trailer from the three circuit van. A powered converter connects to the
battery to avoid additional loading on the original lighting circuits.
Mike "Rocket J Squirrel" Elliott
84 Westfalia: Mellow Yellow ("The Electrical Banana")
74 Westrailia: (Ladybug Trailer company, San Juan Capistrano, Calif.)