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From: firstname.lastname@example.org (Eric)
Subject: wiring harness using BJT (instead of relays) Check design please?
Date: 22 Oct 2002 16:10:15 GMT
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Well, I'm trying to ugprade the wiring on my car. Like the ones found in
www.suvlights.com, I was going to use relays but I found a site,
www.autolamps-online.com that advertised a FET harness. What a neat idea I
thought. They use a FET instead of relays and fuses but I'm going to add a fuse
I chose to use a BJT, a 2N3772. It's rated for 60V, 30A, and 150W. I am
actually thinking of using another one rated for 50V, 80A, and 300W because I
wanted to have it switch on both bulbs together. Another recommendation would
be good. Something small and cheaper the better.
I want to design one for HID lights which isn't what I'm upgrading to yet.
People have told me those pull like 30A at start up but then go down to about
15A. Will the 2N3772 be able to handle that or should I use the other one, I
forgot the part number but I found it in Fairchild Semiconductors product
I'm using a voltage follower configuration.
I have a 12-15V across the collector with no series resistance.(coming straight
off the battery)
I have a 12-15V across the base and ground coming from the existing wiring to
turn the lights on. I also have a 2k ohm resistor in parallel (across base and
ground) to help turn the transistor off.
Across the emitter I have 2-1ohm loads (the bulbs). So the black box resistance
in series is 0.5ohm (Re)
Are there design flaws or design improvements you would recommend? Should I put
them on separate transistors?
Here's the link to the schematic.
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