Message-ID: <01BDE813.171B4E10.ed@testelectronics.com>
From: Ed Armstrong <ed@testelectronics.com>
Reply-To: "ed@testelectronics.com" <ed@testelectronics.com>
To: "Lancair [Lancair.List@Olsusa.Com] (E-mail)" <lancair.list@olsusa.com>
Subject: 24v systems, capacitors for solenoids
Date: Thu, 24 Sep 1998 23:28:51 -0700
Organization: Test Electronics
Mime-Version: 1.0

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1) Yes, you can charge two 12V batteries in series, then use them in series 
to get 24 Volts, and tap off the center between the two batteries to get 
12Volts for the automotive accessories. However, I think the voltage 
choices are 28 Volts and 12 Volts. You can buy a DC-DC converter and get 
12Volts at about 10 Amps from a 28Volt supply. This should be enough to 
power all your fun accessories, if not, get two convertors. I am going to 
use a 12 Volt system. I can not imagine overloading the alternator, using 
more than 40 Amps continuos current in such a small plane.

2) Capacitors will dampen the voltage spike, but they will cause a huge 
inrush current when you actuate the solenoid. You will have to supply 
current to turn on the solenoid plus charge the capacitor. This will put a 
big load on your switch, and wear the contacts out faster. The best 
solution is to add a flyback diode. This is a diode in parallel with the 
solenoid. The cathode should be on the positive side. If you still get 
noise, you can completely isolate the solenoid by putting another diode in 
series with the paralleled solenoid & flyback diode circuit. Keep all the 
diodes as close to the solenoid as possible. I drew the following schematic 
in ASCII. If the spacing comes out wrong, cut and paste this into Notepad 
to see it better.

POS--->I---|--solenoid--|-----GND
           |-----I<-----|

Ed
Watsonville CA