On Mon, 20 Apr 2015 11:48:46 +0000, Martin Gregorie wrote:

I see I left a badly edited mess, so here is another try:

Another way of solving the battery switching problem is to use a break-
before-make change-over switch. These are often rotary switches.
A little searching on RS, Farnells, and similar outlets will find
suitable switches for 12 volts at the sort of currents we require.

If the momentary voltage drop as you
throw the switch causes problems, put a decently sized capacitor
(1000-2000 microfarad) across the output lines of the switch with a
small resistor (10-15 ohms) in series with it. The resistor may be
unnecessary: its mainly there to limit to power surge to about 1A when
the battery is connected.

The season before last I discovered that, if I kicked the XLR plug
feeding power to my panel when I was getting into my cockpit, the
resulting momentary power glitch would switch my radio and main vario off,
which was annoying. Putting a 2000 microfarad capacitor and 10 ohm
resistor across the panel side of the plug has been a permanent cure.

My panel only draws around 400mA with everything on so, even with the
resistor in circuit, the capacitor is able to supply enough voltage and
current to keep the panel alive over the short period while the
connection in the plug is lost: with the capacitor charged to 12v the 10
ohm resistor limits current to 1200 mA. As this is three times more than
the panel requires, it won't affect the operation of any instruments
while the capacitor is powering them.

To check this I put a voltmeter across the panel side of the connector
and, with a stable 12v shown, disconnected the battery. It took about a
second for the voltage to drop to zero. In other words the momentary
disconnection if I kick the plug isn't nearly long enough to drain the
capacitor and cause anything to switch off.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |