Voltage selector

[Pooh Bear]

"N. Thornton" wrote:

Pooh Bear <rabbitsfriendsandrelations@hotmail.com> wrote in message news:<418CA8EB.55EB10FA@hotmail.com>...
"N. Thornton" wrote:
Rich Grise <rich@example.net> wrote in message news:<pan.2004.11.05.18.08.19.998103@example.net>...
On Fri, 05 Nov 2004 09:12:17 -0500, yigiter wrote:

i want to use that circuit in car and the battery of car may be 12V or 24V.
so if the battery is 24V i have to forward it directly to the output but
if the battery is 12V i have to boost it to 24V so i have to forward it to
the converter. and i don't want my circuit to be manual selectable for
battery. when i connect it to battery it must be selected automatically.

Then just use a relay that pulls in at, say, 18 V. I've seen this done
to plug in a battery charger to 110/120 or 220/240, and the switchover
was simply a 220VAC relay.

Exactly. Relay current draw is a non issue for in car use. Will these
people ever learn to tell us what they want to start with? What a
waste of time.

A 24v relay should fail to pull in at 12v, a series zener of a few
volts would make sure it works as desired.

How about a 12V zener in series with a 12V coil ?


Graham

would probably work, but the relay coil would be liable to overheat.
Car system V for nominal 12v is upto around 15v at max. 30v-12=18v,
and on a 12v relay thats a bit steep.

A 24v relay and a 3v zener would have no problems.

I thought that a lead-acid battery on charge should be 13.8 V max.

Making 27.6 V for a 24 V system.

I'm sure a 12 V relay would accept a 3.6 V overvoltage - but your point is well taken.

You'll need to pay attention to 'pull-in' and 'drop-out' voltages too.


Graham