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Voltage selector

Y

yigiter

Jan 1, 1970
0
hello,
i am looking for a circuit. according my project, there are two different
input voltage 12V and 24V. and i want to switch my cicuit accordint to
input. i mean, if the input is 12V it will forward the input to A, if the
input is 24V it will forward the input to B. is there a way to achieve it,
out of using relay? if you can help, it would be very appreciated..
best regards,
 
T

Tim Wescott

Jan 1, 1970
0
yigiter said:
hello,
i am looking for a circuit. according my project, there are two different
input voltage 12V and 24V. and i want to switch my cicuit accordint to
input. i mean, if the input is 12V it will forward the input to A, if the
input is 24V it will forward the input to B. is there a way to achieve it,
out of using relay? if you can help, it would be very appreciated..
best regards,
This may be a good question for sci.electronics.basic.

If by "out of using relay" you mean "using relays", then you can do this
with a 12V relay and a 12V zener diode -- connect the zener in series
with the relay coil and it won't conduct enough to energize the relay
until the input voltage exceeds 12V by a good margin.

If you mean "without using relays", or if you don't want to load your
input very heavily, then you can just use a comparator on the input and
do whatever you want with its output, possibly switching through
P-channel MOSFETS.
 
Y

yigiter

Jan 1, 1970
0
hello tim,
soory for grammar mistake. i mean "without using relays".
 
R

Rich Grise

Jan 1, 1970
0
hello tim,
soory for grammar mistake. i mean "without using relays".

Do a search on "window comparator".

Good Luck!
Rich
 
F

Fred Bloggs

Jan 1, 1970
0
yigiter said:
i have posted a block diagram to make it more clear. also i want it to be
available for high currents(5Amp for 24VDC and 10Amp for 12VDC)
thanks again...
http://s02.imagehost.org/0625/BLOK.jpg

That is just any DC-DC boost converter that stops switching and allows
24VDC to feed through when that is the input.
 
Y

yigiter

Jan 1, 1970
0
i have a dc-dc converter which is not available for 24V input. i don't want
to change it. so i must keep 24V off converter's input. the circuit must
be suitable to topology on the block diagram.
 
Y

yigiter

Jan 1, 1970
0
thank you for your help. you are very kind.....
 
T

Terry Pinnell

Jan 1, 1970
0
yigiter said:
i have posted a block diagram to make it more clear. also i want it to be
available for high currents(5Amp for 24VDC and 10Amp for 12VDC)
thanks again...
http://s02.imagehost.org/0625/BLOK.jpg

What's wrong with just connecting a permanent 24V supply to your
load?! That seems to satisfy the requirement specified in your block
diagram, that the load must *always* have a 24V supply.

Or, contrary to the statement in your original post ("there are two
different input voltage 12V and 24V..."), is there in fact a *third*
possible input voltage, Vin = 0?

If so, is the 24V rail available for the circuit?
 
Y

yigiter

Jan 1, 1970
0
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.
 
N

N. Thornton

Jan 1, 1970
0
yigiter said:
i have a dc-dc converter which is not available for 24V input. i don't want
to change it. so i must keep 24V off converter's input. the circuit must
be suitable to topology on the block diagram.

18v zener, resistor, transistor. One to connect the 24v in to the out,
one to switch the 12v to the booster.
Or relay.

NT
 
R

Rich Grise

Jan 1, 1970
0
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.

Good Luck!
Rich
 
N

N. Thornton

Jan 1, 1970
0
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.
A second slower relay (RC on coil) will keep the 24v off the 12v input
while the main relay decides what to do. A zener/tr on the 12->24
convertor would be a cheaper option for that.

The only reason I can think of to avoid a relay is if youre doing mass
prouction. Did you tell us how many youre making?


NT
 
P

Pooh Bear

Jan 1, 1970
0
N. Thornton said:
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
 
R

Rich Grise

Jan 1, 1970
0
How about a 12V zener in series with a 12V coil ?
Sure. :)
Assuming, of course, that the zener is rated for the coil current or
above. :)

Cheers!
Rich
 
N

N. Thornton

Jan 1, 1970
0
Pooh Bear said:
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.

NT
 
P

Pooh Bear

Jan 1, 1970
0
N. Thornton said:
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
 
K

Ken Smith

Jan 1, 1970
0
"N. Thornton" wrote: [....]
I thought that a lead-acid battery on charge should be 13.8 V max.

There can be a few volts between "should" and "is". It is not uncommon
for the charging system in a car to run up to 14 or even 15V for a while.
There are sometimes spikes much above this but they are too short for the
relay to be troubled by them.
Making 27.6 V for a 24 V system.

When designing for a 28V system, I always allow for voltages up to about
35V.

Simply adding a resistor in series with the realy coil, is a good way to
adapt standard relays to non-standard voltages. It divides the pull in
and hold voltages down by the same amount. Using a zener leaves the same
difference between the two which is not as good.
 
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