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Holding a relay on for a predetermined time? How?

P

Pete

Jan 1, 1970
0
I am experimenting, not having touched electronics for a while.

I have a relay controlling a 240 supply, and a control input carrying 12v

What the control 12v goes live I need the 240 to switch (not a problem).
What I then need to do is to hold the relay in the on position for a
predetermined time (say 10 mins) even if the 12v input goes back to 0v. Each
time the input goes to 12v the time needs to start again.

i.e 12v switches the 240v supply, and allows the 240 to remain on for 10
mins after the 12v feed drops to 0v

What would be the easiest and simplest way to acheive this?
 
B

Bushy

Jan 1, 1970
0
Get a 240Volt time delay relay like they use for downstairs lighting that
can be switched on for a few minutes. Use your small 12 operated relays 240
volt contacts to trigger the time delay relay and have it operate your load.

Available at most combined electrical and lighting stores.

Hope this helps,
Peter
 
C

CWatters

Jan 1, 1970
0
What you are describing sounds like a classic "monostable" application.
There are many IC's that will do this but not so may that will operate from
12V. Try the 555 but note that 10 mins is rather a long time constant for a
monostable of this type (read the applications notes for the 555).

What do you want to happen if the 12 V goes high while the realay is still
on? Do you want....

a) the relay to switch off at the end anyway or
b) extend the time period or
c) Go off and then come back on again?

If b) look for a "retriggerable monostable".

On the other hand I'm sure relays of the type you are looking for must exist
somewhere - try a web search?
 
B

Bill Bowden

Jan 1, 1970
0
Larry G. Nelson Sr. said:
I would build a little circuit and use a small 8 pin pic for the
timer. You need to do a voltage level translation or use a 5V relay
mosfet or magnetic. You can also buy a time delay relay but the choice
is yours. A 555 can be used but long delays are not easy and I find
the PIC solution costs less in parts count and is much more flexible.
They also make mechanically latched relays and you can then use a
supercap to power things wnen the 12VDC goes away.
Larry




Larry G. Nelson Sr.
mailto:[email protected]
http://www.mchipguru.com


All you need is a transistor, a cap and diode.
See the drawing here:

http://ourworld.compuserve.com/homepages/Bill_Bowden/page2.htm#relay_i.gif


-Bill
 
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