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3 second delay


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neo_1975 
Member - Posts: 3
Member spacespace
Joined: March 31, 2011
Posted: March 31, 2011 at 8:48 PM / IP Logged  
I assume I need this to do a 3 second delay. I thought I would try a 4.7uf cap and 1k resistor. It wouldnt even trigger the relay. I i remove the cap and resistor, the relay energizes. Sooo...I am wrong with the cap and resistor. Anyone know what radio shack parts i need for 3 second delay? Thanks for the help
-Neo
3 second delay - Last Post -- posted image.
oldspark 
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: April 01, 2011 at 4:42 AM / IP Logged  
The simplest is six-folding the cap value - ie, to change from 1/2sec to 3 secs means 6x 1kuF = 6mF => 6.8mF or 6,800 uF - say 10,000uF.
The problem is that the resistor forms a voltage divider with the relay (coil's resistance), so that has a limited range of values.
But the resistance value will change with the relay. The original diagram assumes a Bosch type automotive relay of (say) 60-200 Ohms.
The other problem (IMO!) is that being of small resistance value, the capacitor need to be large/ (IE - smaller resistance = higher current = bigger cap to store the required charge (time).)
Except maybe for small relays (coil resistances of 1,000 Ohms or higher), I prefer to use buffers - ie, transistors or FETs. Then small caps and small current resistors (high-Ohmage resistors) can be used and timing is usually independent of the relay... eg:
3 second delay - Last Post -- posted image.
(from brunningsoftware.co.uk - specifically 2 minute on circuit... (down the page).
hotwaterwizard 
Silver - Posts: 1,350
Silver spacespace
Joined: December 11, 2003
Location: California, United States
Posted: April 06, 2011 at 1:13 AM / IP Logged  
3 second delay - Last Post -- posted image.
3 second delay - Last Post -- posted image.
John DeRosa (Hotwaterwizard)
Stockton California
When in doubt, try it out !

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