capacitor size for time delay

Home / the12volt's Install Bay / Relays / capacitor size for time delay (

Topic Closed)

Welcome Guest :)

Members

< Prev Topic Next Topic >

whkoski

Member - Posts: 2

Joined: October 26, 2003

Location: Canada

Posted: March 13, 2007 at 10:15 PM / IP Logged

I'm using the picture below for a time delay circuit, but I'm after a 2 second delay. What size capacitor will I need for that? This is probably a newb question, but I couldn't dig up a solution. I know that when a resistor and capacitor are in series, you can calculate time by resistance multiplied by capacitance, but I'm not sure how to calculate it as shown. Any help is greatly appreciated.

capacitor size for time delay -- posted image.

dualsport

Silver - Posts: 983

Joined: September 27, 2005

Location: United States

Posted: March 14, 2007 at 1:03 AM / IP Logged

That's not a time delay setup, it's for getting a momentary output from a switched input that's steady on after turning on.

The relay will switch on instantly when the switched input is applied.

The capacitor provides the ground path, and as it charges up through the relay coil, it opens up the relay again in a short time, no matter how long the switched 12V input stays on.

The 10K resistor bleeds down the voltage across the cap after the switch signal is turned off, to prep it for the next cycle.

whkoski

Member - Posts: 2

Joined: October 26, 2003

Location: Canada

Posted: March 14, 2007 at 7:38 AM / IP Logged

I apologize for not explaining the change I've made to the circuilt. I'm not using pin 87 of the relay. Instead, the switched 12V output from pin 86 will also connect to pin 87a. When the coil is energized, pin 30 won't see the 12V switched output for aproximately 1/2 second. I'd like to have a 2 second delay, so I'm wondering what size capacitor I need.

hotwaterwizard

Silver - Posts: 1,350

Joined: December 11, 2003

Location: California, United States

Posted: March 14, 2007 at 8:03 AM / IP Logged

Sounds like a timer to me.

A Delayed off timer to be exact.

When power is applied to the Resistor Capacitor network , the fully charged capacitor begins to discharge thru the resistor and the coil of the relay causing the voltage to drop out at a time set by the resistance and capacitance.

It does qualify as a simple timer.

http://www.rwc.uc.edu/koehler/biophys/4g.html

These calculators are for series circuits. The circuit here is not a series circuit it is a Parallel circuit.

http://www.play-hookey.com/dc_theory/rlc_circuits.html

http://www.cvs1.uklinux.net/cgi-bin/calculators/time_const.cgi

http://ourworld.compuserve.com/homepages/Bill_Bowden/rc.htm

John DeRosa (Hotwaterwizard)

Stockton California

When in doubt, try it out !

hotwaterwizard

Silver - Posts: 1,350

Joined: December 11, 2003

Location: California, United States

Posted: March 14, 2007 at 8:26 AM / IP Logged

Looks like 4 capacitors in parallel may do it. 1000 uf each. Not in series.

Capacitors work different than speakers or resistors the add up in Parallel.

Try adding a transistor for better time control.

capacitor size for time delay -- posted image.

John DeRosa (Hotwaterwizard)

Stockton California

When in doubt, try it out !

dualsport

Silver - Posts: 983

Joined: September 27, 2005

Location: United States

Posted: March 14, 2007 at 8:33 AM / IP Logged

Your timer delay is going to be dependent on the resistance of the relay coil you're using, so if you found it's giving you 1/2 sec now, and you want about 2 sec, then you just multiply the cap value by 4.

Using a transistor to switch the relay, and having a resistor-cap network to control the transistor would be much better than directly driving the relay coil in that way, so you don't need to use these big caps. 4000 uF 20V caps aren't all that tiny or cheap.

With the setup you're planning, it's going to momentarily spike up at the start before turning off again for the delay period-

Don't know if that might be a problem for what you're using this for. If it's a mechanical system you're driving, it'd probably be okay, but if you're using it as a trigger for an electronic circuit, it'll trigger it prematurely.

Sorry, you can NOT post a reply.
This topic is closed.

Printable version

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot delete your posts in this forum
You cannot edit your posts in this forum
You cannot create polls in this forum
You cannot vote in polls in this forum

Search the12volt.com

Follow the12volt.com

Disclaimer: *All information on this site ( the12volt.com ) is provided "as is" without any warranty of any kind, either expressed or implied, including but not limited to fitness for a particular use. Any user assumes the entire risk as to the accuracy and use of this information. Please verify all wire colors and diagrams before applying any information.

capacitor size for time delay

Sorry, you can NOT post a reply.This topic is closed.

Sorry, you can NOT post a reply.
This topic is closed.