How about a small PCB mountable ni-cad? Anand Dhuru ----- Original Message ----- From: "Russell McMahon" To: Sent: Thursday, December 20, 2001 04:56 PM Subject: Re: [EE]: Latching relay > > does anyone can send me the formula for latching relay? > > I want to hold the a 12v relay for certain time after power-off (about 20 > > seconds) the relay consume 37mA/12v (I think it's coil resistance is 30-32 > > Ohm) what is the size of the capacitor I should use? > > > > p.s: across the coil there is a 1n4001 diode does it important for > > calculation? > > > At 27 mA and, say 6v drop for a 20 second delay you would need about 4 FARAD > = 4,000,000 microfarad. > This is far too large a load to supply for a long period directly from a > capacitor. > What you need to do is to use some electronics that need much less drive > current to drive the relay. > Then apply the delay to the drive circuit. > > > If you drive the relay with a MOSFET then by placing a 1 mohm resistor from > gate to ground and a 10 uF from gate to ground you will get a circuit that > will stay on for ABOUT 20 seconds after power is removed from the gate. This > is not an ideal circuit but is a starting point. > A better solution would be a digital one as timing delays ca be more > accurately established and altered. A very cheap PIC would do this well or > you could use a counter IC and some extra logic. The capacitor will need to > be a low leakage type. A solid aluminium would be OK. A Tantalum cap would > be useable but select a cap voltage 3 or more times higher than the supply > voltage. > > The reverse biased diode should not affect operation. > > > Russell McMahon > _____________________________ > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. > > > > --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.231 / Virus Database: 112 - Release Date: 12-Feb-01 -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body