from a few miliseconds up to 2 minutes circuit delay:

one cmos 4093 nand gate, two rc and one transistor:
input signal active low on one input, the gate output through a
polarised capacitor ( 1uF...to 4.7uF ) connected to the transistor base;
1M resistor and 1n4148 protection diode against reverse voltage on BE
junction, emiter to ground, colector to VDD through a 10K...250K; from
colector to ground a polarised capacitor ( 1uF to 22uF ), from the same
collector point to the other gate input a negative feedback ( one wire )
Output from gate out. For a relay coil load another transistor is
required. Suplementary protection diode from both gate inputs
to Vdd ( cathode to VDD ). Optional a derivating RC on the input so the
lenght of the input pulse will not count ( the trigger will run on hi-low
input pulse transition and at power up )
It's a stable circuit up to 2 minutes. No PIC inside.

Vasile



On Fri, 21 Dec 2001, Russell McMahon wrote:

> > 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
> _____________________________
>
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>

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