LarryC, I agree with your estimate, except that the capacitor will probably want to be closer to 100,000 uF. For the benefit of the curious, I offer a few more details.
A typical automotive relay, Omron G8W, has an 88-ohm coil and an 8-volt must-operate spec, so with the 20-ohm series resistor it will operate reliably down to 9.8 battery volts.
We have to use the
parallel value of the coil resistance and the series resistor to obtain "R" (88 || 20) = 16.3 ohms. The actual RC time constant is then 16.3 ohms x 100000 uF = 1.63 seconds. But, the operating delay time will depend on the actual relay pull-in voltage, the battery voltage, and component value tolerances.
With a 13.7-volt battery we get a steady-state coil voltage ("Vcoil") of (13.7 x 88/(20+88)) = 11.2 volts. Assuming nominal R and C values and a typical pull-in value ("Voperate") of 6 volts, the delay time ("T") is calculated as:
T = -RC x
ln ((Vcoil-Voperate) / Vcoil)
T = -1.63 x
ln ((11.2-6) / 11.2)
=
1.25 seconds(The
ln () function is "natural logarithm." It's on your scientific pocket calculator.)
Here's the bad news:
1) The operating time will vary with battery voltage. At 11 volts, the same circuit will take 1.8 seconds.
2) The capacitor must be rated at least 16 volts. A 100,000uF 16V capacitor is big (about 1.5" diameter and 2.5" long) and expensive (about $10-12 new).
3) As LarryC mentioned: Assuming the relay is released by simply opening the circuit, the R-C product becomes (88 ohms * 100000 uF) = 8.8 seconds. Assuming the relay drops out at about 2 volts, the release time is:
T = -RC x
ln (Vrelease / Vcoil)
T = -8.8 x
ln (2 / 11.2)
= a whopping
15.2 seconds!You can see why the solid-state alternatives, such as the one suggested by TwinCitySparky, are so desirable.
JoeTestingEngr, the key to using the 555 in an application like this is to forget that it's a timer and just use it as a really good inverting Schmitt trigger with a nice, high-current output. Tie the trigger and threshold pins together as an input, and leave the discharge pin unconnected. The output goes high when the input is below 1/3 VCC and low when the input is above 2/3 VCC.
Oh, and I used GCADD for years before finally biting the bullet and buying AutoCAD LT
Didn't they have a "Miss Manners"-like character answering FAQs in their newsletter?
Edit: Didn't get those italics right the first time... Or the second... [This message has been edited by John Crighton (edited 06-29-2006).]