12 volt delay for relay

[moe]

Hello to the group I am looking for a simple circuit to delay a accessary relay so the relay will close about 10 seconds after I have started the car to prevent any spikes to the radio equipment.Thank you

[gadgeteer]

You won't get simpler than one transistor, one resistor and one capacitor. Use an NPN with sufficient power to drive your relay, emitter tied to ground; then tie the base HIGH with a resistor (limited by your relay current, divided by transistor current gain --- a 10k resistor will provide ~1 mA, if the transistor Hfe is 100 it will supply ~100mA. No reason not to use a DARLINGTON, which is really 2 transistors in one package, so Hfe is several thousand...<p>Now place a capacitor from the transistor base to ground --- maybe 100µF or more. Get the lowest voltage you can find --- the capacitor won't see higher than one volt. Connect the resistor to the accessory line, powered with your key. When you first turn on the key, the capacitor steals all the current as it charges --- and the transistor says "off" until the capacitor approaches 0.6 volts. Larger resistance and larger capacitance translate to longer time, but your resistor has to be low enough to drive the transistor once the capacitor goes away. You might place a couple hundred thousand ohms in parallel with the capacitor (so it discharges quickly and completely), and don't forget to put a diode across your relay coil (diode cathode to the positive lead, anode to negative lead which is driven by your transistor collector). Diode eats the pulse created whenever any coil is switched off...<p>[ May 14, 2003: Message edited by: gadgeteer ]</p>

[gadgeteer]

OH if you use a DARLINGTON transistor, your capacitor will see a volt and a half max. Electrolytic capacitors have a bad habit of drastically changing capacitance over time if they are severely under-voltaged...