555 Spike protection

[ejukated]

Hi,

Ive been building a circuit to replace the Fuel pump relay control output from my FI ECU. I've been having problems with the 555 ic either emitting a puff of smoke and dying or melting Since it happens after repeated cranking and starting, i'm thinking its a voltage spike thats killing the 555. I can run the circuit all day once the car is started and not have any issues.

Would a 15V 5W zener across the power input provide enough protection againts spikes present during cranking/starting in a car?

<img src="http://members.optusnet.com.au/kang5/Saab/untitled.JPG" alt=" - " />

Thanks,
Ben

<small>[ December 03, 2005, 01:32 AM: Message edited by: ejukated ]</small>

[rshayes]

ON Semiconductor (www.onsemi.com) has several application notes (such as AN843) and data sheets on transient voltage suppression (TVS) devices. Also Vishay (www.vishay.com) has the old General Semiconductor line of TVS devices.

These are basically large area zener diodes in small packages. They can handle large peak powers, but the total energy they can handle is usually limited by the package.

They require some source resistance to limit peak currents. This can be a small resistance in series with the power source or the dc resistance of a filter inductor.

Further filtering or regulation may be desirable. A further dropping resistor and zener could be used to provide a regulated voltage for the 555. Something in the neighborhood of 6 to 8 volts would probably be reasonable. Remember that the 555 is quite sensitive to supply voltage changes that occur during the timing interval.

If the dropping resistor is large enough, the zener diode used for regulation may also be capable of providing adequate transient protection.

[ecerfoglio]

Or you can use a voltage regulator (7805 to 7809) to power your 555.

As you have both input and output buffered by transistors Q1 and Q2, the 555 will work OK with less than 12 V.

Connect pin 4 and 8 of the 555 **and R1** to the regulated "+V". Be shure to add decoupling capacitors from the regulated "+V" to ground (10 uF + .1 uF?)

<small>[ December 06, 2005, 01:48 PM: Message edited by: ecerfoglio ]</small>