Diode Relay Circuit

Can a relay's spike damage components?

Im using this radioshack relay:

http://www.radioshack.com/product/index.jsp?parentPage=search&summary=summary&cp=&accessories=accessories&productId=2062482&kw=relay&techSpecs=techSpecs&currentTab=features&custRatings=custRatings&features=features&origkw=relay&support=support&tab=summary

there is no mention that there is a diode protecting it, so is there a risk that it will damage my computer's PSU and compontents in the circuit? How can I prevent this, which diode and how to install it?

The coil in a relay can cause a voltage spike which can cause damage to electronic components. The spike is caused when the current through the relay coil is stopped. The spike is most noticeable with DC powered coils.

Current flowing through the coil creates a magnetic field, so if the current changes so does the magnetic field.Electricity is created when ever a conductor is moved across a magnetic field. Electricity is also created when a magnetic field is moved across a conductor.

When the current in a relay is stopped the magnetic field will collapse. The collapsing magnetic filed cuts across the windings of the coil and generates a current.

The polarity of this spike is in the opposite direction of the current that created the magnetic field.

To control the spike a diode is placed in parallel with the coil. A diode only conducts in one direction. So when the coil is energized to activate the relay the diode is not conducting, but when power is removed from the coil and the reverse polarity spike is generated the diode will conduct. The conducting diode provides a low resistance path which connecting the lead of the coil together. This allows the spike coil potential to equalize through the diode and not the rest of the circuit.

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The Amplifer Adventure part 4 - The function selecting mechanical relay logic circuit

How do I calculate how long it will take for a capacitor to discharge?

I have 12v+ control and diode filling a capacitor, which, when it discharges, will discharge across a 1000 Ohm resistor. The load is a 37mA relay coilHow long will it take for the capacitor to discharge? As in below the transistor's threshold...

I need a circuit that will remain active for at least 5 seconds, if not more. Do I need more resistance? A larger capacitor?

See diagram below:

http://www.closeoutcctv.com/v/vspfiles/assets/images/diagram.jpg

Sorry, I meant to say the load is a 2N,2222 transistor controlling a 37mA relay. It's not the relay itself.
How do I find out what the impedance of my transistor is?

You need a different circuit. The input impedance of a 2n222 common emitter is very very high, much much larger than the 2k resistance in your circuit. What you need is another resistance connected across the capacitor, with an RC time constant that will decay from your control voltage to the cut-off of the transistor.
Look at the curve for your time values

http://www.tpub.com/neets/book2/3d.htm

Google if you want some basic theory

http://www.google.com/search?q=rc+time+constant&ie=utf-8&oe=utf-8&aq=t&rls=com.yahoo:en-US:official&client=firefox

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