Whats a semiconductor substitute for a Relay?

[thecritic]

Thanks for all the help to everyone, especially Sceadwian. I am now going setup the cirucit.
But one last confusion.
Should I necessarily use those so called fast diode ( I don't know about them) or can I get satisfactory result with a 2 amp rectifier diode in parrallel with a fairly large capacitor??

 

[Sceadwian]

You should use a fast diode, rectifier diodes tend to have a decent amount of capacitance with their PN junction cause it needs to be large to pass a lot of power. When the diode is reverse biased that capacitance holds a charge, when the voltage goes forward it takes time for that charge to dissipate, during that point the voltage is building and fast. It's only important in this type of application because the voltage spike the solenoid switching creates is probably on the order of 200+ volts and it's nearly instantaneous. If you keep that conduction path open all you end up with is a small current spike through the coil in the reverse direction when it's switched off. The capacitor may help but it may not help enough, and keep in mind it has to be a non-polar cap. Your call on weather you want to try it that way or not, I wouldn't.

 

[mneary]

A resistor of 1k ohms in series with the gate would slow down the turn off enough (1e3*1.4e-9 = 1.4 microseconds) so diode speed is not an issue.

 

[Sceadwian]

Thanks to mneary looks like you're good to go critic. With the components you're using to it'll probably last forever.

 

[thecritic]

[quote = "mneary"]
A resistor of 1k ohms in series with the gate would slow down the turn off enough (1e3*1.4e-9 = 1.4 microseconds) so diode speed is not an issue.
[/quote]
!!!???
Are you trying to say that I should use a 1k ohms resistor in series between gate of the mosfet and output pin 3 off the timer???

 

[Sceadwian]

Yes, that's what he's saying. The 1k resistor and the capacitance of the fets gate will form an RC network, causing a fast but gradual rise/fall in the fets switching. This will make the voltage spike the solenoid produces a little softer. You'll still need a 10k resistor to ground as well, the gate of a FET should never be left floating, a single static discharge into or from the gate to something else will instantly fry it, the 10k to ground gives current someplace to go besides through the FETs gate junction. Typically the junction will breakdown if it sees anything over +/-20 volts.

 

[thecritic]

Thank you a lot for the help.
I just repaired my damaged relay (I cleaned and filed the burned contacts) and I added a normal high power (3A) rectifier diode across the solenoid, and hurray! no spark!.
Previously the whole relay (transparent casing) used to lit-up in the dark, but now, I don't see any trace of spark.
Also, a simple rectifier diode worked for me, in contrast to the fast diodes which Sceadwian was talking about.