Timing a coil

Lgrant1350

Hi folks,

I'm attempting to learn about electronics on my own so I apologize for a question this basic. I have a relay that I want to time so that it doesn't fall out for approximately 15 seconds after it loses it's feed. The timing isn't critical give or take a second or two.

I put a 1220· microfarad electrolytic capacitor in paralell with it's coil which is
12 volt and 120 ohms. I only got a 1 second delay. so for 10 seconds would I have to go 10 times that for a 12200 uf capacitor? Are capacitors like batteries, the larger the physical size the more storage capacity?

Is there a better way? I tried to buy a 555 today at Radio Shack but they didn't have any. Would that be the way to go. Is the circuit to do it easy? I downloade the data sheet for a 555 but haven't gotten into it yet.

Thanks,

Larry

LoopyByteloose

The 555 IC is the better way to go. Try Google for the many tutorials that are available. Using a capacitor requires a bigger capacitor for more time. At some point you have a very large project. Additionally, the 555 allows you to adjust your timing through a variable resistor. Variable capacitors are almost completely antiques at this point and since they use air for insulation they can be huge.

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G. Herzog in Taiwan

Guenther Daubach

Larry,

I can only agree, the 555 is ideal for that purpose - you can generate delays from miliseconds to hours without very large capacitors. The 555 datasheet specivies the maximum output current at about 200 mA at a moximum of 16 V supply voltage. When the relay coil you want to drive draws more current, or requires a higher voltage, you will need an additional driver transistor. In any case, dont forget to connect a diode, e.g. a 1N4002 across the relay coil with the cathode connected to the more positive end of the coil. The diode is important to protect the transistor or the 555 against reverse transients generated by the coil when turned off.

I have attached a picture showing how the relay can be driven directly, or with a transistor, and how the diode should be connected.

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Greetings from Germany,

G

Lgrant1350

Thanks Herzog and Gunther for taking the time to answer. I'll go with the

555 timer then. Breaking new ground here.· :-)



Thanks again,

Larry