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BJT delay timer problem?

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Full Member level 4
May 11, 2010
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I've built a following circuit:
Voltage at node (2) is about 2V. If I disconnect (R3) at node (3) and the circuit is powered ON then the voltage at the top of cap (C1) is gradually increasing from 0 to 2 volt. But if (R3) is reconnected then the voltage at node (3) never increase more than 0.64 V. The problem in that the transistor never saturate, VCE = approximately 10V and a 12V relay connected to the collector will never activate.
I need two things:
FIRST: How can I make the circuit operates a 12V relay?
SECOND: Where to connect a diode to discharge C1 when the circuit is OFF?
Thank you very much.

Your relay needs a certain amount of voltage/current in the coil, to cause it to close. You must experiment to find out what bias current is enough to turn on the transistor.

The combined R1-R2-R3 need to be low enough ohms, so that enough current goes to the transistor base, to turn on the transistor. Since your relay is rated 12V, and since your supply is 12V, you should turn on the transistor to saturation.

SECOND: Where to connect a diode to discharge C1 when the circuit is OFF?

Put a resistor in parallel with it. The value should be high. You do not want the capacitor to discharge too quickly.

You are NOT providing enough base current.
Use a Darlington transistor. Not only the required base current be orders of magnitude lower, but also the capacitor will charge to about 1.3 V.

You need to add a comparator IC that has a high input impedance to sense the capacitor voltage without loading it down. The output of the comparator can have 4mA as base current to turn on the transistor that drives a relay that needs up to 40mA.
Or use a 555 timer IC to replace your entire circuit.

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