Electrical – 2 seconds delay in analog circuit

analogdelayrelaytransistors

I'm having trouble to design a circuit with delay.

Here's what i did:

enter image description here

(I just tried to play with the transistor.. i know im missing something there)
So what i want to achieve is that when i close the orange switch, the upper relay will power up 2 seconds after the lower relay.
I'm trying to do it with 1 switch, what i mean is that im physically pressing this one switch and all the rest should be automatic.
How can i manage to do this 2 seconds delay between the relays power up. (using only the components used in the picture)

I'll appreciate any help. Thank you.

Best Answer

You have an excellent example of how not to design a timer delay relay.

Three conditions must be met:

  • A) relay coil current normal= ?? or Rc @V
  • B) Ic/Ib = 10~20 (default for all BJT saturated switches)
  • C) Delay =2 sec
    • Ic, Vcc and T tolerances and Vcc = unknown, (learn to define first)

In order to satisfy this chosen design, we can use impedance ratios. Since all voltages are common, consider coil Rc, and worst case, Ic/Ib=10 for an efficient switch

if Rc=330 Ohms , you want your switch to be << 10% of this and with Rb=33k yours transistor and Ic/Ib=20 Rce becomes 33k/20=1.6k and unable to saturate the collector.

Meanwhile C is already huge at 500uF, so can U consider a better way such as CMOS or a 2nd stage BJT to perform both time delay and current drive?

Always define your critical output/input specs, for at least current and impedance BEFORE you waste your time on a design.

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