What is wrong with this circuit?

codemaster11 said:

i think you remove the 20k resistor from the base of the transistor & put a 47uf electrolytic capacitor
at the base of the transistor. replace 4.7k with 2.2k & place a 100uf electrolyte across relay. i believe that it
will solve your problem.

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I think you shouldn't make suggestions that haven't been thoroughly calculated. The 47 uF capacitor would create a delay in a high seconds up to minutes range, hardly useful. The previous suggestion by vfone in more in the useful range, although I'm sure about the resistor value.

the relay chatter may be only due to weak input signal at the base of transistor. using spdt/dpdt relay
is not a best solution for switching in a fraction of second. let say if the Time constant "T = RC" , for R = 2.2k & C = 47uf
then the time it take to fully charge at the base of transistor will be 5T = 5(2200 * 0.000047f) = 0.52sec

KlausST said:

Hi,

Generally 5V supply as well as 24V supply need a bulk capacitor and decoupling capacitor.
But I assume there is another error. Bad solder joint, wrong or defective device....

Klaus

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I'm inclined to agree with this. There's nothing basically wrong with the circuit itself. There must be something wrong that's not shown in the schematic.

The drive to the optocoupler is higher than necessary. The 4N35 has a good CTR characteristic. A value of 1k for R1 should provide more than enough drive to saturate Q1 with a 1kΩ relay as the load. Also, Q1 does not have to have such high ratings (60V, 4A). A BC547 (or the SMD version BC847) will do the job nicely. However, all of this does NOT mean that the OP's circuit is wrong. It should work.

gary36 said:

Observation. When IN is connected to gnd, relay chatters. When IN is connected to 5V, relay gets properly energized and switches ON.

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This doesn't make sense. Nothing should happen when IN is connected to ground. U2 and Q1 should be off then. There must be something wrong with the wiring, the part(s) or the power supply.