Running a 555 based timer with a SMPS power supply

[archat68]

I've built a 555 based 90 sec delay timer. The timer works well with conventional power supply (9V) but when I'm trying to power it through a 9V SMPS power supply it doesn't switches off after 90 sec.
The timer has to be fixed in a place where fixing a transformer for power is not possible.
Can it be powered through a transformerless capacitive power supply?

Or are there any timer circuits which can be built and powered through SMPS / transformerless power supply? the circuit must be compct.

Please help guys!!

 

[Hero999]

What are you switching with the 555 timer?

How is it triggered?

Capacitive power supplies are only suitable for low currents and are not isolated from the mains so the whole circuit, including everything connected to it, needs to be treated as mains and heavily insulated.

The correct solution of course is to add some filtering so it works from the switched mode power supply. At a bare minimum there should be a 100nF capacitor across the supply rails, 10nF between pin 5 and 0V and some ferrite beads on the power supply rails.

 

[archat68]

I'm using the following circuit with a BC548 transistor to drive a solenoid. The R and C values are adjusted so that the solenoid remains powered on for 90 sec and then turns off.

 

[Hero999]

What voltage does the solenoid run off? Unless it's mains then it won't have mains rated insulation so isn't safe to use a capacitive power supply.

How much current does it draw? Capacitive power supplies are only suitable for low currents. Above 20mA or so, a small transformer becomes more compact and cost effective.

There's no decoupling on your current circuit. Follow the suggestions I made in my previous post using a switched mode power supply before going any further,