5V Regulated Power Supply - Filter out Noise

[mvs sarma]

people call it switching noise of diodes.
As the 12V secondary transits the off state of a diode (say 0.6V) it abruptly stops conducting. Even though the circuits show rectification graphs as it each half cycle passes thro' the diode, it only passes above 0.6V approx (depends on type of diode selected and its Vf) and stops just below 0.7 or 0.6V.

This causes twice the power frequency as ripple. this can be reduced by the capacitors across each of the rectifying diodes. Most designers of audio and radio equipments, do this trick while using linear power supply designs .

 

[Nigel Goodwin]

Hi thanks for the points.My load is a motor.

Here is my PCB design.

The yellow colour line is the ground return path of the relay.

1st is the old PCB
2nd is the edited.

Any improvements?

I was trying to spot the 0.1uF's for the 7805, and was unable to do so.

I presume you know that it's VERY important that they are placed as close to the 7805 as possible?, if they are fitted at a distance it's not worth fitting them (as they wouldn't do anything).

 

[Suraj143]

Thanks guys for your information.They were really useful.

Nigel is correct.My 0.1uF and 10uF are located at far end.Its not usefull. I soldered the 10uF direct on regulator pin & soldered 0.1uF ceramic input side of reg.

My major problem was when power OFF & ON again in the circuit the DS1307 RTC is reset condition.The CH bit was always '1',the timer is OFF mode.Even with no load condition. This is due to wall outlet switch noise,relay spikes etc....

After placing decoupling caps on regulator pins the problem was solved.

The ripple scenario is somewhat other,it does not harm much as spike problem.

 

[Suraj143]

Here is the edited circuit.Its working nicely.The ground return path still as old version.

The only thing is there is no any place to fix snubbers on th PCB. For the time being there is no any DS1307 reset problem but don't know how in future behaves the circuit without snubbers