On Wednesday, August 2, 2017 at 1:22:32 PM UTC, Phil Allison wrote:
wrote:

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The OP is clearly a novice and I hope he is aware how
dangerous transformerless PSUs are to work on and takes
all the precautions needed.

If this is a task set by his electronics instructor,
big bad on him or her.




You seem to understand my problem very well.



** Fantastic.

Cos it is bloody obvious YOU do not understand it one tiny bit.

Have you realty built this thing ??

I doubt that VERY much.

Maybe you simmed it.

FFS - you have NOT mentioned the AC supply voltage so far.

Slipped your mind ?



sorry please the cap is 0.66uf and the mains voltage is 220v 50hz.



** The cap is 0.68uF - put your glasses on.

I do hope it is a class X1 or X2 type - rated for mains AC voltage.

It's impedance at 50Hz is 4680 ohms so the average *rectified* current flow is 40mA. Average, full wave, rectified sine wave current = 0.63 times the peak value. Look it up.

The 12V relay must operate reliably at that current, so coil resistance needs to be not more than 300 ohms.

If the relay coil is switched off, there is Zero current flow.

The * HUGE * advantage of using a cap to drop the AC supply voltage to suit the relay is that is dissipates NO power.

A resistor would dissipate nearly 10 watts.



.... Phil

Thanks a lot Phil you were really helpful. All l needed you have provided
I also had a lot more info from hackabay.com. But, but l am surprised the high level of respect for members in this group has gone down. Any way all is not lost yet people like Phil are always there to help.