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i'm constructing a transformerless powersupply, and i don't know how to compute or get the value of a bleeder resistor,storage capacitor and the current limiting resistor.thanks for the help..

Please make your circuit unlisted or public.

Include all information about required input and output voltages and currents and/or powers. Include frequency information if input not at DC.

no circuit + no information = no help.

:)

the input voltage is 220Vac 60hz. and the output is 12Vdc 10mA.. [circuitlab]f8b9kb[curcuitlab]

sorry i don't know how to post the the circuit. https://www.circuitlab.com/circuit/f8b9kb/transformerless-powersupply/

"sorry i don't know how to post the the circuit."

You do now.

:)

First thing to say is that this is potentially an extremely dangerous circuit.

There is no isolation from the mains.

The user is exposed to lethal voltages with no protection.

Unless this circuit is to be used with no possibility of the user being exposed to any part of the circuit, it is not safe.

Sorry but the usual rider of do not attempt to build or use such a circuit unless you are absolutely sure that you know exactly what you are doing clearly applies in your case.

What is your intended end use of this circuit?

..sir i intend to use it on our project at school..i'll be using a 555 timer which needs a dc supply but our project is power failure and resumption alarm,.what is the best thing i should to thanks.

Have a look at this thread:

https://www.circuitlab.com/forums/basic-electronics/topic/2zndkjpx/power-failure-alarm-circuit_please-help/

You can use a design like that triggered from an opto-isolator which is itself driven by a mains input.

The opto-isolator will look roughly like this:

but with a diode such as a 1N4148 wired in anti-parallel across the opto LED (so that the diode conducts for one mains 1/2 cycle and the opto led the other) and the input current source (I1) replaced by the mains supply connected via a series resistor of approx (Vmains_rms/iled), where iled is the average forward current of the LED in the opto-isolator.

Note that this will dissipate approx (Vmains_rms*iled)W.

Using a cheap generic opto such as the cny17-1, the recommended iled = 1mA.

The input side power dissipation can be reduced by replacing a simpleseries resistance with a capacitor in series with a resistor. 100k and 10nF works OK at 230V 50Hz.

If you model the opto input just as two antiparallel diodes (the difference between a LED and an ordinary silicon diode forward drop is negligible in this simulation) then you can see how this works by modelling a 50 or 60 Hz Vsource driving through the series R & C into the anit-parallel diodes and look at the power dissipation in the R and the phase shift of the current through R due to C.

The output of the opto is then used to trigger the alarm circuit.

By using an opto-isolator, you minimise the circuitry exposed to the mains potentials and keep all the clever bits running off a battery.

OK the battery may go flat but with careful design the alarm can be made to consume negligible power unitl the mains goes away.

You could do it all driven off the mains with a capacitor to drive the alarm circuit while the mains goes away but this is much safer.

It is not safe ...it is just less dangerous so you still have to exercise extreme caution!