# Transformerless power supply

1. Sep 21, 2016

### seemasdawange

I designed one transformerless power supply and did lots of calculations for that. can anyone answer me how to calculate value of capacitor using as a smoothing capacitor parallel to zener.

and also tell me if the value of capacitor is larger than required then what changes may occur in the actual circuit.?

2. Sep 21, 2016

### Mark veniaminavich

As I know the higher UF of the capacitor the smoother your power supply will run at the expense of starting time, which means that your power supply will need a slightly longer ( one to two seconds ) time to start working properly, also the input and the output matter of the whole system matter.

Always safety first

3. Sep 21, 2016

### seemasdawange

If i use the higher UF capacitor then reactance of the capacitor becomes low and causes higher current to flow through it. Isn't that make any major change in the circuit? in that case is our circuit operate as per our requirement?

4. Sep 21, 2016

### Staff: Mentor

The smoothing capacitance is determined by the maximum load current and the tolerable ripple voltage. There is a formula for this.
A larger capacitance may cause a larger inrush current at switch-on. This can put a strain on the rectifiers, and the fuse—and also the on/off switch.

5. Sep 21, 2016

### seemasdawange

Thanks dats what i'm expecting.

6. Sep 21, 2016

### Staff: Mentor

Can you post a copy of your schematic? That would help us to answer any questions that you have. There are lots of ways of making a transformerless power supply. Use the UPLOAD button to attach a copy of your schematic to a post. Thanks.

7. Sep 21, 2016

### seemasdawange

Here is the schematic.. further circuit is not here. input supply is 230V AC 50Hz as per our country specifications.
Simulation is done in ISIS Proteus 8.0

8. Sep 21, 2016

### Staff: Mentor

That series 2kΩ resistor means you can disregard the caution I wrote earlier about capacitor inrush currents at switch-on.

9. Sep 21, 2016

### seemasdawange

okay. so what should i do? remove it from series? actually this resister also has power loss in form of heat

10. Sep 21, 2016

### Staff: Mentor

Well, what was your reason for placing it there?

11. Sep 21, 2016

### seemasdawange

To limit the current. if voltage goes too high in that case it may protect capacitor by limiting the current

12. Sep 21, 2016

### Staff: Mentor

Yikes. I see several problems here. Where did you get this circuit? Can you link to the original source? Overall, this circuit makes no sense, IMO.

For example, you have a 50Hz AC Mains source trying to charge a 20uF capacitor through a 470nF capacitor. Do you see any issue with this (and there is the series resistor problem adding to this problem)...?

13. Sep 21, 2016

### Staff: Mentor

Oh, I see—you included it because I mentioned surge current? But you can't make it an arbitrary value—the current through that path becomes the current for the DC side of your power supply. Because you haven't shown what circuits this will be supplying, I can't guess how much DC current it needs to supply. Have you estimated the DC current your loads will draw?

14. Sep 21, 2016

### seemasdawange

No I tried to design it by myself. i read from some source to add series resistance there http://www.designercircuits.com/DesignNote1a.pdf check this pdf u may get the answer.

15. Sep 21, 2016

### seemasdawange

yes of course. my load is a relay of 12 Volts and have power ratings of 0.36Watts. that means load current is 30mAmps

16. Sep 21, 2016

### seemasdawange

check this image. and look at the output waveform.

Is it ok? or should i make some changes here also?

17. Sep 21, 2016

### Staff: Mentor

Before you go further, you do know that modules are available to do all this, more efficiently and probably much more cheaply than you can build it yourself. To wit, that AC capacitor needs mains voltage ratings (600V at least, but I would use 1000V because I know failure will be expensive), so that cap alone will be costly.
Google search, or check out: http://www.digikey.com.au/product-detail/en/cui-inc/PBK-3-12/102-3108-ND/4332645

18. Sep 22, 2016

### seemasdawange

Hmm, u r ryt. can u suggest any module den?

19. Sep 22, 2016

### seemasdawange

You may not understood previous diagram as there was transistor and battery of 12Volt shown. the simplified diagram is here. you can check this. is that ok? or still have some problems?

Last edited by a moderator: Sep 22, 2016
20. Sep 22, 2016

### Staff: Mentor

I can't recommend a particular one. Does the one I linked to have suitable ratings? Or search on google, amazon, ebay, or mouser.

21. Sep 22, 2016

### seemasdawange

Ok Thanx

22. Sep 22, 2016

### Staff: Mentor

No, it still has errors and makes no sense. Why are you insisting on putting a capacitor in series with the AC Mains input? I can think of no reason in this circuit to do that. And you appear now to have deleted the input storage capacitor. Why?

Last edited: Sep 22, 2016
23. Sep 22, 2016

### davenn

indeed ....
It would seem that the OP shouldn't be playing with mains voltages as they appear to lack the knowledge to do it correctly and probably not in a safe way

24. Sep 22, 2016

### Staff: Mentor

The capacitor is there as a low-heat (i.e., low energy-loss) voltage-dropping impedance. This is standard design of this type of low-power transformerless power supply. A voltage-dropping resistor can be used instead, but this is substituting one problem with another: dealing with the heat that resistor produces.

The design itself is a compromise, and is not suitable for applications where a user could come in contact with any part of the power supply or the devices it powers. Opto-isolation is desirable in achieving this.

25. Sep 22, 2016

### Staff: Mentor

Thanks NO. At my work, we actually publish a reference design for low-power non-isolated applications that uses capacitor isolation, so I probably should have recognized that. Still, the design seems to have some issues. I wonder if the OP understands what they are trying to do. It would be good to see some specifications for the design from them...