transformerless power supply

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why zener diode (Vz=12V,1/4w) in this circuit  be very hot?

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Non-isolated, line power devices can be VERY dangerous!!!

Since you are asking a very basic electronics question, you should not be messing with line powered circuits!

Jim

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thanks .OK I will use a isolated power supply.

To answer your question, with no load on the output of the power supply, the zener is acting as a shunt regulator, so all unused power is wasted across it.

About 170mW, for this 250mW device, so it gets hot, assuming this is used with a 240v mains.

Jim

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For this circuit:

Irms [of C] = Vrms / Zc = Vrms / ( 1/2/PI/Freq/C )

Idc [after 4-diode bridge] = 2*SQRT(2)/PI * Irms = 0.9 * Irms

Pzener  (with no load) = Vzener * Idc

For Freq=50Hz and Vrms = 220V , Pzener = 74.64 mW

For Freq=50Hz and Vrms = 240V , Pzener = 81.43 mW

I conclude that if the circuit is built properly (as the one attached), the zener diode (12V, 1/4W) shouldn't be too hot, even with no load.

Kerim

Edited:

Sorry, I thought C=100nF, actually it is 1uF.

For Freq=50Hz and Vrms = 220V , Pzener = 746.4 mW

For Freq=50Hz and Vrms = 240V , Pzener = 814.3 mW

Therefore, the zener diode should be very hot.

For such supply (used for boards that are out of reach by the users, like outdoor light controllers), it is better adding a resistor in series with C to limit the in-rush current (worst case is at Vpeak).

For this circuit, the resistor Rs=220R 3W is somehow acceptable (higher values also work but need higher power).

Prs = Irms * Irms * Rs

Last Edited: Thu. Oct 11, 2018 - 09:26 PM

You mention a 12V 1/4W Zener while the schematic specifies a 12V 1W Zener.

But I completely agree with ki0bk. If you only have such a basic knowledge of electronics you should not (yet) be messing with mains voltages.

Just cruious:

How do you know the zener is getting "too hot"?

It is not safe to touch.

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

Paulvdh wrote:

Just cruious:

How do you know the zener is getting "too hot"?

It is not safe to touch.

Maybe "hot" is in reference to the voltage potential on it :)

KerimF wrote:
... it is better adding a resistor in series with C to limit the in-rush current (worst case is at Vpeak).

For this circuit, the resistor Rs=220R 3W is somehow acceptable (higher values also work but need higher power).

Simple, fusible, functional

An alternative is active current limiting :

via a sell sheet :

Microchip

AC/DC Power Conversion

Low-Power Low-Cost Offline Power Conversion Solutions

via

Microchip Technology

SR10 - Power Management - Inductorless Off-Line Regulator ICs

Capacitor-Coupled, Switched Shunt, (CCSS) Regulator

https://www.microchip.com/wwwproducts/en/SR10

...

datasheet :

...

greater current capability by a much higher frequency non-isolated SMPS :

Power Integrations

AC-DC Converters

P.S.

KerimF wrote:
For such supply (used for boards that are out of reach by the users, ...
though not out of reach of fauna with sharp teeth.

During extreme droughts here, the squirrels will gnaw on nearly anything to try to overcome their hunger.

Teething baby fauna are another "issue".

Feed the fauna to save their lives and your electronics ... enclosures ... cables ...

"Dare to be naïve." - Buckminster Fuller

Is it really worth nowaday's to build a circuit from discrete parts?

What do wall warts cost? The're safe and easy.

(Unless sourced directly from Ebay / Ali / China. Quality then varies between garbage and excellent).

A nice in between are the Hi-Link power modules such as the HLK-PM01

https://www.aliexpress.com/wholesale?SearchText=HLK-PM01

These are quite nicely built and deliver an isolated low output voltage.

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

Thank you, gchapman, for your valuable suggestions.

They are likely very useful to most engineers around the world who are allowed to get (order/buy) the mentioned components, besides being able to be serviced by Microchip.

For instance, I missed a lot the human corporation, ATMEL, because, to me in the least, it was dead when it was acquired by Microchip. The latter has not been allowed to open its doors, since about 16 years, if contacted from where I live. Fortunately, the human brain can always find a solution, when in real need, to build something with what is available.

You're welcome.

There's the higher voltage (greater than 24V SLA) variant of the Black regulator but the BJTs aren't jellybeans and will need a MOV and some capacitance for surge suppression (EFT, lightning)

2-transistor Black Regulator

by Roman Black

http://www.romanblack.com/smps/smps.htm

(3/4 page)

High voltage use

...

KerimF wrote:
...  if contacted from where I live.
Though Jordan may unfortunately be like halfway around the world from you, it's interesting that Microchip Jordan redirects to Microchip France (same with Microchip Lebanon ... I comprehend that a bit)

https://www.microchip.com/distributors/SalesListing.aspx?market=MIDDLE%20EASTERN&region=ME&terr=Jordan

Edit: 2nd URL

"Dare to be naïve." - Buckminster Fuller

Last Edited: Fri. Oct 12, 2018 - 04:26 PM