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#### pharaon

Oct 28, 2014
418
i would like to know what is the advantage and disadvantage of such circuit if it will be used to power on 12v fan

Last edited by a moderator:

#### duke37

Jan 9, 2011
5,364
1. It outputs 9V not ±9V so the 12V fan will run slow.
2. The output is not isolated so will give a shock if touched.
3. Very reliable high voltage components will be necessary. The input capacitor should be 1000V of the type which fails open circuit, not short circuit.
4. It is small, light and cheap.

#### Kiwi

Jan 28, 2013
457
Much safer and easier to use a 12v AC/DC power supply/wall wart of the correct current rating to drive the fan.

#### pharaon

Oct 28, 2014
418
1. It outputs 9V not ±9V so the 12V fan will run slow
That's can be controlled by the zener diode

2. The output is not isolated so will give a shock if touched.
Sure I'll isolate it

3. Very reliable high voltage components will be necessary. The input capacitor should be 1000V of the type which
Can you give me link for such one
The one that used ia ceramic capacitor 400V that run on 220 ac source

#### pharaon

Oct 28, 2014
418
i wonder how to calculate the voltage after the ceramic capacitor

#### Ylli

Jun 19, 2018
396
You would want a Class Y safety capacitor. Example: https://www.digikey.com/product-detail/en/kemet/PHE850ED6100MD18R06L2/399-5422-ND/1927367 .
The output voltage is going to be almost totally dependent on the load, including the load supplied by the Zener diode. With the input capacitive reactance, and the 10K resistor, you have more of a current source than a voltage source.

With the components shown the circuit can probably source around 10 mA.

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#### duke37

Jan 9, 2011
5,364
The input capacitor will define the input current. Use LTspice to determine this.
The current is divided between the output and the zener diode. The output voltage will limited by the 9V zener and will drop if the output current demand is more than the input current.

If you are going to isolate with a transformer as suggested, then get one with the appropriate voltage, An AC transformer and a bridge rectifier should be enough. A smoothig capacitor would only be gilding the lilly.

#### pharaon

Oct 28, 2014
418
If you are going to isolate with a transformer as suggested, then get one with the appropriate voltage, An AC transformer and a bridge rectifier should be enough. A smoothig capacitor would only be gilding the lilly.
i'm gonna use 12 dc fan one of those like pc fan inside my old fridge to make good coolant , and will connect it to the thermostat to run and stop as the fridge work so i need something small

#### Bluejets

Oct 5, 2014
6,583
Sure I'll isolate it

And just how do you intend to do this..?
I'm not certain you understand what an isolated supply comprises.

#### pharaon

Oct 28, 2014
418
And just how do you intend to do this..?
small board to solder the component and then heat shrink

#### Ylli

Jun 19, 2018
396
This isn't going to provide enough current for anything more that the smallest of fans.

#### pharaon

Oct 28, 2014
418
This isn't going to provide enough current for anything more that the smallest of fans.
why?

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,337
1) Do not, never, use this type of "power supply". This is a cheap technique intended to be used in fully isolated housings at best. As soon as there is the slightest risk of exposure of one of the output leads to human touch there is also a risk of a potentially deadly shock. You obviously lack the experience to judge this circuit. Therefore it is mandatory that you use a save power supply.

2) Once you have gained some more experience and still want to use this design (in an isolated, closed housing, of course), consider the impedance of the high voltage capacitor (as per X = 1/(2 × f × C) as a current limiting resistor. This will allow you to make the necessary calculations.

Why do I think you lack the experience? You're getting your units all wrong:
• 1 mΩ is a short circuit. The original design you copied surely calls for 1 MΩ - case matters: m = milli, M = mega.
• The unit of capacitance is F, so it is not 220 µf but 220 µF.
• The output is + 9 V vs. 0 V or 0 V vs. -9 V, but not +. 9 V (as others have mentioned before)
You may think this is nitpicking, but it is not. Electronic design is an engineering trade and requires precise descriptions.

#### pharaon

Oct 28, 2014
418
consider the impedance of the high voltage capacitor (as per X = 1/(2 × f × C) as a current limiting resistor.
thanks for your post, and yes i'm still learning
can you explain this in numbers, for example what is C

Moderator
Nov 17, 2011
13,337

Moderator
Nov 17, 2011
13,337

#### Ylli

Jun 19, 2018
396
Reactance of 0.1 uF capacitor at 60 Hz
Xc = 1/(2piFC) = 1/(2pi*60*0.1e-6) = 26526 ohms
That 10K resistor is also in series, so total source impedance:
Zs = √(10,000² + 26526²) = 28348 ohms.

With a 220 vac source, short circuit current will be:
I = E/Z = 220/28348 = 7.76 mA

Any load voltage will reduce that maximum current capability. With a 12 volt load:
I = E/Z = 208/28348 = 7.33 mA

#### Martaine2005

May 12, 2015
4,674
It doesn’t matter how many times I see the formula and math. It still goes WHOOOSH..

Martin

#### pharaon

Oct 28, 2014
418
Xc = 1/(2piFC) = 1/(2pi*60*0.1e-6) = 26526 ohms
pi stands for?
C stand for? what is e-6?

so which one of those should i use?
also what should be the AC voltage when connect the capacitor in series ?

#### Ylli

Jun 19, 2018
396
pi stands for?
C stand for? what is e-6?

pi = pi. pi is the ratio of a circles circumference to its diameter. pi = 3.1415926535.....
F = Frequency in Hz. For the US line that would be 60 Hz.
e-6 is a shorthand way of saying '10^-6'.
so which one of those should i use?
Neither. Select a more appropriate and safer circuit to power the fan.

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