Questions about a simple variable power supply

[JimB]

The circuit is a load of rubbish!
Don't waste your time on it.

JimB

 

[Speedy]

I thought at first that it was dangerous to use a mosfet, little before it go on, such as dust in the pot with time can be enough. And, no less dangerous when it opens completely when the burner. So agree with you, and have made me think of another and equally simple way as I must try.

 

[Speedy]

Have not had that much time, but has made a fast test. Come to think of controlling it by triac, and it works quite well.

Unstable due to AC power mains fluctuations, and despite I have 2 * 4700uF 450V in parallel connected, I still measure AC on DC line :-/ Tried to connect an AC bulb and on low voltage the light pulse like with in normal 50Hz dimming, but has also regulating down and connected to a LED directly, did not burned and could not see pulsation

Although this covers my needs, I will at some point made a stabilizer and power regulator for it. Since I anyway would test whether the the mosfet in this way could handle it, I thought of inserting the mosfet as a form of regulator, so the output was always around 10v less than the input, as a way to make it stable, but the mosfet burned as fast as in the first.

Made this quick and simple setup and one of the benefits of this rather than the mosfet is if it burns, it's closes automatically and not as the mosfet which opens fully. A security which is a good thing.


(Think there should sit a resistor between 100k and 3m att output to get better stabilization)

 

[audioguru]

Your light dimmer circuit can adjust the voltage but with NO regulation. If the AC input voltage changes or if the load current changes then the DC voltage also changes.
The light dimmer circuit can be turned off then it requires the pot to be turned up fairly high before it starts working again.

 

[Tony Stewart]

My questions are:
1- Why the 12V zener? ( MAintains constant low resistance in MOSFET by Vgs=12 but controls Vs directly from high impedance on pot. better is use MOSFET with logic level threshold (1~1.2V) using 3~5V LEDs or zener
2- If i replace the IRF740 MOSFET by any 400v/10A BJT transistor, what would happen?
note: I know how to connect the BJT. That wont help as BJT is current control current source and FET is (nonlinear) Voltage controlled resistor. But a Triac would help before bridge to lower heatloss and temp rise. to maintain Vdc Min above Vout But then you need 2 bridges, one for low current Bias on Pot, the other either a triac bridge or same bridge with triac front end to drop average voltage just above Vout. But then you need ripple current filter for big cap. like a tungsten bulb or ICL which can handle extreme temps.
3- why C2 is only 10uF dislike C1? Big uF caps cause surge currents inverse to ripple voltage in percent. eg 10%V=10x current, bad for caps.
4- from:
https://avecircuits.blogspot.com/2012/07/0-300v-variable-high-voltage-power.html#.UI3WD2dQaoY
I found a similar circuit:
**broken link removed**
Author said:
How R3 value depends on HFE?

See answers embedded above

old school variable regulators ar e a delecate balance of brute force current pulses and high inefficiency (50%) losses when Vdc in is twice the Vdc out at max current. Which is why you need to learn crude Buck pre-regulator for better efficiency and linear for low noise. use LDO with band gap and bypass MOSFET.

 

[Speedy]

Your light dimmer circuit can adjust the voltage but with NO regulation. If the AC input voltage changes or if the load current changes then the DC voltage also changes.
The light dimmer circuit can be turned off then it requires the pot to be turned up fairly high before it starts working again.

Know, but with load the output is more stable so it is necessary to have a resistance at the output so there constantly is a small load and probably an RC link. This will not switch off.

 

[Speedy]

The reason for this diagram was to find an alternative solution which was just as simple.

We are not talking about an ordinary power supply designed to provide low voltage, but a more high power where this is quite good.

The problem with my setup is the big capacitor which provides a few problems to be for example a constancy load at the output as downregulation otherwise will not regulate the output down before the charging is used, but to high consumption, it is as it should be.

Would say that if one only needs a small amps consumption and therefore with small capacitor and appropriate resistance at the output, is it really good, as long as it does not matter that there is a small oscillation.

 

[Lucas Oliveira]

Hello everybody, I found this topic on google and read it all, I saw that the colleague of the problem is more complicated than mine, maybe you can help me:

I manufacture some sources 220V transformer with primary and secondary 170, place a bridge rectifier 8A and 1.500uF filter capacitor / 400V.

But deivido the peak voltage 170 * 1.4142 = 240VDC I needed some scheme to make it stabilized.

In my region the AC network varies widely, then the voltage rises too from time to time. Would I like some artifice to maintain output at 170VDC and that supports 5A current.
When I turn on a load of 1A tension is on average at 195VDC.

I wanted to use a transistor or mosfet to regulate the output.

I diminir the secondary of the transformer to reduce the power dissipated by a semiconductor that may use.

I thought that these components could be used:
MOSFET IRF740 Canal N 400 / 8A
Zenner 5W 150V
Zenner 5W 91V
Zenner 5W 15V
Zenner 5W 24V

The cannal mosfet N can serve as a regulator? Or only serves as Switch?

I spent all morning looking for something on the net and not found.

I intend to leave the Vin and Vout as little as possible to reduce the power dissipation.

Att,
Thomé Lucas

PS.: Thank you JimB, I translate this text, I know it's not cool, but I think it will take for you to understand.

 

[JimB]

Lucas
This is an English speaking forum, your post in Portuguese will not be widely understood.
I do not know how many Portuguese or Brazilians we have here on ETO, but it would be better if you can re-post in English.

JimB