capacitor power supply switched with mosfet or NPN

[yassin.kraouch]

Hi, i am using capacitive power supply to supply LED ( the Vf of all LED in series is 140V, and the current is 20mA), i want to add a switch in series with LED, so i think to add a mosfet between the LED in the bottom and the ground, what is the requirement on this switch if it is a MOSFET or NPN ? and which one is better ? many thanks

 

[treez]

its up to you, but id go mosfet, as it will need the high voltage capability...also,fet is easier to drive.....needs no current to drive on. (at least not after its switched)..I have nice circuit for youfor cap dropper to give you the bias power for switching the fet if you want.....you do cap dropper?

..and it is your lucky day, because I upload superb cap dropper schematic for you, with fet switch etc

 

[yassin.kraouch]

treeze, thankd for your feedback, bu ti have one question, your schematic looks a little bit complicated, can you please explain ? why we need two NPN transistor ? normally the fet is sufficient to do switch on/off,

 

[treez]

those npns are doing current regulation

 

[yassin.kraouch]

and the inductor ? but your circuit will flicker at 100hz. and the zener diode ?

 

[treez]

I am not bothered about flicker at 100hZ. the zener diode, I think was just in case a led fails open, it provides an alternate path

 

[yassin.kraouch]

and how the current regulation work ?

 

[treez]

voltage across vbe....
whencurrent through resistor makes its voltage go above 0,7v, bjt turns on

 

[yassin.kraouch]

do you have a hole explanation of this circuit, i am curious to know the details,

 

[treez]

I would download ltspice free simulator, put it in and hit "run"..then you will see

 

[FvM]

The discussion lacks a trivial thing, it's never said how you want to control the switch?

So all assumptions about an additional DC power supply are more or less void.

 

[yassin.kraouch]

control the switch via a ucontroller, its easy, but the problem was to what is the specification of the mofet, can i use capcitive power supply to supply the ucontrooller ? and what is the requirement

- - - Updated - - -

I would download ltspice free simulator, put it in and hit "run"..then you will see

i know how to use ltspice, this is not the problem, the probblem is how it works,

 

[treez]

The mosfet must be rated to carry the current....and it must be enough vds to handle the mains peak.....plus see the gate voltage threshold

I think the above schem is pretty much the whole thing for you....you can see where you could get power for your micro...

 

[FvM]

Yes, you didn't mention the power requirements of the microprocessor, which can be quite different, e.g. 50 µA, 1 mA or 20 mA, depending on which code you want to run and what's the control signal source. The difference definitely matters for the design.

 

[yassin.kraouch]

it is a low power ucontroller, it consume with other circuit 30mA max,

 

[treez]

yes but you do simpleled driver...you only use micro to turn ft on...so you don't need it to draw any more than 1 or 2ma, surely

 

[betwixt]

I'm also a bit confused about the circuit. What exactly do D2 and D5 do? As they are in parallel with D8 and D10 they can never have more than their Vf across them and the curent through the capacitors C2 & C3 becomes half sine at best. I have not done an analysis but it would seem that using a single capacitive dropper from the + line at the top of the schematic would do the same job. I assume any supply for a microprocessor would come from across C1.

Brian.

 

[treez]

ill have to run it again but (and this is going back 3 years now) I remember having to put D2 and D5 in to make it work correctly.
The cap dropper is the only bit of this circuit that has changed.....the original circuit (you may have one in your house) has a r/zener/bjt linear regulator to supply the C1 supply.
Oh, sorry, the original cct also has a toggle flip flop toturn the fet on/off.
I'm sure one person will soon know what circuit this is and in which (extremely common) product it exists in. (or at least did do 3 years ago)

 

[betwixt]

Give us a clue - I certainly haven't got one !

Brian.

 

[FvM]

The "capacitor power supply" looks a bit strange, but is working somehow.

You can analyze it as two separate voltage doublers that take the voltage across D8 and D10 as it's input voltage. Depending on the load of the main circuit, the voltage can have different waveforms, possibly with edges, so the series resistors are essential. You can asume that the capacitor Irms and the DC output current will be higher in all situations with output load, but not much higher.