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
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
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,
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
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.
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.
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)
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.