Dimming LED using MOSFET : Circuit Working

Thread Starter

Athul S Nair

Joined Dec 24, 2016
23
Hello All,
Can someone explain to me how this circuit works as shown in the video FET Phaser Light. I know the working of MOSFET, but how it switches on and off with fading effect.
I found some explanation here in the comment section, but not understood completely.
Capture.PNG



Thanks
 

Bernard

Joined Aug 7, 2008
5,479
At start, all Q s off, Cs discharged. Power on; Cs start charging, say Q1 turns on first- red LED on,C2 slowly discharges thru R4., green LED off, C3 continues to charge turning on Q3 which then discharges C1 turning off Q1
allowing C2 to start charging turning Q2 & green LED on. Make any sense??
 

Alec_t

Joined Sep 17, 2013
11,163
Because a FET doesn't have a very sharply defined turn on/off threshold, as the gate voltage rises and falls the LED current doesn't change abruptly, so fading occurs.
 

Thread Starter

Athul S Nair

Joined Dec 24, 2016
23
At start, all Q s off, Cs discharged. Power on; Cs start charging, say Q1 turns on first- red LED on,C2 slowly discharges thru R4., green LED off, C3 continues to charge turning on Q3 which then discharges C1 turning off Q1
allowing C2 to start charging turning Q2 & green LED on. Make any sense??
I understood what you said, but some doubts,

1. Is this the path for capacitor charging?
Capture - Copy.PNG
During this charging process won't LEDs turn ON, since current flows through them or current isn't sufficient to turn these on?

2. The reason Red ights up first is beause of absence of any resistor in its charging path? Since no resistor it charges up quicky compared to other capacitor (C2 & C3)

3. Is this discharging paths for each capacitor. It discharges through respective FETs when they are ON

C2 discharges through Q1
Capture - Copy (2).PNG

C3 discharges through Q2

Capture - Copy (3).PNG

C1 discharges through Q3

Capture - Copy (4).PNG
 

Wolframore

Joined Jan 21, 2019
1,743
C1 charges through LED3, R3 and R6, slowly increasing current through Q1. The MOSFETs have an ohmic region where the LEDs will will come on and get brighter as it charges and dimmer as they discharge.
 

Thread Starter

Athul S Nair

Joined Dec 24, 2016
23
C1 charges through LED3, R3 and R6, slowly increasing current through Q1. The MOSFETs have an ohmic region where the LEDs will will come on and get brighter as it charges and dimmer as they discharge.
Why that path ?
Isn't this the least resistive path?
Capture - Copy (2).PNG

Why through LED3, R3 & R6?

What about remaining capacitors charging and discharging path?
 

ebeowulf17

Joined Aug 12, 2014
3,274
Agreed - there are two common conventions for schematics to indicate whether apparent intersections connect or not, and either one is fine as long as you're consistent in applying it. Here, they've been pretty random in applying a mix of the systems, so you can't read the schematic accurately without understanding the intent and making educated guesses. Pretty annoying.

Here are some images from this website's education section that illustrate the two systems:
Screenshot_20200405-111727.png
(copied from: https://www.allaboutcircuits.com/textbook/reference/chpt-9/wires-and-connections/ )
 

Thread Starter

Athul S Nair

Joined Dec 24, 2016
23
Thanks for all the replies, but one thing still confuses me is, if all capacitors are charging at the same time, and FET, capacitor rating, resistance they are all same, why all FETs are not triggering at the same time? If that happens all leds will dim and light up at the same time, right? Or is there any other tricky part to that?

This sequential effect will happen only if one FET get triggered first.(as explained in the first reply I got)

So whats the reason for triggering only one FET initially? If you check the video comments there are people saying it is due to there wavelengths, but that has no influence on FET triggering.

And it looks like they're all trigering at the same time
1586112737604_1.jpg
But after that they somehow falls in to a sequential order. What causes this?
Also Why leds are not lighting up when capacitors are charging? Is it due to low current?
And which software is this?

And about understanding schematics, you guys figured out actual connection at the first look, so experience matters :)
 
Last edited:

Audioguru again

Joined Oct 21, 2019
1,304
How much current is in a 2V LED when it has a 1M resistor is series and the supply is 6V? 4uA.
If 12mA is fairly bright then how bright is 0.004mA? Maybe producing no light.
 
Top