DImmable mains LED's using NE555P? - Page 1
Forum section: electronics / beginners

author: zerorisers
author_rank: Regular Contributor
timestamp: March 09, 2017, 05:11:47 pm
content: Hey, so I designed a small setup for LED lighting where I am at, but I want to make it dimmable.Here is the schematic I made up. I have it set up to run 6-12 LED filaments at a time. they have a voltage drop of about 80v and are all wired in parallel.

author: StillTrying
author_rank: Super Contributor
timestamp: March 09, 2017, 07:52:11 pm
content: The junction of D9 & D10 should be connected to pins 2, & 6 instead of GND.The mosfet gate needs a bias resistor.I can't really figure the rest.

author: Seekonk
author_rank: Super Contributor
timestamp: March 09, 2017, 09:09:47 pm
content: Don't you think you should learn some basic electronics before you start connecting things to the power line? Or at least look at your schematics before posting them.

author: zerorisers
author_rank: Regular Contributor
timestamp: March 09, 2017, 09:42:26 pm
content: Quote from: Seekonk on March 09, 2017, 09:09:47 pmI have looked over it, but It is difficult for me to pay attention. I go over the schematics 3-4 times at least before building them but I like to check and see if I miss anything. I have autism and ADHD so that is why I posted before it was completely finished.I am fairly comfortable around the power lines, and I have already setup some lights using this method (Without dimmer of course)Quote from: StillTrying on March 09, 2017, 07:52:11 pmTHANK YOU! I already knew about the bias resistor but forgot to mention it in the schematic, but D9&D10 I did not notice I even had connected to ground. (Exactly why I posted on the forum to double check!)EDIT:Ignore schematic below, Too many distractions around me to focus on what I'm connecting stuff to... tells you how I made the first mistake.

author: zerorisers
author_rank: Regular Contributor
timestamp: March 09, 2017, 10:15:02 pm
content: Fixed schematic (SHEET 2_555DIMMER)

author: StillTrying
author_rank: Super Contributor
timestamp: March 09, 2017, 10:46:49 pm
content: Pin 7 is open collector, it would make more sense to connect to bias/pull-up resistor to +12V than the timing components!You can "modify" a previous posting including removing/changing the attachment if you've posted the wrong one."I have it set up to run 6-12 LED filaments at a time. they have a voltage drop of about 80v and are all wired in parallel."That bit doesn't make much sense to me, it's usually a bad idea to wire LEDs in parallel, do they have their own internal resistors? Where does the rest of the 80V go?

author: zerorisers
author_rank: Regular Contributor
timestamp: March 09, 2017, 11:29:32 pm
content: Quote from: StillTrying on March 09, 2017, 10:46:49 pmThe LED filaments I have are from china, they start off with a voltage drop of about 70-75 at low power, but as they approach 1W the voltage drop reaches around 80.The rest of the voltage drop mainly goes across the 2.2uF capacitor (for 6 LED's, for 12 there is 2 2.2uF caps in parallel)They do not have their own internal resistors, I am mainly relying on the reactance of the capacitor to limit current.I do add on resistors to each of the LED's that are in parallel for balancing, but because of their strange behavior with voltage drop they seam to balance themselves.(Assumption based on observation)Pin 7 is not connected to the timing components but I will add in the PULLUP resistor. Thank you.I also removed the PULLDOWN resistor from the MOSFET as it does not seem necessary with the open collector.

author: StillTrying
author_rank: Super Contributor
timestamp: March 10, 2017, 02:12:41 am
content: "The LED filaments I have are from china, they start off with a voltage drop of about 70-75 at low power, but as they approach 1W the voltage drop reaches around 80."OK, I get that now. I think you already know the problems putting the LEDs in parallel, especially if you're running them near their power limits."The rest of the voltage drop mainly goes across the 2.2uF capacitor (for 6 LED's, for 12 there is 2 2.2uF caps in parallel)They do not have their own internal resistors, I am mainly relying on the reactance of the capacitor to limit current."The volt drop due to the reactance of the 2u2 caps depends on the constant current drawn off the 47u cap by the LED strings, if the average current through the LED strings is reduced by a low duty PWM, I think the volts across the 47u 100V cap will go well over 100V, maybe well over 200V as the half-wave rectification acts as a voltage doubler.There should beat least somelow-value-wattage resistance in series with the 2u2 dropper caps for the times when switch on occurs near the mains voltage peaks and the reactance doesn't count, and a very large current spike will flow.I haven't checked the mosfet specs yet, but should be able to LT simulate it to see what happens.I think it's a good idea, but there's still some work to be done to make it safe and long lasting.

author: zerorisers
author_rank: Regular Contributor
timestamp: March 10, 2017, 03:01:19 am
content: Quote from: StillTrying on March 10, 2017, 02:12:41 amI do have a resistor on the neutral line for the purpose of limiting inrush current, and act as a crude fuse in case things go wrong.The negative of the capacitor is connected to "DGND" in order to prevent it charging when the LED's are "off"My calculations show each LED should be run about 5mW under their rated power (considering the Chinese overdrive their LED's I would assume that this should be fine)I should get LT So that I may actually simulate these circuits. I usually just build them up and see if they blow up (using an old motorcycle face shield to cover the board for saftey reasons.Below is the datasheet for the MOSFET I am using.

author: StillTrying
author_rank: Super Contributor
timestamp: March 10, 2017, 11:26:44 pm
content: "I should get LT So that I may actually simulate these circuits."Definitely, and especially as I've already drawn a simulation version for you! There's some fiddly bits but it's probably much easier than you think.Notes.Minor changes to the capacitor dropper circuits, mainly just removing the neutral diode and resistor so that they work.Simulated the LED filaments with ~80 volts worth of zeners, close enough for now.Simulated a 100k pot with 2 resistors 40k+60k, the 2 diodes there should be signal diodes, but power diodes should work up 2 or 3 kHz if you want to use them.I don't think even that half wave ripple on the 12V line (blue trace) would effect the PWM much in practice.Thegreen traceis the 100V LED supply, the half wave ripple on here might be visible on the LED filaments.Being powered from a capacitor dropper there's a problem in that the 100V LED supply varies between about 95V and 160V depending on the PWM duty.Theyellow traceis the volts across the LED filaments, about 80V of course.Thered traceis the current through the 1 LED dropper resistor R5, about 90mA total with quite a bit of half wave ripple.The 115 VAC starts at 270 deg. in an attempt to show worst case switch on current surges.You might as well do the rest of the tweaking yourself, what better start to LT simulation could you have.