Electronics > Beginners

Capacitive Reactance, Inductive Reactance, Power Factor, Switching Hz

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MagicSmoker:
What the heck are V2 and the related components around it supposed to do? Err... I just noticed you have this across the AC side of the mains, not the DC side, so even more perplexing. Just delete all the stuff and try running the simulation again.

TheDood:

--- Quote from: MagicSmoker on December 13, 2019, 06:07:46 pm ---What the heck are V2 and the related components around it supposed to do? Err... I just noticed you have this across the AC side of the mains, not the DC side, so even more perplexing. Just delete all the stuff and try running the simulation again.

--- End quote ---
V2 + FETS are a way to rob some of the constant current produced by the X1 cap. It's the dimming portion of the cct. Maybe it won't work? Just trying to reduce V drop when the current is bypassed and I figured before the bridge was more efficient than after the bridge?


How glitchy is LTspice? I'm pretty sure I've been getting incorrect results or sims. I close it all out, reopen and re-run, and then I get different results. Very frustrating. I thought I was just not remebering correctly the mods I've made in between sims, but I'm pretty sure its been acting up and giving me shit data after awhile?

TheDood:
I've been working on a buck converter if the capacitive dropper is not going to cut it. Ill start a new thread and post it later. I figured if you poured rectified AC into a cap that you could then run a buck converter topology after it to power a load requiring lower V? The sims have been all over the place with op amp outputs greater than the V supplying it, with LEDs at 21V+ but only 6mA current flow, ect ect, taking a break.. Maybe a computer reboot will help..

MagicSmoker:
LTSpice isn't glitchy at all for me. Sometimes it has convergence problems when the ideal diode is used (the default one, unfortunately) but putting a few pF of capacitance in parallel usually fixes that. I also generally run the alternate solver (select Control Panel under the Simulate tab to change) because it gives more accurate results at the expense of taking longer to run.

Of course, it also helps if your circuit makes sense and is solvable by SPICE...  >:D

MagicSmoker:

--- Quote from: TheDood on December 13, 2019, 12:04:35 pm ---...The only way that I can see of maintaining the current flow I had before implementing the VFPFC, is by increasing the VAC input...

--- End quote ---

Increase the value of C1 - it is acting as a "lossless" current limiter due its capacitive reactance. This reactance, in Ohms, is found using the classic formula: 1/(2*Pi*f*C) where f is in Hz and C is in Farads.

12.2uF gives a reactance of approximately 217 ohms which will limit current into a short circuit of 0.55A when supplied by 120Vrms (remember to use the RMS value of the AC voltage here). Note that the higher the voltage drop of the LED string the lower the current the capacitor will let through since it is the difference in voltage between the (rectified) source and LED string that is dropped across the capacitor.

Making this kind of circuit dimmable with a conventional triac-based dimmer is nightmarish. There's probably some specialized ICs already on the market that enable this functionality, but I leave it to you to do that research.

In the meantime, I attached an LTSpice simulation of a rectifier + valley fill PFC + LED load circuit that simulates just fine and rather quickly, even with the alternate solver. I did not take any care in selecting components to leave you with something to do, but it gives you a good - and working - starting point.


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