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High or Low pass filter as voltage divider for LED from house AC

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Electrons-R-Fun
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High or Low pass filter as voltage divider for LED from house AC

Postby Electrons-R-Fun » Wed Jan 31, 2018 3:10 pm

HI,

I had a question, that seems very dangerous to me and I don't want to check without asking.

It appears to me that you can use the 120 Volts(rms) from a house receptacle and feed that into a 1uF cap(if the cap is rated for over 120 AC volts) and a 1k ohm resister to build a voltage divider that could Light an LED through a 100 ohm resistor and the LED would flash at 60 HZ but would work without melting anything. Not sure about the power dissipation yet, but I imagine it would be P=I^2R. Anyway just learning new stuff AC seems very different but has similar behaviors much of the time.

Jason

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Re: High or Low pass filter as voltage divider for LED from house AC

Postby Granz » Thu Feb 01, 2018 10:00 pm

Yeah, that does seem kind of dangerous. My remembrance of impedance is poor, and I do not remember the formulas, but that does not seem right.

Using AllAboutCircuits' impedance calculator (https://www.allaboutcircuits.com/tools/ ... alculator/) gives your capacitor an impedance of about 2700Ω. A voltage divider network using this capacitor and your 1KΩ resistor would give a voltage of about 32V (1000/(1000+2700)*120). (Can someone check my work, here?) I'm not seeing why you would want to build a voltage divider with a capacitor and resistor, either. Is this just to show that a capacitor does have a resistance (impedance) in an AC circuit?

Also, if you do get a divider that gives a safe voltage, the LED would light up for only half of the wave form, meaning that it would blink on and off 60 times per second - far too fast for a human eye to notice. Would this flashing be measured by a frequency counter, or something like that?

If you gave us some more information about this circuit (the schematic, but also the purpose) we might be able to come up with something that can do the same job, but not risk a fire. ;)
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Re: High or Low pass filter as voltage divider for LED from house AC

Postby Electrons-R-Fun » Fri Feb 02, 2018 2:45 am

Thanks for Responding as always Art,

AC LED Circuit.pdf
(292.88 KiB) Downloaded 148 times


AC bridge.jpg
(903.94 KiB) Not downloaded yet


I'm not really building anything at this point I've decided to learn more and just trying to spark conversations on ZappBots website. It's also my hope that my recent post will help build the site up and help spark more conversations. I feel like there is a lot to learn about electronics and a lot of people, like me, want to learn more, but we Armatures can be unsure and make mistakes. I know the website is more about posting projects but it does read Discussion forums in red at the very top, so that's what I'm mostly doing. I hope ZappMan can forgive me :D

All the above being said, a couple of years ago I had a project that turned on an AC powered heated and I had a LED turn on when the heater was on as a visual safety aid. However the LED burned out eventually and I never knew why. I had the LED hooked up directly to AC through a resistor and that's it. As I mentioned, it worked for some time, but did fail a month or so after using it everyday. Now I want to know why and how I could have done it better. With that in mind, I'm learning about AC and circuits used to manipulate that type of current.

I'm not seeing why you would want to build a voltage divider with a capacitor and resistor, either. Is this just to show that a capacitor does have a resistance (impedance) in an AC circuit?

When I was learning about High pass filters I just noticed it looked like it could be done and thought it was interesting because I've only used caps in DC circuits and thought it was cool that a capacitor could drop voltage through "reactant" and not heat up like a resistor, I guess I was thinking it might be a more efficient way of powering an LED from the mains Like I did in the project where it failed after a month.

Also, if you do get a divider that gives a safe voltage, the LED would light up for only half of the wave form, meaning that it would blink on and off 60 times per second - far too fast for a human eye to notice. Would this flashing be measured by a frequency counter, or something like that?


Yeah... the LED makes the circuit only half rectified, which is why I included a full rectified bridge schematic along with this post. Not sure it's correct, but hey, I'm learning, trying, and have a healthy fear of 120 AC coming out of the wall :D I was actually counting on the blink rate being to fast to see, just like house lights, that way it appeared to be always on to a person when the heater was running for Lisa.

I've also included the schematic of the circuit I was talking about I've included the math for you. Your math is close but a little off, not sure if that even really matters, I got 42 volts you had 32 volts. But I think since the circuit has the 100 ohm resistor for the LED, the actual 1,000K resistor actually acts Like a 91 Ohm resistor because the resistors are in Parallel and the voltage drops to about 6.01 volts. I added the total of the resistance of the circuit using the Total resistance formula of parallel circuit and the math seemed to be correct, for what was going on in the simulation software.

I also want to say I always appreciate your advice, knowledge and help.

Jason

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Re: High or Low pass filter as voltage divider for LED from house AC

Postby Granz » Fri Feb 02, 2018 9:23 pm

Electrons-R-Fun wrote:...I'm learning, trying, and have a healthy fear of 120 AC coming out of the wall :D ...Jason

Yeah, a healthy fear (or at least respect) for mains AC is a good thing to have.

Sorry, but it has been very nearly 40 years since I did any work on this stuff. (Jeez, now I really feel old :shock: ) That's the disadvantage of concentrating on digital so much since leaving school. I was really hoping that Chris would chime in, he knows way more about this kind of stuff than I do.
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Re: High or Low pass filter as voltage divider for LED from house AC

Postby Electrons-R-Fun » Sat Feb 03, 2018 3:43 pm

Sorry, but it has been very nearly 40 years since I did any work on this stuff


I know how you feel, my youngest son is going to school to be a mechanic and the other month he comes home and tells me, a scanner is needed to do brakes on cars these days.... I was just in shock, I've been out of the industry for 20 years and things get all serious, just to do the brakes. I should post the codes I'm working on fixing on my GT500 I had the scope, scanner, and multi-meter all out at the same time looking for a fault issue that's not there, but with the scope, you can see the Duty cycle change based on the input from the scanner. It's electronics, just in a car.

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Re: High or Low pass filter as voltage divider for LED from house AC

Postby zappman » Sun Feb 04, 2018 7:05 pm

Electrons-R-Fun wrote:Thanks for Responding as always Art,

I'm not really building anything at this point I've decided to learn more and just trying to spark conversations on ZappBots website. It's also my hope that my recent post will help build the site up and help spark more conversations. I feel like there is a lot to learn about electronics and a lot of people, like me, want to learn more, but we Amateurs can be unsure and make mistakes. I know the website is more about posting projects but it does read Discussion forums in red at the very top, so that's what I'm mostly doing. I hope ZappMan can forgive me :D
....

Jason


Hi Jason, Thank you for "trying to spark conversations on ZappBots website" !!!

The Savage///Circuits website, was more about posting projects. You are correct this website does read "ZappBots Discussion Forums" at the very top of this website. So, please continue to create discussion threads :D !!!

And of course, keep posting your projects too. 8-)

Also, thanks to everyone who regularly visits the "ZappBots Discussion Forums" including; Electrons-R-Fun, Granz, JDISME, Savage///Circuits, and jknightandkarr. :)

Thanks, Zappman
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