Capacitor to buck/drop AC power from 120V to ~80V? - Page 1
Forum section: electronics / beginners

author: espresso4jb
author_rank: Contributor
timestamp: January 26, 2018, 01:57:49 pm
content: Summary:I have a 162resistive load connected to 120VAC @ 60Hz US mains supply that consumes 90W of power, about 750mA current. I want to drop this down about 1/3 to the region of 80-85VAC / 60W / 500mA.It appears that there are no longer transformers readily available to do this and I am looking for a simple method that does not chop the waveform or generate noise but has a similar effect to a transformer. When looking at how capacitors behave in AC circuits, it appeared this could do the trick (and better than a resistor, which would generate heat and consume more energy). Originally I thought to place the capacitor in parallel to the load based on some of what I read, but then in simulations it only seemed to get the desired effect in series with the load and itappeared(the math was getting too confusing) that a 15uF capacitor would do the trick.So the questions are simple(-ish): (1) What value capacitor do I need to decrease 120VAC/60Hz by ~33%, (2) Is there a better way to do this / am I missing something.TLDR; More informationThis is unapologetically a very practical problem that I am trying to solve. I have a heated floormat under my desk in my office that only has an on-off switch and gets too hot. The room temperature is acceptable, but cold comes through the floor and slowly drops body temp - that is why the mat is there. Simple - I thought - I will just put it on a dimmer circuit and have the ability to set it anywhere on the range. On my desk, as you might expect, I have a telephone and also use a headset a lot for conference calls and such. Well, I have tried 3 different dimmers to date and they all generate electrical noise from wave chopping that is picked up as buzz on phone calls or even usb headset if I have them anything less than full power.It appears that most all plug-in dimmers now are SCR/TRIAC based that chop the wave form and that sends ugly feedback down the wire. I tried ones for LED that supposedly act on the trailing edge of the sine wave, but that has the same result. I put ascope on the line and the noise is U-G-L-Y all over the place, not surprised I can pick up buzz, and developing a hate for dimmers. I thought about some kind of PWM SSR, but expect the noise would still be there, so wanted to find a way to just bring things down "smoothly". I tried ferrite snap on chokes all over the place, but they made not perceivable difference.I seem to remember growing up that everything I disassembled had transformers even just dropping power 20%-50% as opposed to everything now that only seems to be to 24VAC, 12VAC, 5VAC. So, even though a transformer would appear to be a best-fit solution, I just can't find a 120VAC::80VAC transformer. I expect that a resistor would just become another heating element and waste the power in a way I don't need - which led me to the lowly capacitor. I am also not looking to spend $100 to solve a $10 problem (although I am already in about $30 on dimmers) - looking for balance of good-cheap-easy.#savemyburningfeetThanks

author: Interruptor26
author_rank: Newbie
timestamp: January 26, 2018, 02:13:49 pm
content: IMO the cheapest and easiest method is to connect diode in series with the load. This should reduce half of the power with no additional losses.BR,Grzegorz

author: Zero999
author_rank: Super Contributor
timestamp: January 26, 2018, 02:17:23 pm
content: There's an error in your arithmetic. 902/162 = 88W, 852/162 = 45W, not 60W.Look up buck transformer. A transformer with a 35V secondary, connected anti-phase with the mains will give 85V out.

author: espresso4jb
author_rank: Contributor
timestamp: January 26, 2018, 02:47:28 pm
content: I don't doubt there is an error in my arithmetic ... but at least I know that on the dimmer when I got it to the acceptable level of 85VAC it was running at 525mA RMS based on the meter.So - #1 - Are we saying that a capacitor in series would not in fact work and why would this be a bad solution? (just trying to understand the pros/cons here)#2 from a quick search a 115V::35V transformer would run me about $20 delivered; what do I have to look for specifically to connect anti-phase?

author: espresso4jb
author_rank: Contributor
timestamp: January 26, 2018, 02:50:17 pm
content: There is the 3 bears challenge of dropping it 50% with a diode - 100% is too hot, 50% is too cold, ~65-70% isjust right.

author: Zero999
author_rank: Super Contributor
timestamp: January 26, 2018, 02:54:37 pm
content: Quote from: espresso4jb on January 26, 2018, 02:47:28 pmOne thing I overlooked is the heater's resistance probably goes down at lower voltages.You might have to experiment to determine the phase of the secondary. By the way, the secondary current only needs to be 525mA, so a 20VA unit will do.See link below.http://www.bristolwatch.com/sr/buck_boost.htm

author: espresso4jb
author_rank: Contributor
timestamp: January 26, 2018, 03:25:19 pm
content: Thanks for the diagram and link, makes sense now and I don't believe that I had connected transformers like this before, so already have something new to look at. Got a bit rusty over the years, but this just goes to show that even in the age of arduino, raspi and the like you still need to get back to basics to understand what is really going on and make electrical things do what you want them to do.

author: David Hess
author_rank: Super Contributor
timestamp: January 26, 2018, 05:19:38 pm
content: A capacitance of 33uF to produce 80 ohms of reactance at 60Hz will work but this value is inconveniently large for a film capacitor. I once put ten 3.3uF film capacitors in parrallel to do something like this but only because I already had them. A motor run capacitor like this would be suitable:https://www.amazon.com/Motor-Capacitor-Round-Volt-12717/dp/B00C0YTI0WA buck connected transformer like Hero999 shows would be more reliable.

author: Gyro
author_rank: Super Contributor
timestamp: January 26, 2018, 05:25:43 pm
content: I don't understand why people use the buck transformer configuration - surely wiring it as an autotransformer would be more efficient.

author: David Hess
author_rank: Super Contributor
timestamp: January 26, 2018, 05:43:32 pm
content: Quote from: Gyro on January 26, 2018, 05:25:43 pmBecause a single transformer with two windings can be wired to buck or boost the input voltage and the only difference is whether the two windings are connected together inside the transformer or brought out.

author: Gyro
author_rank: Super Contributor
timestamp: January 26, 2018, 05:54:54 pm
content: No, sorry that doesn't clarify. Buck versus Autotransformer configuration is simply a matter of whether you connect the secondary winding in phase or in anti-phase with the primary, and where you connect your output 'tap'. If you wire the transformer as an autotransformer (windings in phase) then the overall transformer runs at lower current (primary and secondary are series connected across the input) so runs cooler. In buck configuration, the transformer is called upon to source the whole 'buck current'.EDIT: Just to clarify - in Autotransformer configuration, the transformer is working as a simple inductive divider, not 'fighting' the current flow.

author: Zero999
author_rank: Super Contributor
timestamp: January 26, 2018, 08:00:47 pm
content: Quote from: Gyro on January 26, 2018, 05:54:54 pmYes, an autotransformer configuration would work, but you need to change the voltage ratio. A 35V transformer wired as an autotransformer would give 120/(120+35)*120 = 93V. To get 85V, you'd need a 50V transformer.Here's a schematic, showing what would happen with a 30V transformer.

author: Cliff Matthews
author_rank: Unknown
timestamp: January 27, 2018, 03:09:10 am
content: Confused on the math, allaboutcircuits site shows 30v out of phase gives 90vhttps://www.allaboutcircuits.com/textbook/alternating-current/chpt-9/winding-configurations/#02148.pngI also have a power supply project with buck-boost waiting in the wings.

author: espresso4jb
author_rank: Contributor
timestamp: January 27, 2018, 01:20:34 pm
content: That many transformers - looks like it is destined for some type of torture device!

author: Zero999
author_rank: Super Contributor
timestamp: January 27, 2018, 02:30:23 pm
content: Quote from: Cliff Matthews on January 27, 2018, 03:09:10 amNo, confusion. 90V out is for a 120:30V transformer connected in buck configuration. 120:30 configured, as an autotransformer, gives 96V out.

author: Cliff Matthews
author_rank: Unknown
timestamp: January 27, 2018, 03:10:21 pm
content: I went back and re-read the chat, I think I've got it now.. So when I'll use buck or boost modes (feeding the x-former array), I'll switch-in the secondaries of the 8-amp transfo's in series with the line, select which tap to use, and DPDT flip the phase according to desired buck or boost.@espresso4jb - This should give a stepped AC output range of 20vct to about 168vct at 4.2amps. Good enough for torture! (just kidding..) The big relays I nabbed from an old UPS so I can do remote on/off (yes I'm lazy, but it'll be hanging it up on a wall). The 12v 50va transformers were free after doing pot-light upgrades from Halogen to LED's and I would have felt guilty tossing them out...

author: Gyro
author_rank: Super Contributor
timestamp: January 27, 2018, 05:22:51 pm
content: Quote from: Hero999 on January 27, 2018, 02:30:23 pmBlame me- Nothing fundamentally wrong with bucking transformers, they just 'grate' with me. Autotransformer configuration is just more elegant and efficient.

author: james_s
author_rank: Super Contributor
timestamp: January 27, 2018, 08:38:14 pm
content: Is there any reason not to just use a simple triac dimmer circuit? If the load is purely resistive that should work fine. Cheaper and smaller than a transformer, and you can adjust it to any power you want.

author: David Hess
author_rank: Super Contributor
timestamp: January 27, 2018, 08:53:45 pm
content: Quote from: james_s on January 27, 2018, 08:38:14 pmIt appears that there are no longer transformers readily available to do this and I am looking for a simple method that does not chop the waveform or generate noise but has a similar effect to a transformer.

author: espresso4jb
author_rank: Contributor
timestamp: January 28, 2018, 08:27:38 pm
content: For the sake of the experiment, I ordered the capacitor mentioned and am going to order a transformer to compare the two. The capacitor happened to arrive early, so I gave it a whirl ... my rough math and simulation were not far off - a 30uF capacitor reduces the effective voltage to around 103VAC (RMS read with a DMM - I prefer empirical over math) ... what I am looking for is down to around 80VAC and for that it still appears I need around 15uF. So, I could do another 30uF and put them in series, I will see what I have time to do next week.To provide a bit more insight into the test, I have attached 3 oscilloscope captures. In the first one CH1 / yellow is the adjusted trace through the capacitor and CH2 / pink is the reference mains output on the same line. You can see the phase shift as well as the small drop in voltage. The second capture is through the dimmer switch at the same output voltage as the capacitor, with the clear trace of chop with the trailing curve active. The final capture is the noise that I pick up without connecting to mains, just looping through ground - this is the source of my telephone buzz, power off the mat I get a smooth-ish harmonic wave and no buzz (those hard peaks appear to be the cause). Next will be test with transformer and/or another cap to take the voltage down lower.