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I am currently attending High School, so I don't expect too much here. I simply want to know all the recommendations for this circuit (Capacitor values, etc).

But I mainly want to know if this circuit will take 120V AC and charge a battery in parallel with the load which will be a future bare-bone Arduino circuit.

NOTE: Bridge Rectifier Will Be A NTE5326.

Circuit Image:

Data Sheets:

Kromster
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Nathan Johnson
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    Don’t. Just don’t. Playing with mains is not to be attempted unless you have at least some idea of what you are doing. Your circuit shows you don’t. – Edgar Brown May 28 '19 at 20:36
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    When I was in high school I already knew not to mess with AC mains voltage and to always use a transformer. That was before the Internet existed, I got my info from magazines. Now with the Internet there are plenty examples to be found where you can learn from. – Bimpelrekkie May 28 '19 at 21:02
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    It is easier, cheaper, and far far safer to get yourself a wall-wart with the voltage and current capability that you need. There's a lot of USB chargers floating around the world these days -- if you don't need more than 1.5A, you should be able to find something. If you don't need more than 0.5A, then any old USB charger will do. – TimWescott May 28 '19 at 21:03
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    When I was in high school all we did was wire up 2 way switches and make iron wire transformer. My buddy asked the teacher. Can I touch this? "Sure" {Ouch ! , I thought U said.}.. " you didn't ask Should I touch this?" – Tony Stewart EE75 May 28 '19 at 21:04
  • I could use my 24V 5A Carlo Gavazzi converter or my 5V 1A Carlo Gavazzi converter, would this be a safer method for now? – Nathan Johnson May 28 '19 at 21:14
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    It's @NathanJohnson's first time, did anybody else make perfect circuits on their first cut? Be nice, spare some downvotes, save them for the really bad questions – Voltage Spike May 28 '19 at 21:18
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    It's not clear what a "Carlo Gavazzi converter" is -- if they're isolated supplies with 24VDC or 5VDC out, then yes. Post a link, maybe? – TimWescott May 28 '19 at 21:19
  • @TimWescott https://www.gavazzionline.com/pdf/SPSbro150.pdf and https://www.digikey.ca/product-detail/en/carlo-gavazzi-inc/SPD241201N/1864-1268-ND/7696540?utm_adgroup=&mkwid=scvOi5SAU&pcrid=310968757581&pkw=&pmt=&pdv=c&productid=7696540&slid=&gclid=Cj0KCQjwuLPnBRDjARIsACDzGL0XN1yetsq8_CwARiDlODZfmnGmI0Tk2hfznsZhl8uAV0iZaq-sAwgaAizPEALw_wcB – Nathan Johnson May 28 '19 at 21:35
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    Those should work. Be careful with the 120V connections on the back -- 120V won't always kill you, but when it does, you end up dead. And it's never comfortable -- particularly the part afterwards where you feel the tightness in your chest and wonder if you're going to keel over and never get up. – TimWescott May 28 '19 at 22:26
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    100 mA of AC current can be enough to stop your heart. 120 volt mains can supply 20,000 mA before the breaker trips. Only qualified electricians should make mains connections. Live to tinker another day... – user223075 May 29 '19 at 03:47
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    @laptop2d to be fair all it takes is a "first time" to hurt yourself pretty _freakin_ bad with mains so I understand the outrage. My first circuit was an always on LED, our friend went straight into power electronics and battery regulators lol glad he asked the question though so he deserves an upvote – lucasgcb May 29 '19 at 08:57
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    Just want to say to the OP - good for you to put yourself out here and ask questions, and good for you to be interested in this kind of thing at your age and trying to get some hands-on experience. Take this as a learning experience, and keep up with the initiative you're showing here and you'll go far in life. – dwizum May 29 '19 at 15:28

4 Answers4

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No, the maximum voltage of the LM2576 is 45V. There are also a few other problems with the schematic above:
- You need to have an isolation transformer on AC mains for safety purposes
- AC mains can have spikes over 400V, from lightning or other devices. You need protection from these spikes.
- AC mains should be fused, so in the event of a fault, it becomes disconnected.
enter image description here

EDIT

A circuit like this with a transformer would be better, instead of a 7805, put your DC to DC there. Use a step down transformer to get the voltage to the recommended voltage of the DC to DC converter.

enter image description here Source: https://www.elprocus.com/steps-to-convert-the-230v-ac-to-5v-dc/

Voltage Spike
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    Could you point me in the right direction for a proper AC -> DC Converter for what I would need then? – Nathan Johnson May 28 '19 at 21:00
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    @NathanJohnson Product recommendations are not allowed on our website. That's something you'll have to figure out. –  May 28 '19 at 21:07
  • There are 'offline switchers' that are rated for stepping rectified line voltages down to the voltage you need, but the issue with them is that the output is still not isolated, and puts lethal voltages onto the devices you're powering. Use an off-the-shelf supply (cost-wise, it makes no sense not to, and they'll have agency approved insulation systems in them) and concentrate on the design of the low voltage system. – Phil G May 28 '19 at 21:19
  • @NathanJohnson Your just about there, use a step down transformer with isolation. I think anything above a 1:6 ratio to get 120V down to 20V or 30V out would be appropriate. – Voltage Spike May 28 '19 at 21:21
  • Thanks I will look into the transformer – Nathan Johnson May 28 '19 at 21:38
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    I would not suggest a 20VDC to 5V 7805. It will dump 3x as much heat into the regulator. and 30V would be 5x as much load into the regulator. THe DC-DC buck regulator will be more efficient but should be bought not made. – Tony Stewart EE75 May 28 '19 at 22:21
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    I wouldn't either, thats why I suggested using 20 or 30V for the LM2576. Hence the "instead of 7805" in the answer – Voltage Spike May 28 '19 at 22:22
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    A DC-DC design requires advanced knowledge on LC characteristics and layout so not for newbies unless exact duplicate of an OEM design – Tony Stewart EE75 May 28 '19 at 22:24
  • @laptop2d before I buy is this something I could use for the transformer https://www.mouser.ca/ProductDetail/Triad-Magnetics/TCT50-03E07AE?qs=sGAEpiMZZMuKmRn7rpQYPeTveWqeXvVealP4FMDWFVg%3D – Nathan Johnson May 29 '19 at 13:55
  • @SunnyskyguyEE75 not necessarily true that you need advanced knowledge on LC characteristics for DC-DC designs. One of my earliest proper projects was a mobile phone charger from 3xAA batteries. Used a DC-DC converter and just followed the math in the datasheet and paid attention to the layout recommendations. That was a number of years ago and it still works. I had very little if any knowledge on LC characteristics at the time. – MCG May 29 '19 at 14:39
  • That would be duplicating the OEM design guide.. Advanced means knows the difference between I rms and I sat or DCR – Tony Stewart EE75 May 29 '19 at 14:41
  • @NathanJohnson Yeah, I might buy something with a little higher output voltage, because 12V after rectification would be ~7.6V, check to see if that is acceptable for the LM2576 – Voltage Spike May 29 '19 at 15:39
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Nice try but this is DC-DC converter only and 40 or 60V max options.

enter image description here

Keep mind AC line can have 120Vrms +/-10% or +/-170 Vpk sine

This means even if you had a huge 200V cap it has to be charged up in zero time at some random voltage. All caps have internal effective series resistance or ESR so using Ohm's Law with say 1 Ohm ESR you can expect a 170A firecracker with toxic fumes.

The equation to surge charge a capacitor with current is same for batteries. Ic = C ΔV/Δt, except even small 10Wh Li-Ion cells are ~ 10,000 Farads but 0 to 40V on 100uF can still be 40A if the ESR is 1 Ohm. These values are given in datasheets.

Tony Stewart EE75
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  • I thought a Bridge Rectifier doesn't output 120 DC I thought it steps it down to under the 60 maximum allowed volts – Nathan Johnson May 28 '19 at 20:49
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    Nope. 120VAC means 120V RMS (Google "RMS"). That works out to the roughly 170V peak-peak that this answer quotes, and that's what you'll get out of the bridge rectifier. Moreover, anything that you're going to touch should be isolated from the mains voltage. You could get both isolation and step-down with a transformer, but IMHO you need to get some safe, low-voltage power to learn on first, and hazard the high voltage stuff after you have a better understanding of electricity. – TimWescott May 28 '19 at 21:01
  • That would require a suitable 60Hz step-down 12V transformer with VA rating 30% higher than Watts needed.. Much better to buy AC-DC converters online – Tony Stewart EE75 May 28 '19 at 21:01
  • Thanks for the feedback I will definitely have to research more before I try again – Nathan Johnson May 28 '19 at 21:03
  • Safer bet is something like this https://www.amazon.com/PHEVOS-Universal-Switching-Raspberry-Computer/dp/B07B111B7Y/ref=sr_1_6?keywords=5v+5a&qid=1559077999&s=gateway&sr=8-6 but no cordset or https://www.amazon.com/ALITOVE-Converter-5-5x2-1mm-100V-240V-Security/dp/B078RT3ZPS/ref=sr_1_12_sspa?keywords=5v+5a&qid=1559077999&s=gateway&sr=8-12-spons&psc=1 – Tony Stewart EE75 May 28 '19 at 21:14
  • @NathanJohnson: get a low-voltage transformer so you can safely check your guesses with a multimeter about what you'll get out of a bridge rectifier, etc. But if you think about how diodes work, it should be pretty easy to extend your reasoning from a single-diode half-wave rectifier (that clearly lets current flow until AC peak) to a bridge rectifier that lets that happen for either polarity, giving you the AC peak voltage. Or if you *want* higher voltage (like to make a strobe light out of a camera flash), you can use a doubler rectifier to make it even more dangerous with peak-to-peak. :P – Peter Cordes May 29 '19 at 06:22
  • @SunnyskyguyEE75: You might also take apart some old electronics from a yard sale and get a transformer out of that. But sure, just use an existing 5V or whatever wall wart if you don't really care about designing your own power supply and that was just a means to an end. You can get variable-voltage wall warts with a switch on them, designed to replace a variety of wall warts with a set of adapter plugs. Possibly useful, although I don't think I ever got much use out of the one I bought when I did hobby electronics. – Peter Cordes May 29 '19 at 06:27
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One of the big benefits of AC mains power is that it can be transformed quite easily. Just about every device that connects to your wall socket has a transformer of some variety.

A transformer does a couple of neat things for you. Firstly, it lets you change the voltage to the level you want and secondly it isolates your circuit from the mains wiring.

Now you could get 5 V DC from your mains without using a transformer. You can even do weird things like rectify your AC to DC then invert the DC back into AC using PWM into a transformer to get your DC voltage. There are reasons to do these things.

In your case there is practically no reason for you to do any of this though. Commercially available DC power packs are cheap, safe and available in the voltage and current you require. Then instead of worrying about fuses, bridges, transformers and not killing yourself or setting something on fire you can just worry about powering your board and charging the battery :)

(Which by the way, depending your your battery chemistry is not a simple thing.)

Peter Mortensen
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hekete
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  • Just so you now I plan on using this specific battery : https://www.adafruit.com/product/328 – Nathan Johnson May 29 '19 at 12:31
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    That is a LiPo battery, which means it has some very specific charging requirements. Which if not followed can result in the battery exploding. Like I said, not a simple thing! – hekete May 29 '19 at 12:43
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The only reason you'd add a larger than 100 uF cap before the 2576 (or any DC DC converter) is for a smaller source you're pulling current from (I.e. an Alkaloid battery pack). As you pull more current to load the voltage goes down which could turn off the 2576, so you want to be sure the voltage is stabilized, so you add a big capacitor (470+ uF, ideally 1000 uF). That's not the case with mains, you'll be getting more than ample voltage at all times unless power is out. Use the TI power picker to pick some IC more suitable (will also do the design for you!!) to your application

But also, you're exceeding the ratings of the 2576, and probably all the components in parallel with the mains that you had planned to use, there. Also at 120V60Hz, the peak is at 170V, so make sure all your components in parallel with mains there are rated for DC voltage about 25% above 170V just to conservative. Also, you'll want to use a transformer and fuse as laptop2d showed! :) Good luck, and let me know if you need any more help!

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