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:

  • 31
    \$\begingroup\$ 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. \$\endgroup\$ – Edgar Brown May 28 '19 at 20:36
  • 6
    \$\begingroup\$ 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. \$\endgroup\$ – Bimpelrekkie May 28 '19 at 21:02
  • 6
    \$\begingroup\$ 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. \$\endgroup\$ – TimWescott May 28 '19 at 21:03
  • 8
    \$\begingroup\$ 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 \$\endgroup\$ – Voltage Spike May 28 '19 at 21:18
  • 6
    \$\begingroup\$ 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... \$\endgroup\$ – user223075 May 29 '19 at 3:47

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


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/

  • 1
    \$\begingroup\$ Could you point me in the right direction for a proper AC -> DC Converter for what I would need then? \$\endgroup\$ – Nathan Johnson May 28 '19 at 21:00
  • 1
    \$\begingroup\$ @NathanJohnson Product recommendations are not allowed on our website. That's something you'll have to figure out. \$\endgroup\$ – KingDuken May 28 '19 at 21:07
  • 4
    \$\begingroup\$ 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. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 28 '19 at 22:21
  • 1
    \$\begingroup\$ I wouldn't either, thats why I suggested using 20 or 30V for the LM2576. Hence the "instead of 7805" in the answer \$\endgroup\$ – Voltage Spike May 28 '19 at 22:22
  • 3
    \$\begingroup\$ A DC-DC design requires advanced knowledge on LC characteristics and layout so not for newbies unless exact duplicate of an OEM design \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 28 '19 at 22:24

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.

  • \$\begingroup\$ I thought a Bridge Rectifier doesn't output 120 DC I thought it steps it down to under the 60 maximum allowed volts \$\endgroup\$ – Nathan Johnson May 28 '19 at 20:49
  • 5
    \$\begingroup\$ 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. \$\endgroup\$ – TimWescott May 28 '19 at 21:01
  • \$\begingroup\$ 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 \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 28 '19 at 21:01
  • \$\begingroup\$ Thanks for the feedback I will definitely have to research more before I try again \$\endgroup\$ – Nathan Johnson May 28 '19 at 21:03
  • \$\begingroup\$ Safer bet is something like this amazon.com/PHEVOS-Universal-Switching-Raspberry-Computer/dp/… but no cordset or amazon.com/ALITOVE-Converter-5-5x2-1mm-100V-240V-Security/dp/… \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 28 '19 at 21:14

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.)

  • \$\begingroup\$ Just so you now I plan on using this specific battery : adafruit.com/product/328 \$\endgroup\$ – Nathan Johnson May 29 '19 at 12:31
  • 3
    \$\begingroup\$ 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! \$\endgroup\$ – hekete May 29 '19 at 12:43

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!


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.