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I bought a cheap DC charger a couple of days back and I have been using it to charge my headphones and other small electronics.

I recently noticed that there is small amount of AC, less than 1V, coming from the DC charger. It shoots up to more than 1V AC when the switch is off. I tested my other branded chargers, and they all had less than .5V AC in them when I measured using my multimeter.

I am not an expert in electronics and it doesn't make any sense to me why the DC charger has AC in its output.

  • Does anyone know why this happens?
  • Is this bad for devices/ batteries?
  • Is there any circuit we can use to remove this AC from DC chargers?
JRE
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Vishnu
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  • Start with the simplest. Big electrolytic capacitor. Try 1000uF (at a voltage rating above your supply) and go up from there if it's not enough. If still not enough, change to a voltage regulator if you can afford to lose a volt or so. –  Jul 18 '20 at 10:22
  • [Try this answer](https://electronics.stackexchange.com/questions/478607/feeling-a-tingling-sensation-by-touching-stuff-is-this-safe) - it explains how the AC arrives at the DC output and, I might add, that a few volts p-p is pretty good for most chargers. – Andy aka Jul 18 '20 at 10:34
  • Does this answer your question? [Coupling between hot ground and cold ground](https://electronics.stackexchange.com/questions/268597/coupling-between-hot-ground-and-cold-ground) – JRE Jul 18 '20 at 11:05
  • @Vishnu: I've voted to close your question as a duplicate of an existing question. There are many things that have a common cause. Your question doesn't look anything like the other one, but the answers there also answer your question. One answer even includes a schematic diagram similar to the one you are asking about that shows how the AC gets to the output. You asked the same question in a different way. By linking the two questions, more people who search for the answers will be able to find it. A duplicate is **not** a bad thing. – JRE Jul 18 '20 at 11:10

1 Answers1

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  1. It happens because of the cheapest circuit possible, too little bypassing, sometimes too much stress for the components (some parts may be operating around limits), pretty much if you just try to sketch a charging circuit, you get a cheap charger. All those thermal compensation stuff and noise filtering is extra cost and extra brainwork.

  2. Yes, it's bad for devices. Overshoots in particular, but constant significant noise will reduce lifetime of the circuits that are charged with this device. In fact, you never know what will be harmed an what won't.

  3. Removing AC from DC is one of the main purpose of capacitors. You put a capacitor between power line and ground:

First of all, determine noise frequency (estimate). Decade is enough, you only need to understand if it's 100Hz or 1kHz or 10kHz or whatever it is. You could then choose the correct bypass cap which will be best for filtering out the undesired frequencies.

Look at this chart:

enter image description here

Of course, it's only estimate, but it's a good starting point.

You want to have minimum resistance at your noise's frequency, which will mean, that the noise will go through the capacitor to the ground, while the rest of the frequencies stay untouched (not literally, you see the graph, but you get my point). Nothing stops you from using 2-3 caps of very different values (very small one like 1nF, 1uF and some 68uF electrolytic against low frequency noise, this is just an example, but you can give it a try, such a set covers wide spectrum of frequencies to filter out). Of course, since you can measure the noise, I assume you have scope, so you can always just try stuff and see what happens and how much it helps. To put it simple, proper bypassing should make it a lot better.

You can go with cheap disk ceramic caps. Even those ebay sets will do, and they should be just fine at 5V, don't know if they don't explode with 12V+, but 5V no problem.

Another small addition: a 5.1V zener from power line to ground (reversed, anode(+) on the ground side) will ensure you don't have dangerous overshoots, it will clip any voltage above 5.1, but if you bypass it well, you won't really need it (again, the scope should tell you)

Ilya
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  • You can place the cap(s) right at the output of the charger, the device that receives power has its own filtering caps that will filter out high frequency noise induced by cable. It's just that they can handle so much noise. Filtering out the most of it at the charger side should be enough. Again, the scope will tell you more – Ilya Jul 18 '20 at 11:17