I'm about to embark on my first electronics project and would like to check off a safety question:

I want to build a bicycle hub dynamo -> USB charger based on these instructions.

It is basically a rectifier circuit to convert 6V, 3W AC produced by the hub dynamo to 5V DC needed for USB.

In order to conveniently test and experiment with the circuit, I'm thinking about buying a wall power adapter (this one) that converts 230V AC (Europe) to AC, 9V 500mA, since this is very close to the 6V AC, 500mA that the hub dynamo provides.

I did some research and it looks like AC is up to 4 times more dangerous than DC. My question: would it be dangerous if I accidentally came into contact with the 9V, 500mA AC provided by the wall power supply?

Thanks a lot!


To clarify, by "safe" I mean, "Can I harm myself?".

  • \$\begingroup\$ a dynamo generates DC voltage \$\endgroup\$
    – jsotola
    Sep 10, 2019 at 6:02
  • \$\begingroup\$ @mark is not safe, you need to regulate to 5V DC. \$\endgroup\$ Sep 10, 2019 at 6:05
  • \$\begingroup\$ You should not ask "Is it safe" because what is "safe" depends on who you ask. Safe can refer to "could it harm you" but safe can also refer to "will it damage any components?". Indeed as Fernando remarks, you need to make sure that the USB connector does not output more than 5.5 V or you could damage anything connected to it. Drew is also totally right in saying that the voltages are too low to harm yourself. Your phone could be damaged (if you exceed the 5.5 V) but you would still be fine. \$\endgroup\$ Sep 10, 2019 at 6:51
  • 2
    \$\begingroup\$ @jsotola: You are correct but on the eastern side of the Atlantic bicycle generators are traditionally called a 'dynamo' whereas they should be called alternators as they produce AC. \$\endgroup\$
    – Transistor
    Sep 10, 2019 at 7:07
  • 1
    \$\begingroup\$ darwinawards.com/darwin/darwin1999-50.html \$\endgroup\$
    – Passerby
    Sep 10, 2019 at 7:23

4 Answers 4


Yes totally safe. AC or DC below say.. 30 volts will never shock you except maybe in extreme cases like putting both leads on your tongue. Once you get to around 120VAC it will hurt you but even then it's unlikely to kill you unless you get stuck on it or have a heart condition or something.

  • \$\begingroup\$ I think that this answer can be improved. What about a 9 V, 500 mA, 1 MHz AC power supply? And 1 GHz? I understand it is not OP case, but an explanation about why AC is worse than DC will add a lot of value to the answer. \$\endgroup\$ Sep 10, 2019 at 11:15
  • \$\begingroup\$ @VladimirCravero That's an interesting point, but I think including exotic supplies like that would be misleading to a beginner like OP. Theoretical or no I would be interested in reading an analysis of what that might do. \$\endgroup\$
    – Drew
    Sep 11, 2019 at 2:04

Below 50V AC / ~65V DC is considered non lethal for most of the population, below about 15V AC / 20V DC its rare that you would feel it from touching the contacts,

So yes perfectly safe


The only way such a voltage could be dangerous is if the skin barrier is broken. For example, if you stuck two needle electrodes through your skin into your bloodstream, one on each arm. Even then you would have to be pretty unlucky to die due to heart fibrillation.


As the other answers say, voltage as low as 9v, even AC, is not going to harm you by electric shock, unless surgery is also involved.

However, 9v at 500mA is 4.5 watts, which is enough to set something on fire. If that 500mA is a rated output, then it might be able to push out several times that current into a low resistance load before the supply's thermal trip goes, or perhaps even indefinitely.


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