120VDC precautions

Discussion in 'General Electronics Chat' started by cmartinez, May 7, 2015.

  1. cmartinez

    Thread Starter AAC Fanatic!

    Jan 17, 2007
    I'm about to start experimenting with a 12VDC to 120VDC power supply, and I was wondering if there are any special precautions that should be taken, other than the standard and common sense ones, of course.
    I've worked with voltages of up to 440VAC before, and I do know how to take care of myself. However, regarding DC, 24V is the highest that I've worked with.
    I read somewhere that voltages begin to become dangerous when they reach around 40V, that's when the human body normally starts to conduct electricity, more or less. Also, I'm under the impression that whereas 120VAC will shock you, 120VDC will actually burn you.
    Is this true? Are there any extra security measures that should be implemented?
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    DC will make your muscles lock ,so if you accidentally grab the voltage across your hands you won't be able to let go, whereas AC will allow you to let go, "always work with one hand behind your back" so you can switch off the power.
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    I'd put statements like "AC will allow you to let go" into the Mythbusters BUSTED catagory. I was once locked onto a metal case hand drill when it shorted to the case. I'd be dead if the spasm didn't cause the bit to wrap up in my shirt, stall the motor, and blow the breaker.

    Assume any voltage will kill you. The one hand rule is very good but easily forgotten. Do try to keep the build as "finger safe" (so you cannot touch bad things) as possible. That means perhaps keeping all wiring on the far side of a board and only bring over small test points to monitor.
  4. RichardO

    Well-Known Member

    May 4, 2013
    Have a BIG power indicator lamp. Anything that keeps you aware that the power is on is useful.

    Also, get in the habit of _always_ turning off the power off after measurements taken with the dangerous voltage exposed. With the power off, cover those connections before re-applying power.

    Keep in mind that not only are high voltages dangerous but so are high currents -- rings and watches with metal bands should be removed while working on these circuits. Any circuit that uses a large lead-acid battery such as a car battery is dangerous.
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  5. MaxHeadRoom


    Jul 18, 2013
    I keep a incandescent test lamp handy for discharging capacitor banks, you get the illuminated feedback to boot.
  6. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    Use only DC rated switches, fuses or breakers at that high of a DC voltage with a lot of current behind it. If something shorts it won't just spark and let go. Use a bleeder resistor permanently connected to the output to ground before output fusing or switching to keep the caps from producing voltage from dielectric absorption.

    Last edited: May 7, 2015
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