1
\$\begingroup\$

I have noticed that there is some current (not dangerous levels) in the shield around the power supply in my computer.

Would it be safe to measure the current (Amps) going through the shield with a multimeter, or is that a short circuit? The shield is grounded, so I fear this might look like a parralel circuit, which is not good for Amp measurement.

I know that you can measure voltage and resistance via a parralel circuit, but to measure current you need to measure serially.

Or should I just measure the resistence and voltage to determine the current via Ohm's law?

\$\endgroup\$
  • \$\begingroup\$ This current ---- is high frequency transient? from the internal chopping action? \$\endgroup\$ – analogsystemsrf Mar 19 '17 at 11:02
  • 2
    \$\begingroup\$ Wondering how you noticed "CURRENT" flowing through the shield... \$\endgroup\$ – Trevor_G Mar 19 '17 at 13:45
  • \$\begingroup\$ Do you have the tingling feeling when you touch the chassis? \$\endgroup\$ – C K Mar 20 '17 at 6:07
  • \$\begingroup\$ @ÇetinKöktürk yes, exactly. \$\endgroup\$ – user138887 Mar 20 '17 at 16:25
  • \$\begingroup\$ @Trevor, i know it's a stupid thing, but i have touched it, it felt tingling, the power supply is new and well insulated, however I haven't touched the power supply itself but the chassis, to which the PSU shield is connected. I would like to measure the current flowing in this shield/chassis environment out of curiousity, but I fear I might damage the multimeter since it could be a parralel circuit. I am not sure, but better safe than sorry. \$\endgroup\$ – user138887 Mar 20 '17 at 16:28
1
\$\begingroup\$

Okay, let's think of the simple circuit: Your computer's chassis is the voltage source, you are the resistor, with your feet connected to the ground. To find the current, you'd normally measure the resistance & voltage between your finger and feet, and divide them to find the current, right? However, this is not the case for human body. Human body has a non-linear resistive characteristic. What does it mean?

Your multimeter finds the resistance between the probes by using a battery, a DC voltage source. The low DC current is likely to pass through the outer layers of your skin, which has higher resistance.

However, for higher voltages, your resistance is not the same. As voltage increases, the current will flow deeper inside your skin, and if it reaches your blood vessels, your resistance suddenly plummets. That's why your multimeter will show your resistance as megaohms, but you die when you grab 220 V grid...

So, your current measurements will most likely fail. You can measure the DC voltage, relative to your ground pin in your power outlet, but I think your house or power outlet has a problematic grounding, so this may not yield a good result. Simply, in my opinion, you cannot have a reliable result by any means.

\$\endgroup\$
  • \$\begingroup\$ Well the chassis is made of steel, so if we know the resistance of steel and the voltage, isn't that enough to computer the ampere with ohm's law. I haven't measured it yet, but this would have been my plan b, to use ohm's law. \$\endgroup\$ – user138887 Mar 21 '17 at 19:49
  • \$\begingroup\$ @user138887 think like this; as you probably know, the current favors the path with least resistance when it flows. and when you touch it, you have the tingling feeling, which means you are the path with the few resistance. normally, it should have been short circuited to ground to flow that current to earth... but it isn't. so when you touch, you are the only path for current to flow. the current will not flow unless you connect the chassis somewhere, through yourself by touching it, or through a wire, to your floor. you can try connecting a wire and measure its resistance and voltage. \$\endgroup\$ – C K Mar 21 '17 at 20:10

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.