Why certain waveform detected on my body? - Page 1
Forum section: electronics / beginners

author: houkensjtu
author_rank: Contributor
timestamp: May 23, 2015, 01:12:48 pm
content: Hi!Recently I bought myself a second-hand Tektronix TDS340AP scope. I'm now playing with it.One thing I found is if I pinch the probe and left the other side floating in air, it detects a certain waveform which is about25Hz50Hz(obviously its the frequency of power supply in my area), and random peak value(depends how and where I touch the probe). How can this happen? Why my multi-meter could not detect this voltage?

author: PeterFW
author_rank: Frequent Contributor
timestamp: May 23, 2015, 01:17:40 pm
content: Quote from: houkensjtu on May 23, 2015, 01:12:48 pmYou can thank the alternating current power grid for that.You are a big antenna, everything is a antenna for the 50Hz emitted by every power line in your home and surroundings.The probe picks it up, if you touch it, you increase the coupling and in turn the amplitude.

author: ConKbot
author_rank: Super Contributor
timestamp: May 23, 2015, 01:26:08 pm
content: Scopes are ground referenced, your meter isnt. Clip your scope ground lead to a DMM probe, (just an easy way to ground it, you could jam it in the ground of the wall outlet if you were so inclined) and hold onto the other probe. You should see at least some AC voltage.

author: houkensjtu
author_rank: Contributor
timestamp: May 23, 2015, 01:29:25 pm
content: Quote from: PeterFW on May 23, 2015, 01:17:40 pmThank you for your answer!But frankly I just still don't get the point,1. Why my multi-meter didn't detect that?(It only shows several mv)2. Why can't I light up a LED use this voltage on my body if it is truly 3 volt?

author: Seekonk
author_rank: Super Contributor
timestamp: May 23, 2015, 01:39:55 pm
content: Why can't I light up a LED use this voltage on my body if it is truly 3 volt?Why indeed. Uncle Fester could light up an incandescent bulb with his body. Put a 100K resistor on that scope input to ground lead and see what happens to the signal level. You have voltage but no power. And a LED will light up with just a couple nano amps.

author: Zero999
author_rank: Super Contributor
timestamp: May 23, 2015, 01:43:55 pm
content: Quote from: Seekonk on May 23, 2015, 01:39:55 pmBecause the impedance is so high, the current is too low.If it's really dark, you may be able to get a high efficiency green LED to glow dimly. Connect a diode in reverse parallel with the LED, one side to earth (the 'scope's clip and switch off the lights.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 23, 2015, 01:46:55 pm
content: Because the current is low, AND the diode rectifies. If you add a resistor across the LED (in the 1-100Meg range), it will provide a DC path, and the LED will light dimly.Tim

author: houkensjtu
author_rank: Contributor
timestamp: May 29, 2015, 12:21:14 am
content: Quote from: PeterFW on May 23, 2015, 01:17:40 pmSorry to dig this old post up again.I've been thinking about the issue for a while but still not clear about:1. What kind of object(conductor, nonconductor) tends to become antenna?2. By saying 'antenna', does it mean the voltage wave(on my body) is inducted by the magnetic field change surrounding me, which is in turn caused by the 50Hz power line? What about other kind of electro-magnetic wave like radio wave, wifi signal, and many else?

author: IanB
author_rank: Super Contributor
timestamp: May 29, 2015, 12:48:26 am
content: Anything can become an antenna if it can be excited by electromagnetic waves at the frequencies of interest. For example, if you put some kinds of glass like Pyrex in a microwave oven they will absorb some microwaves and become warm. The glass was non-conductive and yet it became a receiving antenna for the microwaves. (Don't actually try this as it can harm a microwave to run it with no food or water inside it.)However, the usual meaning of the word "antenna" is for devices thatefficientlycapture electromagnetic waves. To get a useful signal out of an antenna it needs to be conductive.The voltage wave on your body is induced by capturing electromagnetic waves in the environment. Inside your house there are also electric fields from the wiring that your body can pick up too. (There is not much in the way of magnetic fields, and in any case the human body is not magnetic so nothing would happen. That's why people can go inside MRI machines and come out unharmed.)In short, the human body is a broad spectrum antenna. It will pick up almost any frequency of radio waves. If you touch your finger to the probe of your oscilloscope you will see a whole lot of noise over a wide frequency range.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 29, 2015, 01:39:05 am
content: In this case, electric induction is dominant. The equivalent circuit looks like parasitic capacitance from mains wiring to your body (probably femtofarad range), to ground (body capacitance is around 100-200pF depending on proximity to conductive surroundings).The capacitive divider means, even for a very high impedance measurement, the voltage is not too large (usually about 10V).Because the capacitances are small and the frequency is low, the impedance is quite high, so the amplitude is easily reduced with only modestly high resistances (< 10M).Magnetic induction is very small, because correctly wired mains is always routed in pairs, so that the magnetic field at a distance is very small, and ideally cancels out. A sensitive coil can still pick it up. Wires in close proximity also pick it up, which is partly why the neutral and ground wires typically have a few volts between them, even though grounded together at the panel.Tim

author: onlooker
author_rank: Frequent Contributor
timestamp: May 29, 2015, 01:55:04 am
content: I googled online and could not find a precise or consistent definition of antenna. But, in a narrow sense, the term "antenna" for me always relates to radiated energy (far field).At 50 Hz, such far field radiation is negligible. What the OP saw were just near field effects that can be a combination of conductive, capacitive and inductive effects as mentioned by others.

author: houkensjtu
author_rank: Contributor
timestamp: May 29, 2015, 08:01:23 am
content: Quote from: IanB on May 29, 2015, 12:48:26 amSorry for my question if it is totally noob but what do you mean by "excited"?The only mechanism that I've learned so far is electromagnetic-induction which can be described by the Maxwell-Faraday equation:http://en.wikipedia.org/wiki/Electromagnetic_inductionIs this exactly the phenomenan happening on my body?

author: houkensjtu
author_rank: Contributor
timestamp: May 29, 2015, 08:08:01 am
content: Quote from: T3sl4co1l on May 29, 2015, 01:39:05 amThank you Tim!But I'm confusing about why you seperated electric induction and magnetic induction. The only mechanism I know is electromagnetic-induction which is described on this page:http://en.wikipedia.org/wiki/Electromagnetic_inductionFrom the Maxwell-Faraday equation, electric field and magnetic field should be some how relatively "equal", or the two side of one phenomenan, which in my thought can not be seperated.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 29, 2015, 03:16:49 pm
content: By "magnetic induction", I mean "voltage generated by a changing magnetic field", i.e., invoking Faraday's law. Although an electric field is created as a result, it is not the dominant part, in the sense of sqrt(E/H) = Z_fields is below Zo (impedance of free space), i.e., H is dominant.Likewise, "electric induction" (or electrostatic) is the voltage generated by a changing electric charge. This is dominant when sqrt(E/H) > Zo.When the frequency and distance scales are well within 1/4 wavelength, this is strictly near-field behavior, so there's no concept of an "antenna" as a device to interface with propagating (electromagnetic, free space, Z_fields = Zo) waves. Rather, it can be expressed as an equivalent circuit with coupling capacitances or mutual inductances. Another way to put it: the physics can be modeled reasonably well by taking the applicable Maxwell's equations, independently -- the assumption and implication being the speed of light is infinite, or that the system is "static" (thus, electrostatics and magnetostatics, separately). ("Static" in quotes, because we're assuming that time derivatives are nonzero -- otherwise there's no induction! "Quasi-static" is probably a more correct term.)Putting the physics aside, as long as we can measure the consequences of that analysis -- namely, an equivalent circuit of capacitances and inductances, we can model pretty accurately what's happening.So that's where it's coming from. At 50/60Hz, we're well within 1/4 wavelength (> 1000 km), pretty much anywhere in the world. So we can model electrostatically induced voltage (high impedances) as capacitive coupling, and magnetostatically induced voltage (low impedances) as inductive coupling. The former only works when there is a  open circuit, because the impedance of the effect is high; the latter only works when there is a  closed circuit, because the impedance of the effect is low.An audio system might have hum introduced in various ways. When plugging in a signal cord, that's most likely electrostatic at work: the cable (and probably the hand gripping the cable) has some 50/60Hz charge on it, which causes the amplifier to BUZZ briefly as the connector mates (assuming RCA connectors, which are made backwards like this). (If the cable is connected to powered, ungrounded equipment, the buzz might be explicitly due to capacitance in the power supply circuit.) That voltage gets shorted out when the grounds connect, because the low impedance of the grounds dominates.However, there may be magnetic induction in the loop formed between amplifiers and sources, if they share multiple ground paths -- creating loops. These loops create voltage drops across the ground conductors in the circuit, so that at any given amplifier input, there may be a voltage difference between signal (no voltage drop!) and ground, which it senses and amplifies.Ground loop may also be resistive in origin, since incidental AC current flow through any of those grounds can also generate that voltage drop directly, in which case the voltage simply distributes through the loop according to Ohm's law. Like I said, there is usually a difference in voltage between mains neutral and ground, and may be due to induction as well as resistive drop.Tim

author: n9zl
author_rank: Unknown
timestamp: May 30, 2015, 10:23:56 pm
content: Sure, radio waves are picked up by your body too. I have an AM radio transmitter not too far from me, and if I up the sensitivity to near max, I can see an amplitude modulated 1400kHz sinewave riding on top of the 60Hz power hum too.