High frequency AC and electrocution - human SOA? - Page 1
Forum section: electronics / beginners

author: Whales
author_rank: Super Contributor
timestamp: August 13, 2021, 12:01:58 pm
content: My vague understanding of direct-contact electrocution parameters:Mains frequency - can cause heart fibrillation if conducted through the bodyLower than 40-100V (depending on which safety standard you read) constant voltage supplies: typically not enough to cause harm if applied externally on skinVery high impedance supplies -typicallysafer even at crazy high voltages. Eg static electricity, plasma balls.Very high frequencies (?) - safer for a given constant voltage & impedance than lower freqs (?)I'm making asimple CCFL driver (bottom design on page)that's currently running a few hundred volts at 20-25Khz (very sinusoidal) and I want to learn more about electrical safety in this range.I know that every human & their skin impedances vary, but is there such thing as a human electrocution Safe Operating Area graph with frequency, voltage & impedance as 3 axes?

author: Whales
author_rank: Super Contributor
timestamp: August 13, 2021, 01:16:30 pm
content: Whilst I'm here: any tips on measuring ~20KHz AC voltages in the hundreds of volts?- DMM Extech MN16: screen flashes (reboot/crash/panic?), until I turn down the power supply and then it starts working at "92VAC"- DMM Parameters Pty Ltd 7080B (1980's?): reads 320VAC- Analog meter Sanwa 460-ED: reads 800VAC- Scope HP 54600A with 10x probe: outside limits (greater than 400Vp-p)If I put resistors in series with my DMMs and scopes: measured numbers drop dramatically, more than they should according to my calcs. Eg adding 10M in series with my 10M scope probe reduces the observed voltages to 30Vp-p, similarly I get strange readings on my DMMs. I suspect intrinsic (or intentional) low-pass filters are being shifted by the impedance increases.EDIT: I might try a 100K+10K resistor voltage divider tomorrow. If it notably drops the output voltage by loading it down: I might be able to assume my directly attached (non-divided) DMMs read a linearly scaled version of the real AC voltage and just calculate the % drop that needs correcting from those.

author: AaronD
author_rank: Frequent Contributor
timestamp: August 13, 2021, 02:28:40 pm
content: Quote from: Whales on August 13, 2021, 01:16:30 pmMost 'scope probes also have about 20pF of capacitance. Your 10M in series with the 10M probe creates a voltage divider (as you know), with 5M impedance to drive that 20p capacitance. 5M and 20pF gives a 1st-order cutoff of 1.6kHz.With a 20dB/decade rolloff above 1.6kHz, you should see about 22dB of attenuation at 20kHz. 22dB above 30Vp-p is 377Vp-p. Does that seem reasonable to you, for what the probe tip is actually doing?Note that the harmonics, and any other signals, will be attenuated differently depending on their frequencies, and will have different phase shifts as well. This will seriously mess up a time-domain waveform, so don't take the shape of the 'scope trace too seriously, unless you can undo that distortion. It's technically possible, but there's a lot of math involved (Fourier transforms or DSP functions, depending on your end goal), and the result is often not intuitive just by looking at the pictures.---Also, the AC scale of most cheap DMM's is only accurate for 50 or 60Hz, depending on the intended market, and they become more and more unreliable as the frequency strays away from that, in either direction. You can get a meter that is accurate at any frequency (within reason), but they're considerably more expensive.

author: bdunham7
author_rank: Super Contributor
timestamp: August 13, 2021, 03:08:05 pm
content: Quote from: Whales on August 13, 2021, 01:16:30 pmUse a 100X scope probe or a DMM that is specified for that frequency and voltage. And read the specs, they often deteriorate as you go up in V-Hz. For example, I have a DMM that will measure TRMS up to 1MHz, but the accuracy drops to 3.3% of reading plus some offset and it is limited to 2*107V*Hz--which is pretty high. 500VRMSat 20kHz would be 1*107V*Hz, so half of what mine can do and likely a lot more than some others can handle. Most Fluke bench meters that aren't antiques and the HP 34401A would work (mind the 400V DC bias limit on the HP), but I would read and understand the specs before trying it out.

author: TimFox
author_rank: Super Contributor
timestamp: August 13, 2021, 04:50:41 pm
content: Note that good commercial high-voltage probes include extra “hilts” to protect the operator’s fingers against accidental contact with high voltage.

author: David Hess
author_rank: Super Contributor
timestamp: August 13, 2021, 05:09:41 pm
content: Quote from: Whales on August 13, 2021, 01:16:30 pmReadLinear Technology application note 65starting at page 60.Note the two warnings in the application note about oscilloscope probes catastrophically failing when making these measurements. Now you will have been warned three times.

author: Whales
author_rank: Super Contributor
timestamp: August 14, 2021, 01:21:23 am
content: I've added a 120Kohm resistor string (10x12K) to provide a convenient 1/10 voltage divider for my scope to measure. This low-ish impedance should (?) keep the lowpass effects of my x10 probe's capacitance away but still give me an effective x100 probe.Whilst lamp struck: 172VACrmsWhilst lamp unstruck: 193VACrmsThe 120Kohm resistor loads the circuit down a bit, reducing the output voltage. To try and counter this I used my Parametric 7080B multimeter (my only DMM that doesn't panic/fail when hooked up across the full output V) to try and work out a relative % fudge factor.Whilst lamp struck: 186VACrms (+8%)Whilst lamp unstruck: 222Vrms (+17%)(This assumes my DMM still reads these voltages linearly, which might not be true if eg some non-linear protection parts are kicking in)

author: Whales
author_rank: Super Contributor
timestamp: August 14, 2021, 01:29:46 am
content: Quote from: AaronD on August 13, 2021, 02:28:40 pmI'm seeing 491Vp-p to 636Vp-p, depending on whether the lamp is on/of and whether the 120Kload is attached. Your number isn't too far off.Is this how I should be seeing the probe circuit (for our earlier example with 10M in series externally)?

author: AaronD
author_rank: Frequent Contributor
timestamp: August 14, 2021, 08:33:31 pm
content: Quote from: Whales on August 14, 2021, 01:29:46 amAccording to a seller's picture on ebay, your 'scope's front panel claims 1M and 13pF. So the math will be different by that much. Otherwise I think you're fairly close. The compensation cap will of course make a big difference too. Use the calibration square wave on your 'scope to adjust it for the most accurate sharp corner you can get. Then, theoretically, you can ignore both caps in your frequency calculations.

author: Zero999
author_rank: Super Contributor
timestamp: August 14, 2021, 08:39:52 pm
content: Quote from: Whales on August 13, 2021, 12:01:58 pmThe main concern at that frequency really isn't electrocution, but burns, which can be deep and very damaging. What's worse is if the current is passing through an area of the body, where there aren't many nerves, it can do a lot of damage before you're in real pain.I've built a similar circuit to that in the past, but wouldn't bother doing it again. Nowadays, I'd use LEDs which are more efficient, don't contain mercury, won't shatter like a glass tube can and run at a much lower, safer voltage.

author: YetAnotherTechie
author_rank: Regular Contributor
timestamp: August 16, 2021, 12:28:27 am
content: My experience and other friends with HF HV is of getting close, not of grabbing and keeping hold of a conductor. In the same aplication, repairing laptop or monitor CCFL power supplies, were even if the current is limited and low, if you get hit by a spark, it's much less a "shocking" feeling but it leaves a very peculiar type of wound, instead of a burn the HF HV pierces deeply through the skin and leaves a hole that takes much longer to heal than normally, and it's thin deep hole, down to the muscle. take care!