A safety hint.

If you decide you've just got to look at the AC Mains with a battery powered oscilloscope, reconsider, if you still decide 'I just have to do this...' then consider using a AC mains to 12 to 30V step down transformer. The transformer will isolate you and pass the 50/60Hz. It will show you the turn on of a SCR/Triac light dimmer.

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If you believe you need higher accuracy, reconsider why? What are you looking for? High frequency noise on the AC mains?

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OK you've decided to play Russian Roulette.

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Obtain a 9K 10W resistor, obtain a 1K 10W resistor, wire in series. I like the Dale aluminum body units because you can mount them on a heatsink that you can connect to the safety/protective ground.

Wire one end of the 9k to the AC Mains hot, wire the other end to the 1K, wire the other end of the 1K to neutral.

Monitor the voltage across the 1K, it will be 1/10 the incoming voltage.

Touching it and ground probably won't hurt or kill you. Just tested mine while sitting on a chair in a 1/4" sheet of Lexan. Used my right finger and thumb. As expected I felt nothing. This is still stupid silly but the safest way to actually directly monitor the AC mains.

However, I doubt your scope will see any HF trash on the AC mains.

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I live to fight EMI/RFI, interference on the AC Mains created by all the switch mode power supplies.

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I have an intrinsically safe AC Mains to communication receiver, spectrum analyzer interface that has been hi-pot tested over night with 10kV on the AC mains side.

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In addition to extreme isolation from the AC Mains voltage, either 120V US, 240V the rest of the world, the output is clamped at +/- 2.1V.

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It was an extremely non trivial project, the actual schematic is bog simple, obtaining components that could withstand 10kV for ~10 hours was the royal PITA.

Capacitors large enough to pass 50kHz with "no loss" and withstand 10kV are rare. And pricey.

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There are commercial units

https://www.reliantemc.com/laplace-voltage-probe-plip/

https://www.onfilter.com/test-and-measurements

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These are serious scientific instruments not "magic hocus pocus dirty power will harm your health or kill you."

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If you can't afford either, and if you don't have the skill set and access to a certified hi-pot tester, to roll your own, don't.

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Again, before monitoring the AC Mains for anything but voltage with a meter and current with a clamp on, really think "Do I need to do this?"

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With "Why do I need to look at the AC Mains."

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Death is for keeps.

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[The afterlife doesn't count because we won't be bothering with such trivial things in heaven or hell of Valhalla or 贵ó濒办惫补苍驳谤. <sorry for the font change there, had to look it up> I'd cover more religions but can't think of their afterlife policies off the top of my head right now, but I'm pretty sure none of them suggest we'll much care about earthly problems.]?

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I lost a friend in college who was trying to look at the AC Mains with a scope and isolation transformer. He was alone so I don't know how he screwed up. Smart guy, not prone to foolishness, yet he died.

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So yea I'm a bit too focused on the safety issues with scopes and AC Mains.

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Going to the funeral for a good friend sucked bilge water.

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Because my scope is the only accurate tool which I have during the last 2 decades. And I had to design and produce various power mains stabilizers.

I am sorry for what happened to your friend.

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You clearly know what you are doing and have an extremely good reason for monitoring the AC Mains.

You need to see near instantaneous changes in the input and output. Very few people have that need.

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My comments were intended for 'normal' people with 'normal' needs not some kid with unclear needs.

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The only time I ever needed to monitor the AC Mains was to check the turn of noise burst of a TRIAC based main studio lighting grid. One dimmer produced obscene amounts of jittery noise that drove the TV camera control units nuts.

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I insisted on a 5 foot square of 3/8" Lexan, two isolation transformers for the scope and the fire department EMTs across the road to be present.

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120V on a 100A circuit still frightens me.?

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I ended up with the Lexan sheet and attenuator when they shut down our department.

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My 9K:1K attenuator is probably the safest, or least dangerous non transformer method to measure the AC mains.

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And the university bought the Lexan and resistors!

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I'm not well versed on the subject, But I wonder if it would help to drive one channel of your scope through a well filtered say 6V transformer,

then the other channel connected to the ac line with a 20 to 1 resistive divider. Now invert one channel and use the variable attenuation knob to cancel all

of the 60Hz signal. Now what is on the scope is the difference between the two signals, that is the noise. You can change the attenuation of both channels to get the gain you need to see the noise.

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? To push this a little further, put the 6V transformer in a box with the Resistive divider, but put a pot as one resistor ( or more complex fine adjust), Connect these out of phase to cancel all but the noise and look at the output with a single scope channel. Then adjust the attenuation to see the noise better. May need to fiddle with the fine tune to cancel all the 60Hz ac.

????????????????????????????????????????????????????? Mikek

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Safety is another issue I'm aware of but, I did use a scope with an open ground plugged into a isolation transformer for 10 years while repairing VCRs in the 80s.

Hi,

I usually stay to low voltage circuits. ?I try to stay away from anything that can kill me (or give me a shock and ruin my day). ? Even so, from time to time I want to scope out a 120v circuit. ? It’s not just scoping 120v mains power waveform.

For example, I once bought an “ultrasonic rodent repellent” (lets not get into if this is effective or not. ? That’s not the point). ?I wanted to probe the circuit to learn. ? This was my early days with an oscilloscope (and I’m completely self taught). ?So… of course I attached the ground of the probe to what looked like the ground of the circuit and….?

…. KABOOM !!! ?THERE WAS A LOUD KABOOM !!!

Besides the safety factor (to my wellbeing), there’s the question of shorted ground when using wall powered oscilloscopes.

?= ?= ?= ?= ?= ?= ?= ?=?

This (and this type of circuits) was one of my reasoning for getting a portable (DMM style) oscilloscope. ? For little bit more than cost of differential probes, I can get a whole battery operated oscilloscope. ?No more worries about shorted ground.

Also, I want to work on my cars. ?Modern cars are full of fast moving signals. ?Lugging out my bench top scope for car repairs… that’s a lot of work. ?

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>> My 9K:1K attenuator is probably the safest….

As somebody else pointed out, this doesn’t mitigate the possibility of connecting the ground of the probe to the wrong polarity of mains circuit.

Jong?

On Mon, Dec 16, 2024 at 03:44 PM, jong kung wrote:

Also, I want to work on my cars. ?Modern cars are full of fast moving signals. ?Lugging out my bench top scope for car repairs… that’s a lot of work. ?

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Check out the CAN/LIN thread and look at CAN readers on Amazon.

Hi,

>> Check out the CAN/LIN thread and look at CAN readers on Amazon.

Actually I was more interested in this new way of quickly diagnosing car problem using something called “pressure pulse sensor” :