is this safe way to measure 220V on oscilloscope  - Page 1
Forum section: electronics / testgear

author: hussamaldean
author_rank: Unknown
timestamp: January 24, 2019, 04:32:56 am
content: Hi allI am using this way to measure the voltage and current across different loads (mainly less than 200mA) to study the effect of different load on power factor, current shape etc. So, I am using an auto transformer to reduce the voltage to 24V (or the rated voltage of the secondary transformer) and then step it up to 200V and measure the voltages across the load and current shunt respectively without using differential probe just making sure the ground probe on the same sidethe schematic can be found in the attachment.Oscilloscope is Rigol DS1054z

author: hussamaldean
author_rank: Unknown
timestamp: January 24, 2019, 04:41:09 am
content: Quote from: blueskull on January 24, 2019, 04:37:51 amthere is no current path through ground at all + the whole lab has RCCB which offers protection for such thing and grounded too

author: helius
author_rank: Super Contributor
timestamp: January 24, 2019, 04:47:08 am
content: That is a dangerous assumption: GFCI/RCD does not protect transformer-isolated circuits at all.

author: hussamaldean
author_rank: Unknown
timestamp: January 24, 2019, 04:48:22 am
content: Quote from: helius on January 24, 2019, 04:47:08 amshall I add one for my load too ?

author: BravoV
author_rank: Super Contributor
timestamp: January 24, 2019, 04:56:52 am
content: You're working with dangerous voltage, one mistake is enough to toast your scope ... or your life.Consider to save money for HV differential probe, even the crappy one, still better than none. As you can afford a DS1054Z, don't see why you can't afford one as they're relatively cheap.Once you owned it, probing mains related devices is sort of worry free, an example for you, the scope is probing the mains line that powered the scope it self.

author: hussamaldean
author_rank: Unknown
timestamp: January 24, 2019, 05:10:53 am
content: Quote from: BravoV on January 24, 2019, 04:56:52 amit is high voltage but the power is very low like 1 to 10 watt max for the transformer (small water pump of around 10W, starting capacitor of induction motor and LED of 5Watt)even at short circuit, the whole current won't be high enough to do any damage

author: BravoV
author_rank: Super Contributor
timestamp: January 24, 2019, 05:55:00 am
content: Quote from: hussamaldean on January 24, 2019, 04:32:56 amQuote from: hussamaldean on January 24, 2019, 05:10:53 amReading above, seems like you're not asking, just want people to agree on what you want to do, right ?Suggesting to proceed what ever you like to do, at least I make you happy.

author: waste
author_rank: Contributor
timestamp: January 24, 2019, 12:55:24 pm
content: An isolation transformer to my understanding can be really helpful to protect your equipment, but it can be a hazard when we talk about yourselfGFCI on the board won't protect you against a short in the isolated part for your transformer, as it can't measure any Δ on the return path (that's the whole idea of isolation). So without an isolation transformer the user is protected from a GFCI even though the equipment can be fried.And even though I understand why everybody tries to warn people from connecting their probes to anything closely related to mains, on the other hand to my understanding there are ways to make things safe for both user and device (Im tilting towards protecting the user more than the device).In that sense using an isolation transformer (and please correct me if Im wrong), creates more potential hazard for the user, in an attempt to protect the earthed oscilloscopes.Countermeasure to that can be a floating scope (battery operated).the other issue I can see is sticking your hands (and probes) where they should not be. If you are using one probe again to my understanding from videos and people here, is more or less safe as long as your oscilloscope is floating or you dont create a direct path from load to earth.With two probes things can be trickier as you have to take into account that the non-isolated alligators essentially are shorted between them (I think the most common mistake people tell is trying to probe both mosfets in a switching power supply)A remedy for this dont put your hands where they should not be, is following a hands-off approach to your measurements. That means. Power off, put your hands (assuming no big capacitors around), place your probes, hands off , power on, take measurement.Personally I don't make so many high voltage measurements to spend 120euros on a cheap differential probe (I agree for everyday use it makes total sense to buy one), but I would like to be able to take the occasional measurement on my capacitive dropper, or some other LED driver in order to detect the potential flicker / ripple etc.I'm really trying to learn from you guys and you have my full respect, I don't have any combative approach on this, I'm just trying to apply common sense and be careful without resorting to exotic (for my hobby) expenses, which is also kind of common for some people here.PS Gyro just showed me a debate about isolation transformers on this link so I place it also here before I read ithttps://www.eevblog.com/forum/beginners/rcd-or-isolation-transformer-what-is-best-to-use-for-safety/

author: taydin
author_rank: Frequent Contributor
timestamp: January 24, 2019, 01:33:16 pm
content: Guys, OK, using a high voltage differential probe will allow the OP to safely use his scope on the mains. But when you compare what a $300 equipment purchase means to YOU and to some hobbyist in the middle east, you will soon realize that a HV probe is out of question. Here in the middle east, we must use what we have and rely on ourselves being careful not to kill us.So let's assume that the OP is careful enough not to kill himself with 240V, and suggest solutions that doesn't involve hunderds of dollars equipment purchaseAnd I'll start with my suggestion. If you power your DUT with an isolation transformer, and if your oscilloscope probe is rated to handle mains voltage, and in case the probe is 1:1, if you oscilloscope input can handle mains voltage, you can safely make measurements on all points of the circuit. If your probe is 10:1, then the oscilloscope input does not have to be rated for mains voltage.Can he still kill himself with this setup? Sure! But the risk is less than floating the scope, and many would take this risk just to be able to make the measurement, and without having to sell their motorbike to get a HV differential probe

author: hussamaldean
author_rank: Unknown
timestamp: January 24, 2019, 05:00:15 pm
content: the problem here is getting differential probe, because it is not available in iraq firstly, and most chines sellers sell cheap and crappyordering from amazon is out of questionI have to rely on this setup to make the measurements.however, I am thinking about this setupQuote from: hussamaldean on December 29, 2018, 05:48:34 am

author: DDunfield
author_rank: Regular Contributor
timestamp: January 24, 2019, 07:04:06 pm
content: FWIW: Some "common sense" about looking at directly mains connected stuff:- Use an isolation transformer on the device under test, NOT the scope. **- If at all possible, keep the DUT grounded (don't isolate from ground even if using xformer). **- Treat any connection (even grounds) as hazardous.- Make sure your scope is well grounded. Measure between scope ground clip and known ground to be sure.- Measure between scope ground clip and what you think is ground on the device under test before connecting the clip.- NEVER connect ground clip to anything other than ground (use differential between two probes if you must measure non-ground relative).- NEVER use less than a 10:1 probe.- NEVER use a probe that can be switched to lower ratio (at some point it WILL get switched).** If you can't isolate the DUT, or can't ground it, then use well insulated (no exposed metal including probe connectors) battery powered scope.

author: hussamaldean
author_rank: Unknown
timestamp: January 24, 2019, 07:07:27 pm
content: Quote from: DDunfield on January 24, 2019, 07:04:06 pmI am isolating the load not the scope

author: taydin
author_rank: Frequent Contributor
timestamp: January 24, 2019, 07:20:09 pm
content: Quote from: DDunfield on January 24, 2019, 07:04:06 pmThis is a confusing statement. Are you saying that he should connect DUT after the isolation transformer, and then he should connect DUT to mains earth?If he does that, he will have completely defeated the purpose of using the isolation transformer! Why do we use the isolation transformer? So that we can FLOAT the DUT. We don't want DUT to have any relation to mains earth, so that when we attach the scope ground pin to anywhere on the circuit, nothing blows up.

author: DDunfield
author_rank: Regular Contributor
timestamp: January 24, 2019, 07:38:59 pm
content: Quote from: taydin on January 24, 2019, 07:20:09 pmHence the "if at all possible" There are times when I want to isolate the hot line but can keep ground connected. One uses an isolation transformer to make a "electrically separate" power system, but often that power system can be grounded. Other times not (If you are isolating ground so that you can clip test equipment ground to something "not ground", then I would think about better ways, but I agree, sometimes there are cases where you need to float everything).Dave

author: vk6zgo
author_rank: Super Contributor
timestamp: January 25, 2019, 03:03:00 pm
content: Quote from: hussamaldean on January 24, 2019, 04:32:56 amIf you aren't testing any kind of actual device, & you just want to see how the waveforms across different parts of quite simple circuitry are affected by changes in those circuit components, the setup shown in your diagram should be useable.I assume you want a source of 220v ac which is isolated from the mains, hence the "back to back" transformers.I am, however, a bit mystified by your reference to an "auto transformer", which is a device with one winding connected across the Mains supply,with this winding being tapped so you can pick off various voltages.Such a device is not isolated from the Mains.https://en.m.wikipedia.org/wiki/AutotransformerDo you mean that you are using a tapping on an auto transformer to feed the 24v winding of the 24:220v transformer?If so, then your diagram is not correct, & there is the possibility of breakdown between the low voltage winding & core of the transformer if one side of it is connected to the 220v live conductor.I would not recommend using an autotransformer for this reason.If it is just a terminology error, & you are actually using two normal "back to back" transformers, it seems reasonable to me.Obviously, you don't want to get hooked up across the 220v you have derived in this manner, but as far as looking at it with a 'scope, I can't really see a problem.Remember, the probe earth clips are all connected together at the 'scope, so you can't place earth clips in multiple places at the same time, when probing different points of your circuit.If you do, the result will be " Bang!".Further results blown up probes, Oscilloscope or yourself!Many folk on this forum have never worked with electronics which use higher voltage than around 12v, & are, understandably, apprehensive about voltages of the order of 220v.Greybeards like myself were brought up on tube technology which uses operating voltages similar to, &, in some cases, quite a bit higher than those you propose to use.On many occasions I have probed anodes with 3 to 4 hundred volts DC on them, whilst trying to find signal voltages of quite low levels..One thing to check, is the maximum voltages your probe is rated for.Modern ones are not always as generously rated as the big old Tektronix ones I probed around with in tube gear.

author: Andreax1985
author_rank: Regular Contributor
timestamp: January 25, 2019, 03:15:25 pm
content: If my scope accepts 400V pk input, I could safely measure mains voltage simply by probing the live contact with a 10x probe (after having removed the ground lead from the probe for the sake of safety). What is wrong with this?

author: taydin
author_rank: Frequent Contributor
timestamp: January 25, 2019, 03:22:27 pm
content: Quote from: Andreax1985 on January 25, 2019, 03:15:25 pmEverything! Voltage is measured between TWO points, but what you are trying to do is measure it on one point.When you only touch the probe's signal tip to the circuit, you will see something, but the amplitude will definitely be wrong. The shape of the signal might be wrong as well, because 50 Hz mains hum will probably be riding on that signal.

author: Andreax1985
author_rank: Regular Contributor
timestamp: January 25, 2019, 03:35:19 pm
content: Quote from: taydin on January 25, 2019, 03:22:27 pmThe scope is referenced to earth, so I should be measuring mains voltage w.r.t. ground.If I want to measure the voltage between live and neutral leads, I could probe both leads in two separate channels and than taking the difference.Why is this wrong?

author: vk6zgo
author_rank: Super Contributor
timestamp: January 25, 2019, 03:41:13 pm
content: Quote from: Andreax1985 on January 25, 2019, 03:15:25 pmIf you just want to have a "rough" look at the Mains voltage to make sure it is there, is the right amplitude, frequency, etc, there is nothing wrong with this.The return leg of the 'scope input connection is via the instrument chassis, the safety earth conductor in the power cord, the earth wiring in the building back to where the neutral & earth are connected, then back again to the neutral side of the Mains connection you are probing.If you are trying to see HF artefacts on the Mains, "all bets are off" due to the substantial extra inductance & capacitance now in the probed signals return path, which will make it hard to get any useful results.

author: taydin
author_rank: Frequent Contributor
timestamp: January 25, 2019, 03:49:57 pm
content: Quote from: Andreax1985 on January 25, 2019, 03:35:19 pmIf the entire path is static (same voltage, resistance, current) between the two measurements, this would work.

author: hussamaldean
author_rank: Unknown
timestamp: January 25, 2019, 04:31:24 pm
content: Quote from: vk6zgo on January 25, 2019, 03:03:00 pmthank you for your explanationhowever, I am using the autotransformer to reduce the voltage from 220 to about 20 to 24 then stepped up once again from 24 to 220Vthe reason behind using autotransformer that I can gradually increase the voltage and monitor bot the output voltage and current the high voltage side of the second transformer using multimeter and clamp meter to insure everything is honky dory. and I also make sure that everything is connected properly before I start by connecting the main to the auto transformer and yeah I am using this setup to see the effect of various load on the current shape and PF