How to measure firing angle after Zero cross with RIGOL1054z - Page 1
Forum section: electronics / beginners

author: anishkgt
author_rank: Frequent Contributor
timestamp: October 14, 2016, 07:36:56 am
content: Hi All,just bought the Rigol 1054z. I have a project in which i need to measure and adjust the firing of TRIAC from an arduino at the peak of a Sine wave after detecting the zero cross.Question is where do i probe to see that on the oscilloscope ? tried watching a lot of tutorial but was not sure what to look for my situation.Any help would be very much appreciated.RegardsGeorge

author: ebclr
author_rank: Super Contributor
timestamp: October 14, 2016, 07:44:45 am
content: Don't connect your instrument on the mains while you are not proficient on the scope use, that's is the easy way to burn the instrument

author: tautech
author_rank: Super Contributor
timestamp: October 14, 2016, 08:11:08 am
content: Use a Differential probe.

author: Marinated
author_rank: Contributor
timestamp: October 14, 2016, 09:07:37 am
content: http://xyproblem.info/Can you say more about your application? What is the load? I guess you're referring to the voltage zero crossing, but in some circumstances it's the current zero crossing that is of interest. If that's the case, a current probe or a temporary solution using a current transformer may be worth considering. If it isn't the case, there may be safe ways to probe the zero crossing on the isolated low voltage side, again depending on your circuit. It all depends on what, exactly, you are trying to do.

author: anishkgt
author_rank: Frequent Contributor
timestamp: October 14, 2016, 10:03:33 am
content: The application is a spot welder Microcontroller. The load is a MOT (Microwave Oven Transformer) and Yes that is the zero voltage crossing. so not sure if i should be looking at the zero crossing voltage or the current. The schematic works and detects the zero crossing and the microcontroller fires a pulse. I want to make sure that the pulse is fired at the peak of the sc sine wave not late and never too early as the load is highly inductive.

author: danadak
author_rank: Super Contributor
timestamp: October 14, 2016, 10:34:49 am
content: This might help -http://electronics.stackexchange.com/questions/98656/using-oscilloscope-safely-with-ac-mainshttp://www.tek.com/dl/51W_10640_1.pdfRegards, Dana.

author: anishkgt
author_rank: Frequent Contributor
timestamp: October 14, 2016, 11:57:03 am
content: thanks but that was specific to the Tek.I did read a lot of them who tried the same idea but am not probing the mains. I am probing the secondary for 9v 1.5A isolated transformer such ashttp://www.newark.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=15003&langId=-1&urlRequestType=Base&partNumber=53X8400&storeId=10194this one.

author: rstofer
author_rank: Super Contributor
timestamp: October 14, 2016, 02:49:31 pm
content: You should probably watch the interrupt input to the AVR on pin 4. To be absolutely accurate, this interrupt will go low about 1 diode drop after zero crossing. As far as the circuit is concerned, this interrupt is zero crossing.You can also fool around with setting the trigger to Source = AC. It didn't seem to me that the trigger occurred at exactly zero crossing but the AC waveform was certainly stable. I didn't fool around with this very much because I didn't anything handy to test.You should be getting a trigger on the start of each half-wave. Turn on channel 3 and touch your finger to the end of the probe. This will provide an AC reference wave. Not a very good one but it isn't life threatening either.If you have an AC wall wart, unrelated to your project, and very low voltage, you can probe that with a separate channel. Unless you have a rare 3 pin mains plug, the output will be floating and you can connect the channel 3 probe across the output leads. If one side is grounded, just probe the ungrounded connection and let the scope ground take care of the common ground. The truly paranoid might put a high value resistor on the probe ground lead - say 1 Meg or so. Just to eliminate the possibility of any ground current. You will be using x10 so you already have a high value resistor in the probe signal lead.There are ways to probe the project transformer but they all assume the project is floating. I would avoid doing this.Get the AC signal working first and then probe AVR pin 4 plus pin 3. This is where 4 channels works great. Channel 1 to pin 4, Channel 2 to pin 3, Channel 3 to the AC source.Or, if you just want to measure phase shift (really, time delay), just probe pins 4 & 3 and leave the AC stuff out of it.

author: anishkgt
author_rank: Frequent Contributor
timestamp: October 14, 2016, 03:48:54 pm
content: So is this suppose to be this complicated.Shouldn't this work without a reference earth. To check phase trigger shouldn't we be probing pin 3 and an AC source like secondary part of the transformer.So if i were to prob the secondary, would it be correct to probe one pin on the secondary and the GND of the O'scope to the other pin on the secondary. On ch2 pin 3 on the IC and the GND to negative of the supply.The attached pic has the yellow (ch1) to pin 3 and ch2 (blue) pin 4

author: oldway
author_rank: Super Contributor
timestamp: October 14, 2016, 06:21:58 pm
content: A low cost, easy and secure solution is to measure the secondary voltage of an auxiliary low voltage transformer connected in // with the primary of your MOT transformer.You can compare this waveform with the secondary ac voltage of your power supply auxiliary transformer.

author: anishkgt
author_rank: Frequent Contributor
timestamp: October 14, 2016, 06:28:11 pm
content: too noob to understand that. Should't it possible to get rough view on ch1 when i probe pin 3 (sends the pulse at the peak) and ch 2 probe a pin on the secondary of the same power supply.

author: 2N3055
author_rank: Super Contributor
timestamp: October 14, 2016, 06:53:07 pm
content: On schematic, you have T1 that provides power to electronics.. From the secondary there is optocoupler that is connected to processor.. that is where uC senses when AC goes trough zero...That is where you can see mains power, but scaled down and safe..Also from output of uC signal goes to MOC3023M that triggers triac.. If you monitor that microcontroller output on channel and signal from secondary of T1 you will see phase relation..Triac will trigger with a bit of delay but that shouldn't be much..Do you control the current too or is it just timer?

author: anishkgt
author_rank: Frequent Contributor
timestamp: October 14, 2016, 07:07:23 pm
content: Spot on. That was how i had been doing but the Scope is something new to me so not sure if am doing it correct.The Spot welder is all about the time nothing to control the current.

author: rstofer
author_rank: Super Contributor
timestamp: October 14, 2016, 07:12:53 pm
content: Quote from: anishkgt on October 14, 2016, 03:48:54 pmThe secondary of the project transformer is floating and it needs to stay that way because one side of the bridge rectifier is project ground. What you don't want to do is ground either side of the input to the rectifier.You can watch the waveform just ahead of the resistor feeding the optocoupler because the LEDs are ground referenced. Connect your scope ground lead to project ground and watch the AC just ahead of the resistor You are going to see full-wave rectified AC so double humps above 0V.You are watching ahead of the resistor because after the resistor, up against the optocoupler, you won't see a signal over 0.7V where the LEDs conduct.The LEDs are NOT ground referenced, my bad!QuoteI missed the signal on yellow. Are you not getting any output?

author: 2N3055
author_rank: Super Contributor
timestamp: October 14, 2016, 07:16:11 pm
content: Quote from: anishkgt on October 14, 2016, 07:07:23 pmPerfect! That makes things easier.. change output MOC to MOC3041.. That one has built in zero detection.. It switches on only full periods.. From zero to zero..In that case it will take care of business.. CPU doesn't have to do anything..To make things simple, you don't have to do anything to the schematics, just change MOC and it will be ok..I asked because I worked on few real ones (spot welders) and they control both current and timing...

author: oldway
author_rank: Super Contributor
timestamp: October 14, 2016, 07:36:35 pm
content: Using ONLY 2 channels of your oscilloscope:Don't use the ground of your project !Probe from channel 1: ground of this probe connected to one side of the secondary of the low voltage transformer which is in // with MOTprobe on the other secondary side of this transformer.Probe from channel 2: ground of this probe connected to one side of the secondary of the transformer of the power supplyprobe on the other secondary side of this transformer.BEWARE: the ground of your project CAN'T BE connected to earth !

author: anishkgt
author_rank: Frequent Contributor
timestamp: October 14, 2016, 07:38:39 pm
content: From Zero to Zero, so it completes one half cycle. So whats the difference between the two. I am guess it helps the TRIAC even better to switch off at the zero as that is necessary when the load highly inductice ? As of now the optocoupler does not fire the TRIAC maybe your suggestion would work once i have it replaced with the MOC3041I get the attached wave form shouldn't the pin on ch 2 be a straight line ?

author: anishkgt
author_rank: Frequent Contributor
timestamp: October 14, 2016, 07:44:06 pm
content: Quote from: oldway on October 14, 2016, 07:36:35 pmSo i don't get it, aren't the GND of the probes connected between each other internally ? my project is not grounded. so how does not connecting to the GND in ch1 and connecting to GND in ch2 make a difference here.

author: 2N3055
author_rank: Super Contributor
timestamp: October 14, 2016, 08:42:42 pm
content: Connecting ground is fine on a secondary side (microprocessor side), that side is galvanically isolated by trafo and optocouplers..I think in this schematic uC senses zero and triggers output.. practically doing what MOC3041 does by itself...Triac shuts OFF EVERY time it's current drops to zero. You don't turn it off. You switch it on and than it is open until current drops to zero.. You can't turn it off like transistor..But since AC goes trough zero.. it shuts it off for you...Every single time you have to trigger it on again for next half wave...That's how it works... So you would have to have same pulses on both channels....MOC I told you about is not necessary if Atmel will do it's job right (rite program in int) MOC3041 is old school, analog, and makes all this zero detection business easy.. You activate LED in MOC, and MOC takes care of zero detection and all that nonsense.. On next halfwave is on.. When you want it of, you switch LED off.. On next halfwave is off..

author: anishkgt
author_rank: Frequent Contributor
timestamp: October 14, 2016, 09:00:38 pm
content: Ah ok ! i get what you mean with how the MOC3041 works. It just switches on at every zero cross. That is not how its required when the load is highly inductive like an MOT. When using such a load it is critical that it switches on the TRIAC at the peak of every sine wave and not at the begining of the zero cross because when the load is switched off at the zero cross there is little current left in the inductor, here the MOT. So that would not not switch on at the exact peak, the difference would be a few 10 or 20ms.The atmel here is programmed to sense the zero cross and fire at the correct time to the MOC3023m. The opto in the circuit fires at random phase meaning it fires when the atmel sends a trigger at the peak of the sine wave. I guess here the MOC3041 would not be best choice this project.Those apart the opto does not switch on the load for some reason, which am trying to figure out.

author: rstofer
author_rank: Super Contributor
timestamp: October 14, 2016, 09:57:33 pm
content: Quote from: anishkgt on October 14, 2016, 07:44:06 pmWith the floating input transformer, how you deal with the scope ground matters a lot. I got confused above, you can't really look at the optocoupler LED with reference to ground. You could do it by using two probes in A-B mode leaving the ground leads off.You can measure all the logic signals referenced to project ground. Yes, the probe grounds are at earth potential and tied together.Stay on the logic side, use your project ground with your scope grounds (and this will earth your project) and everything should be fine.There are complications when using floating transformers and when probing mains powered equipment. Everybody is trying to warn you away from mains and the transformer output. Stay on the logic side. Nothing interesting is happening on the AC side.

author: oldway
author_rank: Super Contributor
timestamp: October 14, 2016, 11:51:12 pm
content: Quote from: anishkgt on October 14, 2016, 07:44:06 pm

author: anishkgt
author_rank: Frequent Contributor
timestamp: October 15, 2016, 04:42:10 am
content: So to sum up the whole probing of the phase angle for the zero crossing,"Also from output of uC signal goes to MOC3023M that triggers triac.. If you monitor that microcontroller output on channel and signal from secondary of T1 you will see phase relation.."The above should be working but like i mentioned the o'scope gives a different output and how can the triggering be set to work on the Rigol DS1054z ?

author: anishkgt
author_rank: Frequent Contributor
timestamp: October 15, 2016, 02:03:15 pm
content: What is the Trigger type that i should go for in here. Pulse was what is was thinking to use. Are there anything else that i should be looking at ?

author: rstofer
author_rank: Super Contributor
timestamp: October 15, 2016, 02:46:34 pm
content: Triggering on the transformer is not necessary. That interrupt signal going into pin 4 is as close to zero crossing as you need to get. As far as the uC is concerned, that IS zero crossing.You know that the zero crossing interrupt is occurring at 10 ms intervals and if you expect your triac to fire at the peak of the waveform, that should be about 5 ms after zero crossing.If you really want to display an AC waveform for reference, use a separate transformer (6.3VAC secondary would work well) and use one of the channels to display it. Make sure the secondary ISN'T grounded. You will be disappointed with the waveform from the project transformer because it is mangled by the rectifier. You can also trigger on AC instead of the channel you use for the AC input. That may result in a stable waveform that triggers very close to zero crossing. After you look at the transformer waveform versus the trigger point using AC triggering, you may decide that you don't really need the AC waveform because everything else can be worked out in the time domain.You should be able to determine the phase delay for the zero crossing interrupt relative to the sine wave zero crossing. That should be a constant and very close to zero. Everything else can be measured relative to the interrupt (pin 4).

author: rstofer
author_rank: Super Contributor
timestamp: October 15, 2016, 02:49:23 pm
content: Quote from: anishkgt on October 15, 2016, 02:03:15 pmWhat signal are you triggering on? If you go to the trigger menu and then select source=AC the scope will automatically trigger on the AC line.If you are triggering on the zero crossing interrupt, I would use edge triggering and the positive going slope.

author: anishkgt
author_rank: Frequent Contributor
timestamp: October 16, 2016, 04:55:02 am
content: Thanks rstofer. I've managed to get this wave formch1 is probed to Pin 4 on the Atmel, showing the pulse during zero cross. ch2 to pin 5 where the trigger is send to the TIRAC via the MOC3023. Ch3 a second transformer as AC sine wave reference. when a pulse is send out to ch2 from pin5 of the ATmel, the wave form is like the one attached. Why is that ? what sort of wave form is that ? how can it be triggered ?

author: alsetalokin4017
author_rank: Super Contributor
timestamp: October 16, 2016, 06:51:24 am
content: (Pin 5 for the CH2 trace? But Pin 5 on the schematic is NC. Do you mean Pin 3 perhaps?)OK, here's what I think. I'm guessing about the programming of the microprocessor.I think the circuit is programmed to turn the triac ON for a certain time interval, which is set by the potentiometer and indicated by lighting one of the LEDs that have time durations noted next to them.So your CH1 yellow trace is showing the zero crossings of the AC source signal. The spikes are 10 ms apart which corresponds to each zero crossing of a 50Hz sine wave. So that's good.Now one of the detected zero crossings tells the microprocessor to turn on, and keep on, the triac for the time interval selected by the potentiometer. This is shown on the CH2 trace (if it is at Pin 3, not Pin 5). Since you are setting your scope at 5 ms/div, you are not seeing the full length of the "on" signal sent to the triac, which is going to be between 100 ms and 450 ms.If I were sitting at your scope I'd set the Timebase at something like 50 ms/div, and use the Edge triggering on the CH2 signal. This should allow the capture of a full "ON" pulse to the triac (2 to 9 divisions). Then you can use the zoom feature to zoom in to see the finer details of where the pulse starts and stops wrt the actual zero crossings.Also, your CH2 trace should probably be DC-coupled and displayed at a more sensitive V/div setting.

author: rstofer
author_rank: Super Contributor
timestamp: October 16, 2016, 02:55:21 pm
content: When you trigger on the edge of Ch2, you can move the trigger point over to the left edge of the screen and get more room to display the entire duration of Ch2.Once you get the entire screen filled with Ch2 ON, you can count the pulses on Ch 1. Then multiply by 10 ms to get the duration. You can also get the ON time from reading the graticule but I would be very interested in the number of pulses as I changed settings.

author: nuno
author_rank: Frequent Contributor
timestamp: October 16, 2016, 03:01:01 pm
content: I have never used it, but doesn't the scope have a trigger mode that automatically triggers on mains zero-crossings? I guess you won't see the wave, but you'll know that the trigger point is the zero crossing.

author: rstofer
author_rank: Super Contributor
timestamp: October 16, 2016, 04:16:54 pm
content: Quote from: nuno on October 16, 2016, 03:01:01 pmYes it does and it works well. Trigger Menu -> Source -> ACI haven't had an opportunity to see how close it triggers to zero crossing.

author: anishkgt
author_rank: Frequent Contributor
timestamp: October 17, 2016, 12:24:20 pm
content: Well i guess i am looking at the correct wave now. is this how it is ? what are the readings i can get from this or what should i understand from this ?I can see that the pulse starts at the peak of the sine.

author: that_guy
author_rank: Contributor
timestamp: October 17, 2016, 04:34:24 pm
content: I'm not normally mr. health and safety but I really get worried when a beginner says he wants to mess with a MOT. Those things have current enough to kill instantly and enough voltage to bridge gaps that would be good insulators at safer voltages.

author: JS
author_rank: Frequent Contributor
timestamp: October 17, 2016, 06:36:59 pm
content: I built a dimmer with a µC few weeks ago, for zero crossing a H11AA1, with a few resistor to mains, then a big resistor on the colector, basically the same approach as shown in the schematic a few posts ago.I then used 2 interrupts, one triggered by the ZCD to reset a timer, a second one with the timer to trigger the opto triac. The triggering time was very short and gave me a hard time trying to estimate the length of the pulse, best approximation was about 50 cycles of the 16MHz µC but could be wrong. The brightness of the lamp was controlled changing the timer comparator value, for min brightness the precision was really great, the last value barely made the filament red, no flickering. At the other end of the range wasn't so stable but I guess was because of not having holding current at the moment it triggered, I guess longer pulses could been used but I didn't see no real reason to do so, just call maximum a few µs longer timer.Normally inductive loads are driven without much of a change in the circuit, but the changes in the power control circuit. you could time the interrupts in your µC to be very precise once you know how your zero crossing works, in the time domain, with a scope shouldn't be a problem, maybe look at the ZCD interrupt moving a pin of your µC and showing it in the scope, to have a number for the delay. If you want to hold it as the max power, leaving the LED on is an option, will trigger as soon as the voltage reaches the minimum and would be very short so the spike isn't terrible, even if it is, the power triac would be triggered for the dV/dt or excess voltage in the worst case. Do you have the smulations of your load (MOT) how behaves when triggering at different times/phases of a sinewave?JS

author: alsetalokin4017
author_rank: Super Contributor
timestamp: October 17, 2016, 08:35:37 pm
content: Quote from: anishkgt on October 17, 2016, 12:24:20 pmOK, you are getting there. (although you have apparently changed the channel connections to the circuit.) Your "on" pulse can be seen to be 100 ms duration, starting and stopping at peaks rather than zero crossings of the CH1 sine wave. Now you need to determine whether that CH1 sine wave is really "in phase" with the actual power that the triac is supposed to be controlling.Also, why are you still using AC coupling on the CH1 signal? Normally we would use DC coupling unless there is a very good reason for using AC coupling. In this case it probably isn't very important (AC coupling removes any DC offset that may be present and there may not be any DC offset in this actual signal) but it is better to be sure rather than guessing. AC coupling isn't "for measuring AC", rather it is (mostly) used for removing DC offset.

author: anishkgt
author_rank: Frequent Contributor
timestamp: October 18, 2016, 01:18:39 pm
content: Hi JS,QuoteDid not understand what you meant by how it works ? the zero crossing is detected by the H11AA1 and the scope is probed on pin5 of it which goes to pin4 on the ATmel.QuoteMoving a pin - you mean move it detect on another pin ?. There is actually two pulse the pic shows the first pulse.nope no simulation and did not understand the last paragraphHi alsetalokin4017,QuoteFrom what i learned is that an inductive load should not be switched on at the zero cross, so now from what you said it should switched OFF at the zero cross, am i correct ? i've corrected the switch off at zero cross now. pic attached. How do i check if ch1 is in sync with the TRIAC ?Thanks guy for the answers.

author: oldway
author_rank: Super Contributor
timestamp: October 18, 2016, 08:54:26 pm
content: You have only control on "switch on" time of the triac.When gate signal goes to 0, the triac does not "switch off" imediately, the triac stay conducting until the MT1/MT2 current drop below the holding current for a sufficient time.

author: JS
author_rank: Frequent Contributor
timestamp: October 18, 2016, 09:36:16 pm
content: Quote from: anishkgt on October 18, 2016, 01:18:39 pmYou are probing your analog signal, that way you know how the H11FF1 works, nothing about how fast the µC responds. Make an ISR when the µC detects the zero crossing and with it write a pulse in an output pin. Then time the difference between one signal and the other, or even better, the actual zero and the µC output. That is the time you have as delay, it should be short enough it doesn't represent a problem working at 120 pulses/second, from the 60Hz mains, but is good to be sure.About how it works I'm referring to the timing, once you have enough data to do the timing between the analog and digital behavior, that behavior should be consistent. Note that there isn't any more info in plotting 12 pulses than just 2, but when plotting 2 pulses you can see much better what's going on, also you should use some markers one the sine wave to compare how well you are timing from it to the pulses.I hope that clears the digital part a bit, for the power handling, which was the last paragraph all about. When working with inductive loads, the thing to be carefull about is when you switch off, since it kicks back as a mule, trying to keep the same current raising in voltage as much as is needed to do so, a 5V relay can kick several times those 5V if disconnected without using the protection diodes. When you are working on AC you can't just hook up a diode to solve this, so it's easier to just switch when the current is low enough so the kickback isn't a problem. Good luck the triac doesn't turn off till the current is low enough, so using a triac solves that by it self, you only need to be little careful. If you go to the datasheet of the triac you are using you will see the typical application circuit (Figure 13 in the link) which has 2 extra resistors and 2 caps that the one resistive load doesn't need. For the turn on, there are no spikes for voltage or currents with an inductive load, the transient of the inductor will take care of that, for capacitive loads you could have a huge current spike, look at the initial charge of a filtered rectifier.http://www.mouser.com/ds/2/149/MOC3023M-196223.pdfYou would probably still need some trial and error to see the optimal times for your interrupts to trigger the triac, it will depend on how inductive the inductive load it is, for resistive loads the fastest you turn it on (as far as the holding current is reached) the higher the voltage in the load. If the load is only inductive, the zero crossing current will be at the peak of the voltage, so you need to trigger after that to have the holding current, also the triggering pulse should be longer (than a resistive load) as the inductive load will take longer to reach the holding current.You still have the problem of your load changing as what you are doing with the other side, the load with the secondary open will be very inductive, the load with the secondary shorted (while soldering) will be very resistive. Maybe is a good idea to sense the zero crossing in current, maybe both.JS

author: anishkgt
author_rank: Frequent Contributor
timestamp: October 18, 2016, 11:19:39 pm
content: Thanks sounds clearerNow to see how long the TRAIC is switched on and when it switches off would it be ok to connect a 1.5k resistor at MT2 and another at the neutral side then probe it ?

author: anishkgt
author_rank: Frequent Contributor
timestamp: October 19, 2016, 09:27:40 am
content: silly me, 1.5k wouldn't work, maybe 1M.

author: JS
author_rank: Frequent Contributor
timestamp: October 19, 2016, 02:16:51 pm
content: As you have a transformer I'd recommend to probe the secondary which is insulated, and the voltage should follow pretty closely the primary. You still have a 4 channel scope with math capability, you can use 2 channels as a differential one and make the subtraction in software. Of course you should only use the signals to do the probing (in 10X for this application) and leave the grounds alone as they are already connected internally.Another option to work safe with mains measurements is to use a transformer, maybe a tiny 12V would do the job, first measure the mains and the secondary of the transformer, make the division and write the number on the transformer, this would be the turns ratio, then you just plug the primary to mains and the insulated sec to your instruments. The limitation with this is the bandwidth, which should extend up to a few kHz. Other limitations of this method is the low input impedance which is not a problem for mains, there could be saturations as the transformer is unloaded and not at nominal load and as I said the bandwidth. What I'm trying to say here, the method isn't good for anything else other than mains measurements, but for the kind of things one is looking to see in mains the transformer is a valid method.I'm not saying the resistors wouldn't work but, it isn't safe, you would have no idea what are you measuring level wise as the attenuator ratio would be unknown, etcJS

author: anishkgt
author_rank: Frequent Contributor
timestamp: October 19, 2016, 04:55:38 pm
content: Thanks JS.The transformer that you mean is the MOT, correct ? It has about 600 to 800Amps. would that be a problem for the O'scope even when measured at 10X ? or can it measure that much amps ?Quotehow is that done ?

author: JS
author_rank: Frequent Contributor
timestamp: October 19, 2016, 06:50:04 pm
content: Quote from: anishkgt on October 19, 2016, 04:55:38 pmO'scopes doesn't care about no currents bro, they just care about voltage, if your sec is unloaded the sec would have a tiny fraction of the pri voltage and no current, even if it can handle 1kA. If your sec is shorted the voltage won't be any good as you are in the limit (or out) of the transformer's confort zone.Press the button on your scope that says 'math', from there the menu should help you do the rest. Do not connect the grounds to anything. Just the tip of your probes to whatever you are measuring, let's say channel 1 and 2. Then you select math, and there you'd have the option to subtract, select the 2 channels 1 and 2 and it will plot a new signal in a new color representing Cmath=C1-C2. You can turn off the plots of C1 and C2 as they no longer represent any information for you, and just use the Cmath to work as usual. Input triggering, gain, sensitivity and time base should still be made over the physical channels, not the math one. Remember to use the same sensitivity range in both input channels for the difference to be true. In any case you should give it a proper read to the user manual of the scope, it will explain all this plus a lot of other functions it has, some of them referring how not to kill the dude using it. It's not as door handles, you aren't supposed to know how to use one of this, it's ok to go to the manual even if you have used a bunch of scopes, as the manual will revel some functions you didn't even know were there.JS

author: anishkgt
author_rank: Frequent Contributor
timestamp: October 19, 2016, 07:12:34 pm
content: So i've connected the MOT secondary on ch4 and here is how it looks. The MOT switches on at the peak but as suspected switches off half way after zc.i heard aternistors like thehttp://www.farnell.com/datasheets/1723975.pdf?_ga=1.14963187.479192878.1474453029are better at switching off at the zc and they are said to be snuberless.How good would those be here other than it can handle high current ?