voltage on oscilloscope - Page 1
Forum section: electronics / beginners

author: guitar309
author_rank: Contributor
timestamp: January 24, 2011, 02:37:10 pm
content: I have asked a question about this before... but... I just bought a new instek oscilloscope... I would like to use it for biasing tubes in a guitar amp... I purchased a dummy load which has a bnc output for an oscilloscope... so my question is... This scope says it can handle 300V max... but is that 300V with the probes turned to 10x?... The reason I am asking this is because I wanted to buy a bnc to bnc connector... but if the dummy load has say.. 100V on the output to the oscilloscope and the connectors are only 1x... I didn't want to burn anything up..Sorry .. a bit hard to explain....

author: scrat
author_rank: Frequent Contributor
timestamp: January 24, 2011, 04:26:15 pm
content: It should be rated at 300V on the input, so with the BNC directly, too.Since this is a safety matter, and I'm not 200% sure, just wait for another answer (and take another look at the scope's datasheet, where you can guess the right answer).

author: saturation
author_rank: Super Contributor
timestamp: January 24, 2011, 04:46:49 pm
content: Mains powered scope makers specify max voltage between high and earth ground input of the scope. You also need to consider the CAT limits of the probe separately, and choose whichever is lower. Working on an amp I presume that is after the main side of the power supply, which is CAT I. Since the probe divides by 10, the input presented to the scope are less than 40Vpp. However, if there is a non-isolated connection between mains supply and the low output like the Y capacitor in a switching power supply, its prudent to consider the maximum rating to CAT II level.For example on the Rigol:Analog channel maximum input voltageCAT I 300Vrms, 1000Vpk; instantaneous voltage 1000Vpk.CAT II 100Vrms, 1000Vpk.CAT II 300Vrms, when use probe RP2200 10:1, RP3200 10:1 or RP3300 10:1.I recently troubleshooted a malfunctioning switching power supply, on the low power side. It was adjustable and capable of delivering 300Vdc. I was surprised to see rare voltage spikes to nearly 900Vpp when it was set to 150Vdc, at the limits of the transient response of my probes. To avoid damaging a more sensitive instrument like a Rigol 1052E, I use a separate low cost hand held scope just to examine a known circuit that has > 40V on it, you'll never know what's in there, and you can may find a decent one like the Velleman HPS40 for much less than a set of isolation probes. Also, I use higher voltage safety procedures such as using only one hand when probing or no hands, by not actively probing when the device is powered on.

author: Unknown
author_rank: Unknown
timestamp: January 24, 2011, 06:19:15 pm
content: Quote from: guitar309 on January 24, 2011, 02:37:10 pmBoth the scope and probes have maximum voltage ratings you should observe. With 10x probe, your probes see the full voltage (so for 300V, they have to be rated for 300V), the scope only sees 10% of it, or 30V. This does not mean that with a 10x probe and a scope rated for 300V, you can safely measure 3kV, since the probe is unlikely to be rated for 3kV.One thing to note: the max. DC voltage with AC coupling. AC coupling works by putting a cap in series with the probe. Since the DC impedance of the cap is almost infinite compared to the 9 Mohm resistor, the cap will see almost the full voltage, not just 10%. No such problem with DC coupling. The scope will usually have a max. DC voltage rating on AC coupling (max voltage of the cap).You also have to observe overvoltage categories according to IEC 61010 (eg. CAT II/III for residential mains), so for a CAT II 300V circuit, you need a CAT II 300V (or better) 10x probe and a CAT II 30V (or better) scope.

author: xian
author_rank: Newbie
timestamp: May 29, 2011, 08:53:49 pm
content: So let me see if I understand correctly.The Rigol 1052E is rated for 300V input. That means I can measure maximum 300V with a 1x probe , 3kV with a 10x probe and 30kv with a 100x probe ? Assuming that the respective probe is actually rated for that voltage (The default Rigol probes are rated for maximum input of 150V AC on 1x and 300V AC on 10x so is not suited for high voltage. )Using the 100x probe from DX (rated for 1200v) should be safe to measure 1kv source ?@David: you really should make a tutorial about safety before somebody will blow his head off. Or even worst, blowing up his oscilloscope.Yap, I am serious about it, I really don't mind to win the Darwin Awards as long as my Rigol live to tell the tale. So if you do this please put a lot of accent in how to protect the equipment.

author: Bored@Work
author_rank: Super Contributor
timestamp: May 29, 2011, 09:42:20 pm
content: Quote from: xian on May 29, 2011, 08:53:49 pmNo, because you don't know if the unknown manufacturer just pulled the rating out of his ass, or if it is real.If you talk about this probehttp://www.dealextreme.com/p/250mhz-oscilloscope-probe-max-1200v-35610, just note that they don't have any special hand guard, which would be my second reason not to trust it.Further, typically the voltage rating depends on the frequency of the input signal. It goes down with frequency. There is no such information about this on the DX website, maybe it comes with the probe. But already because this is missing on the website I wouldn't trust the probe. Strike three, and out.But if you don't have enough, note the BNC plug is not isolated. That should work, if your scope is properly grounded and if you connect everything properly and in the right order. But if you e.g. first connect the probe to the DUT, if you mix up signal and ground, and if you then touch the plug to connect it to the scope ...

author: tekfan
author_rank: Frequent Contributor
timestamp: May 29, 2011, 10:29:29 pm
content: Quote from: xian on May 29, 2011, 08:53:49 pmThe voltage rating on the scope is basically the maximum that the scope will survive without damaging the front end. The maximum attenuator setting (minimum sensitivity) is usually 5V/div (10V/div) on some scopes. Almost all analog (even most digital) oscilloscopes have 8 vertical divisions so:8 divisions x 5 volts per division = 40 volts peak to peakThis is basically the maximum amplitude that the signal can be to fit fully onto the CRT. You could put in a 100V p-p signal but then more than half of it will be displayed off the screen.With a 10x probe you could measure 400 volts but be careful because most probes are only rated to 600V. The 10x probe has a 9 Mohm resistor inside it and the scope has a 1 Mohm resistor inside thus forming a 10:1 voltage divider.There are many 100x probes rated to only 1kV. This may seem like a useless thing but it's really not becuse when measuring high impedance circuits the probe loads the circuit less. The 100x probe has a 99 Mohm resistor + the 1 Mohm scope input resistance thus forms a 100:1 divider. So when you probe a circuit with a 100x probe it sees only a 100 Mohm load.You really have to watch out for the voltage rating for the probes otherwise you can get arcs from the tip to the grounding sleeve on the probe.Quote from: guitar309 on January 24, 2011, 02:37:10 pmWhat is the resistance and wattage rating of the dummy load?If it's 8 ohms then you have no worries about damaging the scope because you need a lot of current to have 300 volts across an 8 ohm resistor.Example:If you've got a 100W amplifier and you are measuring the voltage across the load resistor when the amp is at full power, you'll get about 28 volts across the resistor.In this case you can hook up the dummy load directly to the oscilloscope. If you have higher wattage amplifiers then I would measure the dummy load with a 10x probe.

author: EEVblog
author_rank: Unknown
timestamp: May 30, 2011, 12:49:22 am
content: The 300V rating on the scope input is an absolute maxim figure without damage.It does not mean the scope can actually measure 300V, say if it only has 5V/div maximum. Having your signal off the scoep screenAnd BTW, you can't fool the scope by switching it to x10 mode and using a x1 probe!You need a proper high voltage probe for this kind of work.Dave.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 30, 2011, 01:57:14 am
content: First of all, determine what kind of level you see out of the BNC for a given input power.Use a 10X probe initially connected to a chopped off BNC coax lead,or whatever else you can do to connect itacross the BNC output.Connect an amplifier to the load,& wind it up till you see a signal displayed on your Oscilloscope.Continue to increase the power on the amplifier until you reach the maximum you would normally test to.- Make a note of this (peak to peak) level somewhere.Many modern Oscilloscopes automatically correct for the 10X probe,if not multiply the 'scope readings by 10 to give you thepeak to peak voltages present at the BNC connector.The maximum peak voltage you will present to your Oscilloscope input will be 1/2 the peak to peak voltage across the test load at the maximum amplifier output, divided by 10.If the BNC is intended for direct connection to an Oscilloscope,it will probably be attenuated with respect to the voltageactually present across the amplifier load.( Which you should be able to calculate,knowing the amplifier output & load resistance).If it is not attenuated,buy a BNC to terminals adaptor,connect your X10 probe to the terminals,& you are good to go!VK6ZGO

author: Psi
author_rank: Super Contributor
timestamp: May 30, 2011, 08:35:06 am
content: Whatever you do,donotget this DX probehttp://www.dealextreme.com/p/high-voltage-probe-for-oscilloscope-1-1m-cable-34519It's got no attenuation at all, if you wire it up to your oscilloscope and try to measure 30kV you will put 30kV directly into your oscilloscope.Or.. if you do try it, make sure you make a video so we can watch the sparksPersonally I would never trust any probe from DX that says its rated for more than 300V.DX deal in ultra cheap items, not the sort of place you want to buy anything that requires quality to be safe to use.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 30, 2011, 09:03:32 am
content: Dave has a video on DIY 100/1000X? probe, check it out. check jahonnen's website as well. diy is better than this unknown manufacturer probe.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 30, 2011, 09:54:00 am
content: Quote from: Psi on May 30, 2011, 08:35:06 amThat isverynasty!It looks just like a real High Voltage probe.I wonder if someone who knew nothing bought a bunch of bits surplus,& just tacked them together.I have used real HV probes many times over the years,& they are usually very well engineered & do exactly what they say they do.I have never seen one for an Oscilloscope. 'Scopes are normally 1M ohm input Z ,whereas DMMS are 10-11Mohm.VK6ZGO

author: xian
author_rank: Newbie
timestamp: May 30, 2011, 06:57:50 pm
content: Quote from: BoredAtWork on May 29, 2011, 09:42:20 pmThis was strictly theoretically, the actual quality of the probe was not considered.Quote from: BoredAtWork on May 29, 2011, 09:42:20 pmI know the quality of the DX products is questionable, but from my experience I can say are a lot better then what I can get locally (Romania) at a similar price. So I order one anyway.Offtopic: For electronics are just two kind of products here: The professional high end equipment( Only the one the cost a arm and a leg) and is costing twice here because of taxes and stuff (I really love the EU) and the extremely low end chinese (sold by by the pound) stuff rebranded with german names. You just can not get anything in between.Quote from: BoredAtWork on May 29, 2011, 09:42:20 pmThat was one of the things that made me think twice before ordering one, but again, I can get anything better under 100$.And if I mix up the signal and ground when probing a 1KV source using a cheap chinese probe I consider myself worthy for the Darwin Awards.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 31, 2011, 06:08:51 am
content: We have got way off topic on this,the OP was trying to look at Audio levels across a Dummy load (Substitute for a speaker),to see if the signal was distorted(Adjusting bias on the output devices).Some guitar amps purport to be in the 1kW class,but whether this is average power,or "Pretend Power",I'm not sure.My guess is that the dummy load audio level at the BNC is attenuated to a more convenient voltage for an Oscilloscope.We used to test amplifiers this way ,back in the day,with Noise & distortion sets bridges across a dummy load.The dummy loads we used were all made in house,so I don't know what a commercially made one would be like,but commonsense would seem to point to an attenuated output.Normal X10 probes are commonly used in the internal circuitry of TVs etc, at around 400v levels,so they should be able tohandle a few hundred volts of audio.HV probes are another story,however, & are usually used with a DMM,or way back when,with electrostatic voltmeters & VTVMs.One of the comments on the link posted says that "HV" probe has NO series resistor string,which if true,means it is extremely dangerous.VK6ZGO

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 31, 2011, 07:14:11 am
content: check the OP date. he maybe on honeymoon in hawaii right now. but keep the info coming

author: Bored@Work
author_rank: Super Contributor
timestamp: May 31, 2011, 03:13:50 pm
content: Quote from: xian on May 30, 2011, 06:57:50 pmSo you don't want to listen but just waste our time.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 01, 2011, 01:21:15 am
content: Quote from: BoredAtWork on May 31, 2011, 03:13:50 pmThe probe xian is getting is the 100X Oscilloscope probe,not the very dangerous so-called HV probe.The 100X probe is normally for looking at high impedance circuits which would be adversely affected by the parallel Z of a X10,rather than for looking at high voltages.That said,the normal way of using a 'scope probe to look at higher voltages is to clip it on the test point,attach theearth lead to a convenient earth point,all with the probe BNC already connectedto the 'scope,thenturn the DUT on.If you are in doubt about the ability of the probe to handle voltages of this order,a test jig could be made up,with a BNCmounted in a diecast box with a solid mains earth,& a voltage divider across it totalling 1Mohm,but with a lower leveltap that a DMM or X1 probe could be connected across.Attach the probe's BNC to the test jig BNC.Attach the X100 to the 1kV source, without any meter or 'scope attached & see what happens!If it goes zap! all you have damaged is the probe & a couple of resistors.If it seems OK,measure across the tap on the voltage divider & see what voltage you see.You should be able to tell if the thing really is 100X,& not 89X, or 112X or something.VK6ZGO

author: Bored@Work
author_rank: Super Contributor
timestamp: June 01, 2011, 05:59:30 pm
content: Quote from: vk6zgo on June 01, 2011, 01:21:15 amI know. But again, why is the idiot asking if he doesn't listen?

author: tecman
author_rank: Frequent Contributor
timestamp: June 01, 2011, 10:40:07 pm
content: Quote from: guitar309 on January 24, 2011, 02:37:10 pmIf you are interested, I have a Tektronix P6013A 1000X high voltage probe that I will sell.Drop me a PMPaul

author: scrat
author_rank: Frequent Contributor
timestamp: June 03, 2011, 08:32:25 am
content: Quote from: BoredAtWork on June 01, 2011, 05:59:30 pmSorry, but I don't understand this aggressive way.I think people here is free to ask for advice, and free to decide wether or not to follow it. Otherwise it won't be an advice, it would be an order.If our time is so precious, we're free not to give any advice.

author: Mechatrommer
author_rank: Super Contributor
timestamp: June 03, 2011, 04:34:43 pm
content: Quote from: scrat on June 03, 2011, 08:32:25 ami dont know how many time i have told the wifey this! but she still dont get it i think.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 04, 2011, 07:04:49 am
content: With wifey,itisan order!!VK6ZGO

author: Mechatrommer
author_rank: Super Contributor
timestamp: June 04, 2011, 09:17:17 am
content: Quote from: vk6zgo on June 04, 2011, 07:04:49 amno! she keep saying it is an advice!

author: vk6zgo
author_rank: Super Contributor
timestamp: June 04, 2011, 11:33:14 am
content: But isn't that the art of being a wife?Guide the old bloke without him realising he is being guided.VK6ZGO

author: xian
author_rank: Newbie
timestamp: June 05, 2011, 10:35:29 pm
content: Quote from: BoredAtWork on June 01, 2011, 05:59:30 pmSir, yes, sir ! I heard you loud and clear, sir !I will toss the probe as soon as it arrive, sir !I am sorry I disobeyed your orders, sir !Permission to speak freely, Sir !First of all, I have already order the probe before I asked the question. Second of all, a 100x probe can use for multiple porpoises not only for HV. Sometimes the standard 10x is just drawing to much current for the circuit to handle. Most of the time the 10x probe will do just fine, so there's no point in investing in expensive 100x probe that I will use once in a blue moon. For what I needed just a simple just some resistors did just fine.Now I am sorry I mention the source of the probe. Because my question was about the voltage that the oscilloscope is able to handle on it's own and what's the theoretical difference between probes, not about the quality of the probe.And BTW, I am not your wife, sir !Quote from: vk6zgo on June 01, 2011, 01:21:15 amThank you vk6zgo for your advice, this is really helpful.

author: Bored@Work
author_rank: Super Contributor
timestamp: June 06, 2011, 05:57:27 am
content: Quote from: xian on June 05, 2011, 10:35:29 pmYou ask for buying advice after you already bought? Thanks for confirming you are an asshole and your only intention was to waste our time.

author: Simon
author_rank: Unknown
timestamp: June 06, 2011, 06:49:19 am
content: can we cool it please, I think all points have been made well enough by now.If we are talking rigol scopes (DS1xxxE/D series) the maximum that you can measure is 30V, 300V with a 10X probe and therefore 3000V with a 100X probe.The 300V figure on the front panel is part of the CAT rating and means that if you give it a spike of more than 300V it will probably deservedly die as it is 10X the measuring capacity.

author: xian
author_rank: Newbie
timestamp: June 06, 2011, 08:51:52 am
content: Quote from: BoredAtWork on June 06, 2011, 05:57:27 amYou, sir, are and idiot.