How much scope to I need for this? - Page 1
Forum section: electronics / testgear

author: JACIII
author_rank: Newbie
timestamp: November 02, 2013, 08:38:39 pm
content: Posted this is the Beginners section and no one would touch this part:Hey Y'all, need to troubleshoot and insure the consistency of a roughly 25 millisecond 130v/125A weld arc. It's a one shot event every three seconds so I have used single shot trigger to capture the form. I am looking at voltage, current, and segments of the arc time (1 millisecond to 50+milliseconds range) and tenths of a millisecond matter for measuring the initial spike.I have used a Fluke 199c with a current sensing ring in the past on this, but the Fluke belongs to the company I work for, is so expensive I hate to use it, I don't have immediate on demand access to it, and it's easy to get lost in the menus.So the question is how fancy (or cheap) do I need to go with a scope to take these measurements?Thanks,JACIII

author: Dark Prognosis
author_rank: Regular Contributor
timestamp: November 02, 2013, 08:43:45 pm
content: Reciprocal of 25us would be the bare minimum but for comfort I would say twice that.To clarify a little - with a 25-micro-second period, F(Hz) = 1 / .000025 = 40,000 Hz or 40 KHz so a 1mhz scope would be very comfortable in handling this signal.

author: JACIII
author_rank: Newbie
timestamp: November 02, 2013, 08:52:24 pm
content: Excellent! Thanks for the info!There's a guy nearby with a TEK TDS1002 for sale so I think I'll give him a call.Thanks again!JACIII

author: Dark Prognosis
author_rank: Regular Contributor
timestamp: November 02, 2013, 09:06:44 pm
content: Quote from: JACIII on November 02, 2013, 08:52:24 pmIf you can get that on the cheap you are set and can zoom in on the signal with ease.

author: Unknown
author_rank: Unknown
timestamp: November 02, 2013, 11:50:32 pm
content: Quote from: Dark Prognosis on November 02, 2013, 08:43:45 pm1 mHz would be way too little. I would recommend at least 200 kHz of bandwidth. You'd have a hard time finding a decent scope with less than 20 MHz of bandwidth, though.A major limitations of the TDS1002 is record depth: only 2.5 kpts. This limits the amount of zooming in you can do on a single shot trace compared to more modern designs that have 1 Mpts of memory. It's not a terrible scope, but it's passive TFT, slow update rate and short memory make it inferior to modern low-end scopes like the Rigol DS1052E. Make sure you pay well under the $300 a DS1052E costs. Around $200 with probes would be a fair deal in my opinion if you don't need the extra features modern scopes offer.Keep in mind that a current probe, if you need to buy one as opposed to a DIY design, may easily cost as much if not more as the scope. Also note that a normal bench scope will have all channels referenced to ground (unlike the ScopeMeter) and standard probes will have a voltage limit of about 300 V CAT II, which may be lower than the probes you have with the ScopeMeter.

author: JACIII
author_rank: Newbie
timestamp: November 03, 2013, 12:22:03 am
content: Quote from: alm on November 02, 2013, 11:50:32 pmYeah,The scopemeterwillspoil ya'. I don't like using my employer's gazillion dollar scope, but I don't mind using their current sensing ring even a little. Will bear in mind the reference to ground.Thanks for the tips,JACIII

author: Dark Prognosis
author_rank: Regular Contributor
timestamp: November 03, 2013, 10:24:07 am
content: Quote from: alm on November 02, 2013, 11:50:32 pmThat 1mhz was a typo I meant 10mhz but most modern scopes are 25mhz minimum and these days they are 100mhz which can eat a 40khz signal for lunch and have room for a 5 course meal too. Still, I have to ask, even with the typo, 200khz bandwidth is less than 1mhz, lol, so why wouldn't a 1mhz scope be enough for a 40khz signal?The points for zooming in is why I said if he can get it on the cheap which is about 200 else pay $397 and get an 1102E.

author: Retep
author_rank: Regular Contributor
timestamp: November 03, 2013, 10:54:13 am
content: Quote from: Dark Prognosis on November 03, 2013, 10:24:07 amAlm is correct; 200khz > 1 mHz and 1 mHz < 40 kHz. Capitals matter: mHz means milli Hertz (1/1000Hertz), whereas MHz means Mega Hertz (1000000 Hertz). You will have a hard time finding a scope that doesn't have a bandwidth that is many orders of magnitude higher than 25 mHz or even 100 mHz.

author: Dark Prognosis
author_rank: Regular Contributor
timestamp: November 03, 2013, 02:26:43 pm
content: Ahhh, I meant 1 megahertz not milli and why I put them all in lower case had it been mHZ then I could see it but it should be all caps as MHZ since the M needs to be capitalized but when talking about scopes it would be extremely foolish to have, in the context I wrote, milli. I mean lets get real and you tell me what scope out there has a maximum bandwidth of 1 milli hertz?