Oscilloscope capabilities or alternatives - Page 1
Forum section: electronics / beginners

author: realist123456
author_rank: Newbie
timestamp: June 29, 2012, 04:58:38 am
content: Hi all,I'm a newbie just getting into the world of electronics. I'd like to learn as much as I can simply because I find it interesting but also with a view towards repairing simple household appliances.However, I have a question that I've been hunting around the net trying to see an answer to but haven't come up with a satisfactory result yet and I was wondering if some of the more knowledgeable members of the forum might be able to help.This is the situation:I am a photographer and I use large flash packs on location. These store from 1200 to 6400 watt-seconds of power and discharge through high powered zenon flash tubes.They are normally powered from the mains but increasingly I find myself shooting on location with no mains power available.So I've recently purchased some generators off Ebay (a 4200kva and 7200kva) in order to power my lights on location.The specs of the generators say that they deliver a perfect sign wave (being the inverter type of genny).However before I connect up my expensive gear (these flash packs are very finnicky about having clean power) I want to make sure that the claims of the genny's delivering pure sign wave electricity are true.So I'm thinking about getting an oscilloscope to test them with. However, I've been reading a bit about doing that and it seems that it's not a simple (or as safe) as one might first suspect.Suggestions are to use an isolation transformer or to use a step down transformer to convert 240v to maybe 24v and test the signal on that. The idea is that the waveform coming from the 24v transformer would be the same as from the 240v source (only smaller of course).So my question is this: Does that sound feasible or is there another way to test the purity of the supply maybe without using an oscilloscope?Oh and BTW, would the waveform change if the genny is under load as opposed to no load?Thanks in advance for any suggestions.

author: Psi
author_rank: Super Contributor
timestamp: June 29, 2012, 06:16:39 am
content: Quote from: realist123456 on June 29, 2012, 04:58:38 amYes, a 240V - 9/12/24V transformer is a good safe option. You could use an AC wall wart.Quote from: realist123456 on June 29, 2012, 04:58:38 amIt probably won't change that much, unless your loading the generator with a large inductive/capacitive load.It's easy to check, just measure it in both situations.

author: realist123456
author_rank: Newbie
timestamp: June 29, 2012, 06:44:06 am
content: Thanks PSI,I looked up what a wall wart is and see what you mean. Excuse the ignorance of a newbie.All the ones I saw looked like AC to DC transformers but I guess there are AC to AC versions too.Just a question about putting load on the genny and then checking it. I would be loading it with a large capacitive load - by which I mean that the massive capacitors in the flash packs suck up huge amounts of current for the several seconds they take to charge.Obviously they need straight 240 volts @ 10 amps to power them but if I stuck a double adapter in the same socket as that charging the packs and used one side to charge the packs and the other running to an AC - AC wall wart then I'm presuming the waveform from the wall wart would mimic the waveform of the 240 circuit on the other side of the adapter which is under load?This is just me guessing so correct me if I'm wrong thanks.

author: Psi
author_rank: Super Contributor
timestamp: June 29, 2012, 08:41:05 am
content: Yeah, i don't particularly like the name "wall wart" but everyone else on here seem to call them that.Yep, there are AC versions too, which is what you want. A DC version wont give you a waveform to look at.I really don't know how your flash units work but i would have expected them to use their own internal switchmode system to take the 230v mains and generate the 400v or so needed for the flash tubes.Switchmode supplies do tend to clip the peaks of the mains waveform a little, however i doubt it will be a problem.It is possible that your flash tubes use a voltage multiplier system to get ~460V from 230V, in which case yes they might distort the waveform a little from all the capacitance.At some point you'll just have to plug them in and see if they work.I would check your genny is producing a sine wave first though.Any well designed product should be able to handle genny power without being damaged.You should probably read up about earthing and generators as there are a few things that can cause problems if you have interconnecting gear running from generators and mains.Quote from: realist123456 on June 29, 2012, 06:44:06 amYes, the waveform will be the same on both sides of the double adapter.

author: Rerouter
author_rank: Super Contributor
timestamp: June 29, 2012, 09:26:13 am
content: an isolation transformer is far from ideal for checking the shape of a waveform, as it colours it,use a resistor divider if you can, (mains 230/240v peaks at about 340v so make sure your scope can handle it)say 2x 1.8M resistors in parrellel and 1x 100k in series, for a 10 to 1 divider, ground tied to neutral, and scope input at there junction,(you can build it into a closed plug with 2 wires coming out to measure)

author: Psi
author_rank: Super Contributor
timestamp: June 29, 2012, 09:31:22 am
content: It should be fine to tell if the waveform is square or sine though

author: Psi
author_rank: Super Contributor
timestamp: June 29, 2012, 10:01:47 am
content: Here you go, i took a pic for the output of my square wave inverter, both before and after an isolating transformer (60w load)Its pretty obvious when you have a square wave inverter

author: realist123456
author_rank: Newbie
timestamp: June 29, 2012, 12:52:49 pm
content: Thanks for those responses guys.Much appreciated.I'm thinking the double adapter and AC-AC step down transformer sounds like the easiest way to go.The 'scope I'd be looking at would be an Atten 1102 which is rated to 600v. Not enough I suspect to comfortably handle the more than 300v peak coming out of the mains (By the time you take into account multiplication factors).So if I were to use the double adapter and effectively "bleed off" a small amount of current it sounds like that would be safe to work with.It would mimic the 240 main current waveform and with the added benefit of showing me what the waveform was like with the genny operating and pulling load.I'm still kinda new at this so I might get a bit more learning under my belt before I attempt that though.BTW, just an off topic question you might know the answer to: I've seen people using insulated gloves while working with electrics (even though they've been switched off).The gloves look a bit like black tradie's gloves but they are apparently insulated and rated to 600v. (the really big thick ones are rated much higher but you could never work with delicate equipment with those on as it would be like working in welding gloves). Anyway, the 600v type look like they are thin enough to work with most of the time. Do you think they are worthwhile as a way of possibly avoiding shock if one were to touch something live at 240V?Thanks.

author: Psi
author_rank: Super Contributor
timestamp: June 29, 2012, 01:25:03 pm
content: Most of the time gloves are worn for line men working on high energy switchgear, things that have hundreds of amps available. People working on household 230v don't normally use them as far as i know.But yes, if you can find insulated gloves intended for high voltage that are thin enough to work with then it will definitely make things safer.However you have to be careful that they don't make you complacent. The most important thing to remember when working with high voltage is to respect it. You definitely don't want to be wearing gloves so heavy they make working difficult because then you're more likely to make a mistake.It's best to keep away from mains voltages unless you feel pretty confident that you can work with it safely.Also, about your scope voltage. If you use a 10x scope probe it will reduce the input voltage to the scope.

author: T4P
author_rank: Super Contributor
timestamp: June 29, 2012, 01:41:27 pm
content: In any case, around HV(to us of course, 1kV is HV to a sparky)it's much safer to use 10x attentuation on a probe

author: digsys
author_rank: Unknown
timestamp: June 29, 2012, 03:02:11 pm
content: I totally agree with PSI. First off, 240VAC is not considered a lethal enviroment. We grew up being taught towork on 240V live. Gloves can actually BE a hazard. They can suddenly get caught on structures, havematerials fall inside, and reduce your "feel" of what you're working on. Learning best practices is FAR moreimportant. ie Only using one hand (right) on a live item, proper footwear, NO rings, watches, jewellery !!!Insulated tools, IF working live etc etc The isolation transformer will bring it all to a safe level anyway.

author: G7PSK
author_rank: Super Contributor
timestamp: June 29, 2012, 05:59:57 pm
content: If you are really concerned about the power from these generators, you can get power conditioner transformers which will round up the waveform and remove transients they are often used to protect electronics when running on small generators. My experience has been that you are far more likely to get a good waveform from a straight alternator than one of the small inverter generator sets which tend to have the waveform as a large number of steps, the inverter units often give a better voltage and frequency stability in the small size units but voltage stability can be obtained by the use of an electronic AVR board something I have not seen on the cheap Chinese alternator sets But fitted on most European manufactured units.

author: realist123456
author_rank: Newbie
timestamp: July 01, 2012, 02:42:42 am
content: Thanks for all the replies.They are appreciated.It would appear that I have a lot more research to do before I can jump into testing these gennys.Isolation transformers, 10x probes, power conditioners, alternators, stepdown transformers etc.Its all a bit of a muddle at the moment as I don't know enough about what these things are, how they work in detail and how to use them, let alone know which one would be best for my situation and the power I'm trying to test.I think I'll put a few months or a year into learning just what I'm dealing with here as the more I learn, the more complex everything seems.Like most things I guess, there's more to it then might appear at first glance and when it comes to electronics, more disastrous consequences if you get things wrong.I'd prefer not to blow anything up. Especially me.

author: T4P
author_rank: Super Contributor
timestamp: July 01, 2012, 09:12:31 am
content: “experience is proportional with the amount of parts ruinedexperience is proprtional with the number of projects that don’t work right…experience is proprtional with the number of projects that do work right…if something doesn’t work it is not a failure. you were just successful at making something that did not work !electronics hobbyists are like little kids. only the price of the toys has gone up …” – free_electron