EEVBlog #87: The Electronics Design Merry-Go-Round - Page 1
Forum section: electronics / general

author: ModernRonin
author_rank: Contributor
timestamp: May 15, 2010, 01:34:16 am
content: I really liked this one. I haven't personally played the optimization / iteration game, but I've known many people who have.From a technical perspective, I'm curious if a high-side current monitor based on an opamp is possible in this particular application.I was looking into circuits to sense mains current a while back, and eventually concluded that I'd probably end up having to use an expensiveisolation amplifierif I wanted to do high-side current measurement directly. (I assume that some kind of isolation amp is what's inside those expensive high-side current monitor ICs.) But there's got to be a better way, right?A couple of ideas occurred to me on how to do it. One, is there some way to very accurately level-shift the high-side sense inputs down to lower voltages? (Resistive voltage dividers? Precision resistors are expensive...) Two, is there a way to power the op-amp from a mains-like voltage so the high-side inputs won't be far off common-mode voltage?I like the EEVBlog because it make me think about stuff like this.

author: ModernRonin
author_rank: Contributor
timestamp: May 15, 2010, 03:06:15 am
content: You can read the "Applications" section of the data sheet:http://www.zetex.com/3.0/pdf/ZXCT1009.pdfSeems to be mostly related to monitoring batteries. My guess is it's mainly about recording how much current they're presently discharging, or monitoring how much current a charger is putting into them.

author: Zad
author_rank: Super Contributor
timestamp: May 15, 2010, 04:22:38 am
content: Much appreciation from me for this Vblog. Much of electronic engineering is this "admin" kinda work. It is engineering because you are optimising your components for the given task. You need to know a lot about a lot in order to speed it up much, and they just don't teach it in Universities and Colleges, yet it is the part of the job that has the potential to save employers the most money (excepting extreme circumstances of course).In the past I have generally used the resistive divider method of high-side current measurement. Looking at the Farnell site, you can get 0.1% for £0:05p. That sort of accuracy is unnecessary in most systems, so if you go to 1% and you find them at 0.4 of a penny. Of course the resistive divider isn't suitable for all systems, but that's engineering for you. "Time, Spec, Budget. Pick 2."

author: hans
author_rank: Super Contributor
timestamp: May 15, 2010, 09:32:29 am
content: A lot of read stuff for low and high side current sensing:http://focus.ti.com/analog/docs/microsite.tsp?sectionId=560&tabId=2180&micrositeId=7A big advantage of high-side current mode is you aren't measuring against ground, but just a voltage drop on a resistor. You can also wire up the load the way you want, the shunt resistor can be placed anywhere as long as it's in series with the current you want to measure.

author: TheWelly888
author_rank: Frequent Contributor
timestamp: May 15, 2010, 02:55:25 pm
content: Quote from: hans on May 15, 2010, 09:32:29 amOh dear! The polarity of the voltage developed across the Rshuntis incorrect ( assuming that the current is conventional rather than electron! )Anyway back to Dave's blog - I would have thought that one useful parameter for microcontroller selection would be minimum ADC conversion time and maximum PWM output frequency?

author: Unknown
author_rank: Unknown
timestamp: May 15, 2010, 03:38:53 pm
content: ADC speed can be an issue. The ATmega (and probably also the ATtiny) is really slow, max. 4kS/s or so without overclocking the ADC clock (which will reduce resolution). I believe PIC's are much better in that regard.An issue I have with parametric search, especially at Mouser, is that often I find that only 20% of the devices I look at have specs beyond type/package/price, so that makes it almost useless. I usually use Digikey to search based on specs, and Mouser if price if my main concern, eg: 'give me the cheapest jellybean part you can find', for example if I need a DAC with at least 8-bit resolution and a 1kS/s sample rate. I also use a Greasemonkey script for Firefox that lets you sort based on price on Digikey (and also filter out results like 'call for price' or 1k minimum order). Digikey is usually much better at normalizing the specs, Mouser might offer you .1A, .10A and 100mA.Another annoyance is that NXP uses package names different from anybody else, so I have to memorize that SOT-146 is DIP, SOT-163 is SOIC, and so on. Fortunately Mouser does include the standard names.

author: TheDirty
author_rank: Frequent Contributor
timestamp: May 15, 2010, 10:00:01 pm
content: Quote from: alm on May 15, 2010, 03:38:53 pmI was going to mention this. The Sort by Price script by Alex Leone is very nice. You need Firefox, the GreaseMonkey Add-On and the script. All the links are here:http://www.sparkfun.com/commerce/news.php?id=355

author: EEVblog
author_rank: Unknown
timestamp: May 15, 2010, 11:52:57 pm
content: Quote from: TheWelly888 on May 15, 2010, 02:55:25 pmNot for this application. A few Hz ADC sampling would suffice, and I can work around almost any PWM frequency.Dave.

author: TheDirty
author_rank: Frequent Contributor
timestamp: May 16, 2010, 03:00:54 am
content: This is another blog that hasn't shown up in the standard RSS feed. At least it's not showing up in my Miro player.

author: RayJones
author_rank: Frequent Contributor
timestamp: May 16, 2010, 07:29:49 am
content: High side current monitors come in very useful when you want to monitor 2 currents that share a nominally identical ground.If sensing on the low (earthy) side, the multiple potential ground references makes for an interesting, if not impossible grounding of the ADC system without one current inducing biases into the the other's measurement.One constant though, irrespective of high or low side sensing, is the current sense resistor itself.Low burden voltage is often desirable, so the low milliohm valued shunt usually costs more than the semiconductor device!

author: EEVblog
author_rank: Unknown
timestamp: May 16, 2010, 08:08:58 am
content: Quote from: TheDirty on May 16, 2010, 03:00:54 amAnother Wordpress quirk.I had to delete it and re-do, please try again.Dave.

author: hans
author_rank: Super Contributor
timestamp: May 16, 2010, 08:50:35 am
content: Now the comments are goneOh well, not too bad. What I said there is that a lot of suppliers have bad parametric searches. Last week I looked for a DC-DC convertor. 5V out fine, like 300 left. Ranges that support 12V in also fine, still got like 80 found. Until I went over to the currents, I only counted like 25 converters that will match at all if I would require any current. In other words , only 30% or so of the products have full parameters. If you go to deep you potentially throw away a lot of products that could work, be cheaper and etcetera. Just like alm mentoined too, 100mA can be 0,1A 0,10A or 100.0mA etc.I find Farnell and RS so bad for searching parts, I might go and try Digikey and Mouser more often, but I'm afraid that if I found my part and go back to find it on RS or Farnell it isn't there. Do you all recognize the same problems

author: Unknown
author_rank: Unknown
timestamp: May 16, 2010, 10:31:27 am
content: If I recall, the comments had a fairly low signal to noise ratio anyway. Anyway you can somehow consolidate the comments? Having comments on three different places means many users don't get to see them all.

author: EEVblog
author_rank: Unknown
timestamp: May 16, 2010, 12:45:01 pm
content: Sorry, a bug in Wordpress means I have to delete the post and start a new one to fix the podcast feed issue. That means all comments get lost in the process, no way to preserve them.I don't know of any way to consolidate Youtube comments, Wordpress comments, and forum comments. If anyone does let me know.The signal-to-noise ratio on Youtube is much lower than on wordpress!Dave.

author: Unknown
author_rank: Unknown
timestamp: May 16, 2010, 03:01:17 pm
content: Quote from: EEVblog on May 16, 2010, 12:45:01 pmThe only solution I can think of is to disable comments and post a link to the forum thread/Wordpress. I don't think there's any better way, short of writing your own script that copies comments (I don't think they're available as RSS feed, but I could be wrong).People need a Youtube account to post comments on Youtube (I usually watch on Youtube because of the higher resolution, but don't have an account). And people need a forum account to post on the forums. It might lead to more registrations for the forum, although it's debatable whether this is a good thing.

author: ModernRonin
author_rank: Contributor
timestamp: May 16, 2010, 07:58:43 pm
content: Looking into my mains current measuring problem again, I found this great article at EDN:http://www.edn.com/article/CA434873.htmlAnd the corresponding PDF:http://www.edn.com/contents/images/434873f1.pdfBasically, a zener diode and a large value resistor in series make a reasonable regulator to track the voltage of the mains. You can then tap off power and ground from around the zener.There's also a couple of transistors involved to level-shift the output of the monitor IC down to ground.

author: Unknown
author_rank: Unknown
timestamp: May 16, 2010, 08:29:07 pm
content: Make sure to treat everything as directly connected to live mains voltage, i.e. insulation, clearance, make sure no part can be touched. Since there's no galvanic isolation, there's no protection from transients and other dangerous stuff. There's a reason why they don't make power supplies like that anymore in consumer electronics.

author: r
author_rank: Contributor
timestamp: May 17, 2010, 02:38:32 pm
content: it bin about 1 day but it has steel not came up in itunes

author: tuta
author_rank: Newbie
timestamp: May 18, 2010, 05:42:07 pm
content: HiDoes anybody else have trouble viewing episode 87 on YouTube? I tried it from at least 3 different computers and get a blurry picture every time. I'd love to watch itCheers

author: ModernRonin
author_rank: Contributor
timestamp: May 18, 2010, 06:03:46 pm
content: I watched viaand it's looking fine to me. Taking forever to load, as these usually do, but looks fine once it loads up.

author: r
author_rank: Contributor
timestamp: May 18, 2010, 06:14:13 pm
content: it steel not on itunes

author: EEVblog
author_rank: Unknown
timestamp: May 18, 2010, 10:23:09 pm
content: Quote from: tuta on May 18, 2010, 05:42:07 pmYou must have watched it before it finished processing.Either that or you are expecting a 1280x720 screen capture to be sharp and readable at 640x360 which is the minimum Youtube resolution.Hit the 720p button bottom right and expand the window if you want to watch it in full resolution.Dave.

author: tuta
author_rank: Newbie
timestamp: May 19, 2010, 08:50:46 am
content: That helped! Thanks a bunch. Why did'nt I try this by myself?:-PCheersTuta

author: saturation
author_rank: Super Contributor
timestamp: May 19, 2010, 12:04:02 pm
content: Other that parametrics, one way to speed up your online work is get the biggest LCD or equivalent monitor you can, or get a video card that supports multiple monitors. That way you can see and compare multiple sites simultaneously, thus faster, without flipping between pages.

author: EEVblog
author_rank: Unknown
timestamp: May 19, 2010, 12:14:07 pm
content: Quote from: saturation on May 19, 2010, 12:04:02 pmEveryone at my works has 3 monitors. 30" + 2x21"Some have 5 monitors!Dave.

author: saturation
author_rank: Super Contributor
timestamp: May 19, 2010, 02:47:51 pm
content: Way to go. I'd do the 3 but I currently only have space for 2.Quote from: EEVblog on May 19, 2010, 12:14:07 pm

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 20, 2010, 12:09:42 am
content: A few comments on this whole process which I know all to well.....DK/Mouser/Farnell aren't the be-all & end-all of component suppliers. Franchised specialist distis can often be significantly cheaper once you get into the 100+ or full reel/tray quantities. They are typically a bit less easy to deal with, and you often get a better price if you phone them up than online. You also often get this "Target Price" bullshit to which I always answer "As cheap as possible". They tend to also add delivery charges and refuse to use anything but UPS/Fedex etc. however savings can be significant enough to justify the hassle.DK has about the best parametric search but still falls down occasionally, an example being input voltage range on regulators, where it needs a 'min' and a 'max' input rather than seperately listing all the different ranges. the way they list the same part multiple times for different tape/reel options also produces an unnecessarily large number of search results.Farnell's parametric search is much poorer due to bad/inconsistent data entry, e.g. you get seperate entries for "5V, 5v, 5.00VDC" etc. The data has obviously been entered by people who don't understand what the figures mean so you frequently need to do multiple selections to avoid missing things. And then you still often miss things due to strangeness in the way they've been categorized.What they have ALL so far failed to implement is a way to specify the quantity at which you want to sort by price. Diferent manufacturers (who often are the ones dictating the price breaks) have different, sometimes vastly different qty/price functions, so the ability to sort by 100x price would be somewhat useful if ou are looking to buy 100 parts.And don't get me started on buying FPGAs - what's all this "Supported price" bollocks all about.. I just wanna buy the goddamn things not jump through a load of hoops created by some marketing dickhead.

author: Simon
author_rank: Unknown
timestamp: May 20, 2010, 06:58:11 am
content: that chip looks interesting, looks like it will do as the MAX4172 but much cheaper ?