Half Ohm - a uCurrent lookalike for Ohms - Page 1
Forum section: electronics / general

author: bingo600
author_rank: Super Contributor
timestamp: June 16, 2012, 08:23:47 am
content: Just saw this one on HD - 14.80€http://hackaday.com/2012/06/15/multimeter-add-on-lets-you-measure-tiny-resistance-values/http://jaanus.tech-thing.org/half-ohm/half-ohm-milliohm-adapter/http://shop.tech-thing.org/Designhttp://jaanus.tech-thing.org/half-ohm/half-ohm-final-design/http://jaanus.tech-thing.org/half-ohm/the-concept-of-half-ohm-low-resistance-meter/Precision is only 1% , but it might be interesting for folks with a lower cost DMM.And the price seems fair.But for me .... I guess i'm better off with my Agilent 34401A , in 4-wire mode./Bingo

author: ejeffrey
author_rank: Super Contributor
timestamp: June 16, 2012, 09:22:32 am
content: Quote from: bingo600 on June 16, 2012, 08:23:47 amAccuracy is only 1%... the precision is as high as your DMM.I've made a similar circuit a while back, but also with a polarity reversal switch that flips both the excitation current and the lead polarity to allow you to do a manual "AC" resistance measurement to cancel small offset voltages due to current flowing in the circuit. It would have been better to use a 555 and a CMOS switch to flip at ~20 Hz so you didn't have to average in your head.

author: rr100
author_rank: Frequent Contributor
timestamp: June 16, 2012, 11:13:41 am
content: Looks good, I rather like this "adapter" design, better than separate case and extra wires for ucurent - although both designs have pro and cons.The shop is broken, otherwise the price would be really ok, especially that it includes shipping and it's shipped from EU. Assuming they can really deliver which is in doubt if even the site isn't working.

author: EEVblog
author_rank: Unknown
timestamp: June 16, 2012, 12:08:15 pm
content: I originally considered that sort of approach for the uCurrent, but it's tricky housing wise (which is why they don't have one), and that sort of approach I've never found to be very rugged.Dave.

author: rr100
author_rank: Frequent Contributor
timestamp: June 16, 2012, 12:53:55 pm
content: Yea, they aren't for heavy use/naked transport. I have a small plastic box in my bag for such naked boards that might come in handy. In fact these might be the best of the bunch as they won't jump around the bench once stuck in the meter.

author: free_electron
author_rank: Super Contributor
timestamp: June 16, 2012, 01:37:51 pm
content: Come on.. Housing problem... You're kidding right ?Pomona has a little sturdy plastic box with two banana plugs on one side and two banana jacks on the other side. Specifically made for this kind of fun. They have the same for bnc to bnc and other mixes...http://www.pomona.cc/PressReleases/PE1092eng.htmlThere you go. Now go build it dammit !Pomona 2259Pomona 2098Heres the pagehttp://www.pomonaelectronics.com/index.php?i=prodsub&parent=BOX&cat=NONCOND&getDetails=If you click on the stockcheck it lists all large distris that have stock and they throw you on their site so you can check price.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 16, 2012, 03:50:33 pm
content: Quote from: free_electron on June 16, 2012, 01:37:51 pmThe double banana plug to binding post box is $30/each.

author: free_electron
author_rank: Super Contributor
timestamp: June 16, 2012, 04:04:40 pm
content: And your point is ? That it costs money ? So does eating, breathing, walking amd anything else in life.If you want something nice it may cost a bit, yes. So what ? Any hobby costs money. Golfplayers pay 30$ to walk a couple of hours on the grass to put a little ball in a hole... And at the end of the day they have nothing. Scubadivers pay 40$ to 50$ for someone with a rickety boat to take them on a bumpy ride to spend two tanks of air blowing bubbles and watch a couple of fish for two hours...We would at least have something that will serve us for years. I say 30$ well spent. ( by the way, i do scubadive so i know what i am talking about.. )

author: Monkeh
author_rank: Super Contributor
timestamp: June 16, 2012, 04:07:25 pm
content: Quote from: free_electron on June 16, 2012, 04:04:40 pmOr you could buy the small plastic box, binding posts, banana plugs, and drill the holes yourself, for about $10.

author: free_electron
author_rank: Super Contributor
timestamp: June 16, 2012, 04:10:36 pm
content: That you can do too. My mechanical skills are zilch... Holes invariable turn out triangular or square, not lined up, and undoubtedly the plastic box fill split when i drill it. And the banana plugs will not sit at exactly 3/4 inch pitch either.No thanks, i'll buy the box.

author: Monkeh
author_rank: Super Contributor
timestamp: June 16, 2012, 04:14:10 pm
content: Quote from: free_electron on June 16, 2012, 04:10:36 pmIndeed, we are not all gifted with the ability to use a ruler.If you're getting triangular holes in plastic you're drilling at the wrong speed or feeding it too fast (or both).

author: krivx
author_rank: Frequent Contributor
timestamp: June 16, 2012, 04:22:07 pm
content: If you're getting square holes with a normal drillbit please let the rest of know how you're managing to do it

author: Monkeh
author_rank: Super Contributor
timestamp: June 16, 2012, 04:27:49 pm
content: Quote from: krivx on June 16, 2012, 04:22:07 pmIndeed, that would be quite useful to know!

author: bilko
author_rank: Frequent Contributor
timestamp: June 16, 2012, 05:14:23 pm
content: Anybody thought about contact resistance between the banana sockets and test leads ?I would imagine that you would have to 'calibrate' this unit every time you plugged in the leads else you end up with nonsense values, particularly with low quality connectors

author: rr100
author_rank: Frequent Contributor
timestamp: June 16, 2012, 05:37:26 pm
content: resistance measured is proportional with voltage provided to meter, so you can zero it out/calibrate it on the multimeter.

author: Bored@Work
author_rank: Super Contributor
timestamp: June 16, 2012, 05:44:04 pm
content: Quote from: ejeffrey on June 16, 2012, 03:50:33 pmPomona obviously priced it for the professionals who have to weight the cost of the ready-made enclosure against the cost of having a technician running around, getting a blank enclosure, drilling it and mounting the sockets.

author: Monkeh
author_rank: Super Contributor
timestamp: June 16, 2012, 05:46:35 pm
content: Not to mention producing and stocking a relatively low volume product.

author: Bored@Work
author_rank: Super Contributor
timestamp: June 16, 2012, 06:34:07 pm
content: Quote from: Monkeh on June 16, 2012, 05:46:35 pmI am not so sure about that one. A lot of their other products are relative low volume, too. They should have good processes to produce and stock low volume products. I think these enclosures are maybe three to four times more expensive then they need to be for Pomona to still making a bit of money from them.But as you can see from one reaction, there are people who consider it a good deal, so Pomona got their pricing right.It is a bit of a gamble, of course. Maybe some Chinese company picks up the idea, and floods the market with cheap, low quality copies. Then Pomona has a problem.

author: bilko
author_rank: Frequent Contributor
timestamp: June 16, 2012, 06:56:56 pm
content: Quote from: Bored@Work on June 16, 2012, 06:34:07 pmI always considered that Pomana manufactured high quality products. Anybody buying a cheap chinese copy probably wouldn't have bought Pomona anyway, so really no loss to them.

author: free_electron
author_rank: Super Contributor
timestamp: June 16, 2012, 07:29:51 pm
content: Pomona is the oem for a lot of testcables sold by other brands...

author: Unknown
author_rank: Unknown
timestamp: June 16, 2012, 07:59:36 pm
content: If I would design something especially for low resistance measurement, I would have designed it for four-wire measurement. You can correct to some degree for contact resistance with the relative function, except that at low resistances the probes on a pcb can act as a decent force sensor (higher contact pressure = lower resistance). This can easily account for 1 mOhm difference (100% error if the resistance is also 1 mOhm, or hundred times worse than spec).

author: ejeffrey
author_rank: Super Contributor
timestamp: June 16, 2012, 08:23:45 pm
content: Quote from: bilko on June 16, 2012, 05:14:23 pmThat should be very low. Banana connections in good condition have fairly low resistance. A much bigger problem is the contact resistance between the probe tip and the DUT.4-wire measurement would fix both of these problems completely, but you have to weigh that against the additional complexity of performing a 4 point measurement with only two hands.

author: free_electron
author_rank: Super Contributor
timestamp: June 16, 2012, 08:51:36 pm
content: Who says you need 4 hands ? Never heard of kelvin clips ? Looks just like ordinary crocodile clips but both halves of the beak are elctrically isolated. So yo have 4 probes with 2 clips.Real milliohm meters use either ac curent as excitation and perform an in- sync measurement to eliminate noise. Or they use a pulsed current source to avoid heating of the testsubject.

author: Unknown
author_rank: Unknown
timestamp: June 16, 2012, 08:59:52 pm
content: There are also probes that combine two tips in one probe, for example:Parrot Kelvin push probes(could do with sharper/harder tips)Keithley spring-loaded Kelvin probes(retail price is way too high, however)

author: icon
author_rank: Regular Contributor
timestamp: June 16, 2012, 09:14:53 pm
content: Quote from: free_electron on June 16, 2012, 04:10:36 pmThere's nothing wrong with your mechanical skills. You're (I presume) trying to drill round holes using an ordinary jobber drill. Why wouldn't you? Drills are for making holes, right? Sadly not in the case of thin materials. What happens is that when the tip breaks through, one edge of the drill catches and the flexibility of the drill allows it to pivot around that edge (instead of the centre, where there is no longer any support). When the first edge breaks free the opposite edge catches, it pivots round that edge, etc. Noise, vibration, ragged out-of-position and (commonly) three sided holes.What you need is a conical or stepped conical drill bit, designed for drilling thin metal and plastic. Ruko make good ones. The stepped ones give you specific hole sizes, but I like the plain conical ones - they're both metric and imperial! I use the shank of a jobber drill as a plug gauge to get the right size hole.Incidentally, square holes are drilled with triangular drill bits.RegardsJohn

author: bilko
author_rank: Frequent Contributor
timestamp: June 16, 2012, 10:09:22 pm
content: Quote from: ejeffrey on June 16, 2012, 08:23:45 pmAgreed, but this gadget is quoted at 1% accuracy at two orders of magnitude lower resistance than the contact resistance variation. Very difficult to achieve and results probably not repeatable without a proper 4 wire device and kelvin probes.

author: free_electron
author_rank: Super Contributor
timestamp: June 17, 2012, 01:25:08 am
content: Quote from: icon on June 16, 2012, 09:14:53 pmAha, thanks. I'll go looking for one of those. I know what you mean.For nice frontpanels and stuff i have those lasercut or waterjetted.There's a techshop in town that has that equipment. You can use that if you are a member.I dont have the patience to make nice cutouts for displays and stuff. Export from the pcb tool to dxf so the dimensions and positions are exact. Export to the lasercutter , slide in a sheet of acrylic or abs and cutting a way it goes.

author: Monkeh
author_rank: Super Contributor
timestamp: June 17, 2012, 02:19:20 am
content: Yeah, cone drills are very handy, especially for odd (imperial hilarity.) sizes. I got a set of three a while back which, while cheap, are more than capable of cutting 1.2mm sheet steel.I still maintain that you can make a good hole in a plastic project box with a normal twist drill. You just have to be careful.

author: SeanB
author_rank: Super Contributor
timestamp: June 17, 2012, 06:31:05 am
content: Imperial size tooling is often much cheaper, as there is no real demand for them, and they are often old stock just sitting in a shelf for decades. I needed a taper reamer last year, and the metric 1:10 was 15 times the price of the Imperial 1:12 one I got ( and needed) while being the same size. The taper pins for Imperial size though are harder to get, but the metric ones can be persuaded to fit with gentle application ( Small hammer instead of the sledge hammer).

author: T4P
author_rank: Super Contributor
timestamp: June 17, 2012, 10:05:29 am
content: I bought one from ebay a long time ago (Titanium Nitride ones) works really goodAmazon has those too but those aren't bad at allhttp://www.amazon.com/Neiko-3-Piece-Titanium-Drill-Steel/dp/B001OEPYWK/ref=sr_1_2?ie=UTF8&qid=1339927386&sr=8-2&keywords=step+drillAliexpress do have but that one is like 22$

author: G7PSK
author_rank: Super Contributor
timestamp: June 17, 2012, 10:44:14 am
content: For drilling accurate holes in plastic I have found that "brad point" twist drills that are designed for wood are very good they do not slip off center when starting due to their needle point and the rest of the cutting geometry means that a clean hole is cut with an easy breakthrough that leaves a small plastic washer on the drill tip and no burr on the plastic. I keep a set specially for plastic as they need to be very sharp. If I need a big hole I will start with a small brad point (3mm) and then use a step drill, cone drills do not leave an accurate size hole repeatedly and are tapered.This is the sort of drill I use.http://www.screwfix.com/p/dewalt-brad-point-drill-bit-set-5-pc/64247

author: SeanB
author_rank: Super Contributor
timestamp: June 17, 2012, 11:11:05 am
content: Careful, there are plenty of fake nitrided drills around. The real ones are hard to tell apart until you drill a hole. I prefer the cobalt steel drills though.

author: T4P
author_rank: Super Contributor
timestamp: June 17, 2012, 11:15:55 am
content: Quote from: SeanB on June 17, 2012, 11:11:05 amMine's still okay, but then again i dont really know what's "okay"I drill a seriously thick piece of wood (Beyond what my drill is rated for) and it only gets slightly hotand the nitride didn't come off when i was drilling 1590BB's neither did it leave a ugly burr ( the key thing of a proper stepped drill bit is that if it works properly a nice burr-free hole should be there )

author: ejeffrey
author_rank: Super Contributor
timestamp: June 17, 2012, 12:12:40 pm
content: I've never used a TiN covered tool for anything but metalwork. Does it actually make a difference on wood? I don't think so, but I don't do a lot of wood work.

author: pbendel
author_rank: Contributor
timestamp: January 29, 2013, 11:28:38 pm
content: Has anybody thought of combining the uCurrent and halfOhm designs into one unit? That sounds like each might have parts in common to make the whole thing simpler to build. Or maybe Dave can add a Revision 4 to his uCurrent to do uOhms?