EEVblog #133 - Dodgy Digikey Components

[tyblu]

EEVblog #133 - Dodgy Digikey Components

This is a fantastic blog -- I didn't know you could do 4-term measurements with DMMs. Too bad you got burned! Must have been a 'Friday' batch from Burns!

Seems like D. Jones let out his frustration by following a crazy EE's train of thought:
1 - gah, 3.5% off!
2 - this resistor is 3.5% off!
3 - but wait, is it really? (measure 2X)
4 - ... is it really really? (measure 3X)
5 - ... you've got to be shitting me. (solder up 4term measurement setup and measure 4X)
6 - hey, i didn't have to solder that up at all (measure 5X)
7 - now that the camera is off ... (measure 6X, 7X)

Quite enjoyable!

[EEVblog]

 

I had actually already shot another blog about a dodgy Maxim component just the day before, so that's twice in almost as many days for me!
The Digikey one trumped it though as it's more pertinent.

Dave.

[mikeselectricstuff]

Dave - why are you ranting at Digikey when it's pretty obvious it's a manufacturer problem? Do you expect Digikey to test every reel that passes through their warehouse?
Have you contacted DK and/or Bourns yet?

[EEVblog]

I ranted at both of them, not that it was much of a rant anyway. I was mostly telling the story of what happened.
It is not entirely obvious who is at fault, it could very well be Digikey for all I know.
In any case Digikey are in the best position to investigate it, that's why I'm asking them what's going on.

No, of course I do not expect Digikey to test every reel, that is silly.
Yes, I have contacted Digikey about it.

Dave.

[Alex]

Oh well, it happened now. What about the units that were sent out? Any plans for those?

I remember in the past there were issues with fake components, especially power transistors (eg 2n3055). The die inside was half the size. The part would work fine when you test it but then blow up when installed. Are such parts still in circulation?

[Strube09]

Dave,

Just ignore the criticism of the video comments. Facts are you got bad components and it sucks. You never said it was anyone companies fault, but Digikey is where you bought them so they should fix it simple as that.

Thanks for telling the story and still keeping some education in there showing how the 4-wire works.

I would like to point out that I think this is the first time we have ever seen you use a bench meter... Maybe there are some pluses to a bench meter

Strube

[JohnS_AZ]

This video serves well as a 'real life' object lesson to all of the EE students who watch Dave's videos. I remember back in my Motorola days when we built about $125,000 worth of VME boards using diodes that had reversed markings from the manufacturer. It took 4 hours to build them, and a week and a half to rework them all.

Shit happens.

(BTW... VERY cool tip on the dmm/4-wire technique)

[TheWelly888]

I echo the others about the 4 wire resistance measurement - it made the concept so much clearer for me! Thanks, Dave!

It has just struck me, through hole resistors usually have a colour band denoting tolerance, SMD resistors certainly have no space for any kind of tolerance indication so no one can be sure exactly what has been despatched either from Bournes or Digikey!

[Fraser]

I found the blog very interesting and do feel sympathy for Dave on this one. I have come to respect the high quality that is now available in mass produced SMT and do not check the tolerance on every component I use but then mine are usually alpha prototypes and issues can be diagnosed during testing. What I find interesting though, are the comments on the YouTube Blog that say Dave should have tested every single uCurrent for performance before sale rather than batch testing. I find that view hilarious because Dave has produced these units at a wallet friendly price as a favour to the community... I really don't think he needs the money and feel sure he certainly doesn't need the grief that this matter has caused him. As I have stated in my Blog comment, time is money and if Dave had tested each uCurrent for performance I would expect the cost to rise in order to cover his time. Sadly that is why some relatively simple professional electronics cost significant sums of money, you are often paying for the testing that was done for specification compliance.

The uCurrent appears good value for money and I for one wish to thank Dave for making it available and also for raising this matter of component value deviation from specification. I found the 4 wire resistance testing very interesting as I own several bench meters that are equipped with that facility and I now fully appreciate it's usefulness. 

[EEVblog]

What I find interesting though, are the comments on the YouTube Blog that say Dave should have tested every single uCurrent for performance before sale rather than batch testing. I find that view hilarious because Dave has produced these units at a wallet friendly price as a favour to the community... I really don't think he needs the money and feel sure he certainly doesn't need the grief that this matter has caused him.

Correct on all counts. I never really wanted to do this extra batch, but a few people kept asking and I had some spare boards so I relented.
I originally only sold them to support the silicon chip article, and I originally did kits too which were a pain in the arse.
Not that any of that is an excuse for shipping some units out of spec.

If I ever got into a selling stuff like this full time I'd have some more time and enthusiasm for sustaining the products.
As it is, I generally come out with a new project and sell an associated kit because it is fun.
I actually think it's a bit unkind to produce a project and show it off and then when people ask "where can I buy it?" I have to say "sorry, I can't be bothered"
It was like that with the uCalc project.

I don't make much money out of this stuff. If I counted my time as money, I would make even less.

Dave.

[paulpthcom]

Dave,

Somewhat unrelated to the other comments, but how's that Keithley 225 you used?  Do you find that you use it often for things other then testing out the uCurrents?

[EEVblog]

Somewhat unrelated to the other comments, but how's that Keithley 225 you used?  Do you find that you use it often for things other then testing out the uCurrents?

Yes, it can be quite handy. LED testing is one example, so you know how bright a led will be for a given current, just dial it up.

I also have another Keithley current source that goes down to sub picoamps, not quite as useful, but nice!

Dave.

[Bored@Work]

Somewhat unrelated to the other comments, but how's that Keithley 225 you used?  Do you find that you use it often for things other then testing out the uCurrents?

If you want to build your own, simpler but useful version of a current source (partly doubles as load), see this article http://www.edn-europe.com/programmablecurrentsourcerequiresnopowersupply+article+2251+Europe.html
(the "no powersupply" in the article is wrong).

Just an LM317, a handful of resistors, a few BCD switches you can get off eBay http://shop.ebay.com/?_nkw=BCD+switch&_sacat=See-All-Categories plus enclosure, heatsink, terminals (and a power supply) gives you a nice retro-style current source.

[PeterG]

BoredAtWork, nice find, looks doable in an hour or so with parts in hand.

[Bored@Work]

BoredAtWork, nice find, looks doable in an hour or so with parts in hand.

It took me a little longer, including drilling of the enclosure and  front plate "art" work.

[paulpthcom]

I received my uCurrent today and the resistor is definitely off in the 10.2/10.3 range.  I didn't bother with the 4 wire test, since well doesn't seem like there's much of a point.

[mikeselectricstuff]

Of course being on the high side, you can fix it with a parallel trim resistor to bring it down to the correct value

[PeterG]

I have to say, its things like this that make me happy i have element 14 just 3km away.

Regards

[allanw]

Anyone else notice the high-pitched sound going on through the whole video?

[tyblu]

Not I, allan. Don't think the dog did either.

[ziq8tsi]

Of course being on the high side, you can fix it with a parallel trim resistor to bring it down to the correct value

You can not make a 0.1% tolerance resistor (with the required temperature coefficient and long term stability, etc) by paralleling resistors of higher or unknown tolerance.

Anyway, given that the dodgy batch of 10R0s appear to have values all over the place, measuring and calculating the required trim value for each unit would surely be more work than unsoldering the existing resistor and fitting one of the replacement 10R0s that digikey will doubtless provide.

[austyp]

I ordered the same part from Digikey to build my own uCurrent, P/N: CRT0805-BY-10R0ELFCD-ND, date on my packaging is 16-Nov-2010.  I measured a couple of resistors in my batch using my Agilent U1253A and they were 10.12 and 10.13, so I suspect that the parts I have are from the same reel.

I haven't contacted Digikey, but will be interested to know the outcome of Dave's investigation.

For the exercise I created my own schematic and pcb for the uCurrent.  If it's not some kind of infringement on copyright, I'm happy to give people the Eagle schematic and board files if they're interested in making their own.  I used BatchPCB, think each board cost about US$15 from them, parts quite a bit more once freight to Oz factored in.  This was my first board layout, and there's some things I'd do differently next time around.  I haven't even fully verified the operation of the assembled design but someone may find it useful.

Regards
Austin

[mikeselectricstuff]

Of course being on the high side, you can fix it with a parallel trim resistor to bring it down to the correct value

You can not make a 0.1% tolerance resistor (with the required temperature coefficient and long term stability, etc) by paralleling resistors of higher or unknown tolerance.

Yes you can, provided the trim is large relative to the nominal value. The trim required in this case is only a few percent, so any error and TC of, say a 1% trimming resistor will be less than 1% of  a few percent , i.e. insignificant.

Anyway, given that the dodgy batch of 10R0s appear to have values all over the place, measuring and calculating the required trim value for each unit would surely be more work than unsoldering the existing resistor and fitting one of the replacement 10R0s that digikey will doubtless provide.

You can calculate the require trim resistor from the current reading error. Or use a resistance box. The trim R will be several K so contact resistance won;t be a problem.
 

[alm]

You can not make a 0.1% tolerance resistor (with the required temperature coefficient and long term stability, etc) by paralleling resistors of higher or unknown tolerance.

Yes you can, provided the trim is large relative to the nominal value. The trim required in this case is only a few percent, so any error and TC of, say a 1% trimming resistor will be less than 1% of  a few percent , i.e. insignificant.

This is correct, but the issue is that the unknown tolerance/TC is the '10R0 0.1%' resistor, which might be a crap 5% thick film resistor for all we know.

[mikeselectricstuff]

Seems unlikely, given the identical markings and (ISTR) 3 digit mantissa.