EEVblog #643 - Mailbag

[EEVblog]

[Simon123]

Did u used 3V on meter to power it on or 1.5V.
2R10 is 3V battery.

[Mark Hennessy]

I bought my fx7000G in 1989 for my A-levels, and still use it daily. I have tried many newer Casio calculators, but keep going back to the 7000. I guess that after 25 years, I'm used to it. Recognising that it's getting old, I bought a couple second-hand, and they are in pretty mint condition. Good job I did, as no-doubt the prices will shoot up now

[kcs]

I am very curious how did he got solder mask between those pins. I have used IC in the same package recently and I was not able to get solder mask because of the small pitch. I have used the same PCB manufacturing service as that guy.

Datasheet for IC recommends:
 - Distance between pads - 8 mils
 - Pad width - 17,5 mils

PCB manufacturer's rules:
 - Min solder mask expansion - 3 mils

So we are left only with 2 mils for solder mask.

[nathanpc]

I like the new format, it looks a lot better since the camera isn't too close to the box you're trying to open and you have more room to show the contents before changing to the usual angle.

[rob77]

the new format is great ! 

[Dr. Frank]

Hi Dave,

The new location and perspective (angle) of your videos is really very nice.

The background now is very pleasant (not so uneasy any more, as in the foregoing video), as you cleaned up the racks neatly.

On Long Shots, the picture pane should be a little bit bigger (more wide angled), as your head is cut off periodically. That's quite distracting, during watching your video.

The sound is quite ok, but could be better. I think it sounds a bit hollow now, maybe you had more trebly (?) sound in other videos.

Frank

[Laertes]

I am very curious how did he got solder mask between those pins. I have used IC in the same package recently and I was not able to get solder mask because of the small pitch. I have used the same PCB manufacturing service as that guy.

Datasheet for IC recommends:
 - Distance between pads - 8 mils
 - Pad width - 17,5 mils

PCB manufacturer's rules:
 - Min solder mask expansion - 3 mils

So we are left only with 2 mils for solder mask.

These boards were made by OSHPark, apparently. They don't really specify minimum soldermask width but I'd venture a guess it's going to be 4mils. Make the soldermask expansion only 2 mils and there you go.
In my experience, soldermask expansion specs are pretty loose by the manufacturers, with OSHPark or my local source here, I've never run into an issue with 2 mils or occasionally just 1 mil. My local manufacturer actually recommends going for 4mils.

[Laertes]

Oh and I forgot: The new format is great! 

[kcs]

Laertes, yes, I know it is OSH Park. And yes, they do specify min solder mask expansion, just load DRC file they provide.
My points was, if I follow the DRC rules, I cannot put solder mask between those pins. Of course I can choose not to follow them and accept the risk.

[Alexei.Polkhanov]

Those MOSFETs are impressive but 0.19 ohm RDS On won't put them into "world best in TO220 package" category. I recently ordered
IRFB41110 ($6 on Digikey) - RDS ON is 3.7 mOhm - that is milliohm. http://www.irf.com/product-info/datasheets/data/irfb4110pbf.pdf

[rob77]

Those MOSFETs are impressive but 0.19 ohm RDS On won't put them into "world best in TO220 package" category. I recently ordered
IRFB41110 ($6 on Digikey) - RDS ON is 3.7 mOhm - that is milliohm. http://www.irf.com/product-info/datasheets/data/irfb4110pbf.pdf

i bet those 3.7mOhm are not rated for 600V , are they ?

[free_electron]

pff. the lab is too clean  if a cluttered desk is a sign of a cluttered mind, then what is an empty desk a sign off.
a lab is oly a lab if there's oodles of cables running between and to/from instruments and a mystery board.

equipment sitting on a shelf means it is not being used, so basically ballast and occupying valuable space. time to clean out the 'surplus' and give it away  > ( no, i don't want any , unless it's got a HP or Agilent logo on it, and even then if it's got the old clickety pushbuttons i don't want it. )

[Laertes]

Laertes, yes, I know it is OSH Park. And yes, they do specify min solder mask expansion, just load DRC file they provide.
My points was, if I follow the DRC rules, I cannot put solder mask between those pins. Of course I can choose not to follow them and accept the risk.

Sorry, I can't view that due to lack of Eagle(why don't they put all the info on their site... ). So that's expansion, do they specify minimum solder mask width(or sliver, as Altium calls it, or whatever Eagle calls it)
Well anyway, from experience: Yes you can go with 2 mils and I've definitely soldered a few hundred pins with 2 mils on OSHPark boards and I can't recall a single issue with soldermask...

[rob77]

@ free_electron:
i think the stock of equipment on the shelves is an excellent background for the videos. i would definitely keep it as it is

[sonnytiger]

I am almost certain I used to have the thinner version that Dave spoke of, I had it when i was a kit, it was awesome. I wish i still had it, haha.

[German_EE]

    

Three thumbs up for the new video format. I presume it's just camera angles that's stopping you from doing mailbag teardowns the same way?

[max666]

That universal dimmer is interesting, I would have expected only a Phase-fired Controller with a triac a diac two caps and a couple of resistors, but that thing is packed with components. 

[djacobow]

I'd be interesting in learning more, too!

I've built a few AC dimming type devices now, and none were exactly this complex. I've done reverse phase angle devices using just a TRIAC (and I've torn down a few wall box dimmers that work this way) and I've also done the FET-in-a-bridge type. In both cases, I used a opto to do ZC detection and the timing for firing the dimmer was done by a microcontroller. I guess the uC can replace a lot of bits and bobs of analog. It's kind of insane to use a computer to do the job of a few transistors, but for a hobbyist, it's definitely cost effective and the computer enables some really cool features like "soft" on/off.

Anyway FET-in-a-bridge enables forward phase angle dimming (or any shape, really: tails only, peak only, etc) but I'd be curious to know why they needed two FETS. Are they in parallel to reduce Rds?

I wonder if this device actually measures the current waveform and does some adaptive magic to make the dimmer just work "better" with various loads.

That universal dimmer is interesting, I would have expected only a Phase-fired Controller with a triac a diac two caps and a couple of resistors, but that thing is packed with components. 

[superUnknown]

So how is that dimmer actually working with only two mosfets? Those diodes ensure that each fet sees half the waveform and then in turn the uC clips it to suit?

[djacobow]

It can work with just one. You put the FET at the "DC" terminals of a bridge rectifier. The FET always sees voltage the same way. The load is in series with the "AC" terminals of the rectifier. In my experience, driving the FET can be somewhat complicated depending on what you are referencing your driver circuit to.

I can see using two FETs in a not-bridge rectifier, where each handles on half of the cycle. This might be better; I haven't really thought about it.

So how is that dimmer actually working with only two mosfets? Those diodes ensure that each fet sees half the waveform and then in turn the uC clips it to suit?

[max666]

Looking at the bottom board in Daves video, I can not see how those four diodes can form a bridge rectifier, I mean the AC input goes straight to the centre pins of the MOSFETs, which would be drain.

I think each MOSFET handles one half of the cycle, like djacobow suggested.

[smashIt]

dave, the first grapghics calculator was the fx-7000g, not the fx-7000gb

i own a fx-7000ga, so there were quite a few versions of the fx-7000

[FreudianSip]

The B you see in German street names is actually not a B- it's pronounced as a double s sound. StraBe is pronounced "strasse"!

[david77]

That Ohmmeter is called a "Leitungsprüfer" which literally means line tester. These crude devices were commonly used by telecomms people to check their POTS wiring, relais and strowger switches back in the day. A bit more sophisticated than a bulb and battery but not in any way a precision instrument. West German companies like Gossen, Metrawatt, Siemens also made similar devices but not until 1989 I suspect.

The 2R10 battery was quite common and usually called a flashlight battery in Europe, it contains two 1,5V cells stacked in a paper or plastic shroud, these single cells fit into these meters.

I nabbed a pic off the interwebs:


I'd suspect the case is bakelite or something similar, so I don't think it'd really survive a fall...
These things were mostly obsolete by 1989, at least in the west. It probably had something to do with the unique way East Germany was run that they still made them to the end.

Also quite interesting to note who made that thing: VEB Klingenthaler Harmonikawerke
That was a state owned (VEB means Volkseigener Betrieb - state owned company) maker of musical instruments mainly harmonicas, a bit of PA gear and electronic organs. Again, a sign of a state managed economy.