EEVblog #353 - Heaps o' mail

[nitro2k01]

[nitro2k01]

Comments on the video:

Dave needs to be better at interpreting symbols on stamps. The airplane means it's an airmail stamp. The Swedish stamp has a butterfly symbol, and sure enough, those are closeups of butterfly wings.

THe battery charger letter mentions it can charge at 500 or 100 mA per hour. No! Ampere is already a "per time" unit. Saying x mA per hour is like saying 70 km/h, per hour.

Then there's that unused opamp on the IO board. The problem with tying both pins to ground is that the ground on the non-inverting input (with no feedback) will make the opamp want to drive the output low, while putting ground on the inverting input (again, with no feedback) will make the opamp want to drive the output high, as near the positive rail as possible. Just about any interference, including changed load or input signal on the used opamp, can make the unused opamp go into hysteresis and go from one state to the other, which can cause problems.

My recommendation would be to the non-inverting output (+) to ground, or whatever else you like, such as the same input as the other opamp, and connect the the output to the inverting output (-). Ie, connect it as a voltage follower. This should avoid any problems.

[SeanB]

Also using it as a voltage follower and using it as a buffer amp will help slightly with the filter, at the expense of a slightly higher noise figure and a poorer response to transients, along with not quite being able to get to 0V as close as before.

[nitro2k01]

SeanB, I mean simply letting the second opamp follow the input signal, but not connect the output anywhere (except the feedback loop as mentioned.) Or letting it follow ground. Anything to avoid the potential problems involved in tying both + and - to ground.

[SeanB]

I know, but to leave a reasonable opamp unused..........

[samgab]

1. Those Wiha drivers are great. Excellent brand and quality. I gotta get some really nice good quality screwdrivers soon. Thinking either Wiha or Wera.

2. I don't know if that new one is a genuine Apple branded charger or a Shenzhen copy, but here are some teardowns of originals. See if it looks like this inside:
http://www.arcfn.com/2012/05/apple-iphone-charger-teardown-quality.html


3. I really like the idea of soldering kit videos, with a bit of off the cuff commentary along the way.

[samgab]

PS, this is how the markings should look:


Also, Flextronics' patent for this:
http://www.google.com/patents?id=x3jnAQAAEBAJ&zoom=4&pg=PA1#v=onepage&q&f=false

[Precisiontools]

Whoa! Whoa! Did you loose your PB Swiss drivers Dave? When did this happen?

[poodyp]

I chuckled when I saw that lipo charger, cuz I saw it here and thought I should share it, but forgot about it. I think 12.85 is a fair price considering the likes of sparkfun and adafruit charge $10-15 for the same. Also it looks like he used the OSHPark PCB service, but I'm not sure they deliver to germany.

The rest of the site certainly looks interesting, like an etsy for electronics.

Check out the set of Wiha screwdrivers here, $30 for a set of 6 is pretty awesome. Back when I was working on low voltage wiring my co-workers kept telling me to get klein tools, but those're like $10 to $15 a piece, and like $25 for the insulated ones .

[EEVblog]

Dave needs to be better at interpreting symbols on stamps. The airplane means it's an airmail stamp.

We do not have such monetary "airmail stamps" here.
You get regular stamps, and a separate airmail sticker.

Dave.

[Psi]

Whoa! Whoa! Did you loose your PB Swiss drivers Dave? When did this happen?

He mentioned that he couldn't find them in a previous video.

Typical lab problem, they eventually turn up

[nitro2k01]

I know, but to leave a reasonable opamp unused..........

Well like you said, it would reduce voltage range and increase noise ever so slightly. I would personally have broken these out with solder pads or similar, and connected them as described by default with cuttable traces, so the unused opamp could be reused by someone who wanted to.

A good to use the second opamp in series might also be some sort of improved filter topology, but I think a simple Sallen-Key or similar active topology using just one opamp would be superior to the current one pole LP, still without using the second opamp.

Henrik, are you reading this thread, btw?

[nitro2k01]

Dave needs to be better at interpreting symbols on stamps. The airplane means it's an airmail stamp.

We do not have such monetary "airmail stamps" here.
You get regular stamps, and a separate airmail sticker.

Dave.

Yeah, I don't live in the US, and I had never seen that but I thought it was immediately obvious. Why else would there be a plane on the stamp. (Yeah I know, 20/20 hindsight and all that.)

[EEVblog]

Yeah, I don't live in the US, and I had never seen that but I thought it was immediately obvious. Why else would there be a plane on the stamp. (Yeah I know, 20/20 hindsight and all that.)

The plane could have had some symbolic meaning for the state or county or something, I don't know.
Just for airmail seems like a silly idea to me.
e.g. you'd have to ultimately produce both "airmail" and "non-airmail" versions of the same monetary value stamp.

Dave.

[H.O]

Henrik, are you reading this thread, btw?

I just did ;-)
Thank you all for the feedback regarding the OP-amp. Using a single pole lowpass filter serves the intended purpose here. The PWM frequency being put out by the stock firmware is ~8kHz so it's well above the cutoff frequency of that simple filter. The intended main use of the analog output of the board is to drive an analog input on a variable frequency drive or other motordrive so, like the text on my website says, the circuit serves its purpose even though it's not super accurate and/or ultra linear.

Repurposing the board for some other use than it was initally designed for could of course benefit from a better filter....next revision!

Geez, tying both inputs of the spare OP-amp to ground obviously wasn't a good move on my behalf. I haven't actually noticed any problems but that's definitely not a good design decision. Definitely a change for next revision, thanks alot for pointing that out!

/Henrik.

[Bored@Work]

Just for airmail seems like a silly idea to me.
e.g. you'd have to ultimately produce both "airmail" and "non-airmail" versions of the same monetary value stamp.

Not if the postage for airmail and surface/sea mail are different. You print the little plane on the stamps for airmail postage, and that's it. Then everyone knows to put the right postage on airmail.

But even if some stamp values fit well on airmail and surface mail, it isn't really an issue to print different versions. Any self respecting postal authority prints heaps of different stamp series with the same value. Collector series with limited print runs like http://shop.auspost.com.au/collectables/stamps-issues and the common large print runs.

All a postal authority has to do is to look into their statistics to learn the distribution of how many airmail and surface mail they transport. Then they print plane and non-plane stamps using the same distribution.

[robrenz]

I live in Lancaster county Pennsylvania.  That stamp looks like the view from my house.