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# LNMP Finishing touches - wiring the battery pack and enclosing - Page 5

Ha, I was going to cheat and do that anyway, since I had my voltage divider set up last night and I was sooo close.

This morning, I can't get it to work. I thought I could build a divider circuit. Is my math off? Here's my setup:

signal in - 1M resistor - meter probe red - meter - meter probe black - 1K resistor - signal ground (out)

I'm getting ... holy cats, is it the laptop not running on battery that is causing me to get inconsistent readings?

You must have missed my recommendation that the total resistance of the divider be around 1K.

And yes, you probably should be running the laptop on battery here, if only as a matter of principle. It removes a potential source of problems, so you don't have to even ask the question of whether it's a risk.

Edited by tangent - 2/20/13 at 10:13am

Oops. Bad math. Always the factors of ten, they are getting me. Well I have the divider correct (if ugly), and the input signal is just right:

Measuring at the output with SW1 jumped and unjumped, I get a difference that is a factor of ten.

So far, so good. This is what scares me from the assembly guide:

"The procedure to trim the amp’s gain is conceptually simple"

I'm going to re-(nth) read the next section... any hints?

Thanks, Tangent!

-Sara

What scares me is that this is what I get out of the output:

Final PICS!!!

I have a feeling I'll be searching for or starting a new thread on.... how to use it!

So you pushed on through the calibration and found out what was giving the odd reading, and corrected it?

Hi! Back from the dead. Actually I got pregnant and I don't work on projects with a passenger on board. But he's weaned now so I'm anxious to go back and familiarize myself with everything and let you know... I've been going crazy missing the work. Apologies for the disappearance, but propagating the species makes me into a narcoleptic who cries, barfs and yells every day all the way through. I thought I'd spare as many innocents from that as possible. Tangent, are you still out there?
Edited by saraengelstad - 3/28/15 at 4:14am

Here's the product of my distraction.
Quote:

Tangent, are you still out there?

Yes.

I'm still uncertain whether you finished building and testing the LNMP, and whether it works correctly.

I am eager to finish but haven't.
Just a day or two after my last post was when we got the ah, surprise baby news. It was quite a shock and derailed all sorts of things for a while. Again, my apologies for leaving you hanging.

I tried picking it up last summer, spent a day performing initial trials on the units the LNMP is meant to be used on, to weed out any that might have obvious problems. I think you used the word 'exciting' at one time to describe a cap failing? It certainly is. A tantalum popped just like a firecracker, sprayed my chest and neck with tiny bits that I had to dig out of my skin. While I did that, I considered how I was still breastfeeding and worried enough to put it aside until that was over.
But now I'm finally free to get back to it. I'm brushing up now... so much fell out of my brain that I need to study a few days.
I'm including a pic of the largest bit of that cap I could find. It WAS a thrill!

Safety glasses are our friends.
Edited by saraengelstad - 3/28/15 at 8:32pm

The cap almost certainly popped because you put it in backwards. The instructions in the LNMP assembly guide told you to be sure to get the polarity right, but you either missed that, or you didn't understand the significance of what it was telling you, or you didn't look up the meaning of the markings on the cap you chose.

If you're trying to learn to swim by jumping straight into the deep end of the pool, you're going about it the right way.

If you're just trying to get a working LNMP, there are probably more expedient paths to your goal.

Apologies....I was unclear, the firecracker wasn't in the LNMP, mercifully. It was in one of the units the LNMP was built to examine.

I have a test procedure to help catch things like polarity errors. Care to poke holes in it for me?

Since I usually build multiples;

Step 1: Line 'em all up with the same orientation and inspect, checking for:
joint solder is shiny and flowed through nicely on both sides of the board
resistors should be oriented uniformly, markings readable, ratings identical.
diodes- all stripes on the same ends
Caps grouped by type, cans first, tants second, etc. Polarity and/or markings should all face the same way.
ICs- double check markings and orientation.
Test points all present
LED flat spots on the bases all oriented the same way

Step 2: check for consistent component altitude above the board. I used to crank them down tightly with pliers. Bad idea. I wasn't thinking about heat, expansion or other stresses. I got good desoldering practice after that.

Step 3: connectors and sockets look clean, good, right number of positions, etc.

Some other things- I run my fingers over the bottom of the board to find any irregularities - this helps me find iffy solder work, uncleaned flux, anything that stands out.
I look for anything crooked, leaning, or bent...

Unless I surpass myself and, for example, put ALL the diodes in backwards, this screen saves a lot of unpleasantness later.

Here's what remained on the board of my firecracker (pic)

Looks right to me... do you think it was a faulty part?
Edited by saraengelstad - 3/29/15 at 2:21pm
Quote:

I have a test procedure to help catch things like polarity errors. Care to poke holes in it for me?

Since I usually build multiples;

I would be happier with that procedure if you had a known-good instance in the group for comparison. As you say, it is possible that you could get them all backwards.

Quote:

Looks right to me... do you think it was a faulty part?

I doubt it. I would guess that the regulator isn't regulating for some reason, so that the full rectified DC voltage is getting through, and that voltage is too much for the cap.

If that's an LM317 you've got bent partly out of the frame, I would cut the remains of C8 off the board then power it back up again — still being careful! — and check the DC voltage with at those pins or between TP2 and TP3.

If it's some other type of regulator, the cap may be necessary for its reliable startup, so you may not be able to do this test without replacing it first. If you do, be prepared for a repeat performance.

She already said she had to dig out bits of it from her neck and chest!  Maybe this would be prudent advice: Do Not Use Tantalum Capacitors.  Replace that thing with a ceramic, film or electrolytic - whatever has a similar rating and can be pigeon-holed in those pads.  Perhaps Tangent can help with an alternative.

Just my personal opinion, but I think tantalums should be outlawed - certainly for DIY.  They're too dangerous.

P.S. Nice Rugrat, Sara!  I had two female versions of those, but it's been a while.   Anyway, good luck on the LNMP.  It's a great device and it looks like you built a great example of Tangent's design.

We have so many different capacitor types because they each have their advantages and disadvantages. Typically the only thing that can replace a tantalum is an aluminum electrolytic, which is much more susceptible to heat, a serious consideration for that particular cap, being right next to a voltage regulator.

Cap types that aren't still useful typically get dropped from the manufacturers' lines. Tants are still being made despite the difficulty of getting the materials because they have some uncommon properties that makes them more useful in some areas than their closest alternative.

Tantalum capacitors have other properties that make replacement with an aluminum electrolytic difficult. The National Semiconductor LM317 datasheet says that you need about a 25 µF aluminum electrolytic to give equivalent performance to a 1 µF tantalum. If the problem here is overvoltage, though, that will also kill an aluminum electrolytic cap.

Edited by tangent - 3/29/15 at 9:21pm
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