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Questions relating to designing PCBs
By HarrisonHJones
#49587
Lots of "firsts" for me.

First PCB:
I just got my first PCB board I've ever designed from BatchPCB. Interestingly, I got three of them instead of the two I ordered which is cool. Sadly, I only skimmed BatchPCB's eagle tutorial and forgot to widen the silk screens so there's no pretty white text of them(More on that later) but I can live with that. I received all my components for the Li-ion charger(Based on a Max1555) about three weeks before today and I was overjoyed when I opened the little package containing my PCB's. Then.... I remembered that in my insistence that my charger look "real" I used all SMD parts(except the dc jack) for the design. This leads me into my second "first"

First SMD Soldering:
I felt pretty confident soldering "normal" components(ie, the big stuff). I have soldering lots of things before but I had never soldering SMD. To get ready, I read up on alot of articles. In the end I decided to try to solder all my SMD parts like I do my "normal" parts: Clean the tip of the soldering iron. "Wet" it with allittle solder, put alittle solder on one of the SMD pads, use tweezers to position the SMD part, and tack down the part with the small amount of solder I put on the one pad. Once the SMD part was down I wet my tip again and put it to the other pins, the solder wicked up, and the part apears to be soldered. After all the parts were soldered(Mini USB series B's are hard as hell to solder w/o bridges by the way) I had my completed USB/DC 1Cell Li-ion Battery Charger based on the Max1555.

First Li-Ion Charger,
With the charger soldered, I of course wanted to test it. I got out my RCR123A rechargable 1 Cell Li-Ion battery(from BatteryStation), checked the polarity, and wired it to the charger's Power and Gnd pins. I plugged in the USB cable and.... Nothing happened. The light didn't turn on but on the bright side, nothing smoked, or blew up. I was a little sad but then decided that if anything was to go wrong, it was that I put in that pesky SMD LED in backwards. So, I checked the polarity, both ways, the LED didn't light up. I thought: "DAMN, I overheated that stupid resistor" and I bypassed the resistor and the LED lit up. After a little more experimenting, I found out that the resistors I ordered were 330 times too resistant! I needed 1k I ordered 330K. Stupid me. luckily I had a few spare SMD resistors from a salvage mission(gotta love RC cars) and I soldered one in that worked at 3.3v(ie the LED lit up w/ that resistor w/ 3.3v). I went back to my station and the little thing worked! The led lit up when connected to power. I was so overjoyed that I couldn't wait to show it to my brother. I took it in the other room, hooked up the battery and was suddenly hit with an intense "holy **** it's hot down here" sensation from my finger. Seeing smoke and feeling heat impulsed me to quickly remove battery power: I had reversed the stupid polarity(sounds like a crappy star trek episode) because I forgot which was power and gnd(Darn that missing silkscreen). Thinking that all was lost I made a "oh hell, what could it hurt" decesion and reconnected it correctly; It seems to work: the led came back on and there is no smoke.

So, are my thoughts:

Firstly: How do I check to see if the charger is actually charging? The LED is on, but is that good enough? Is there another way to check?

Secondly: Does anybody see a problem with my way of soldering SMD? I don't use solder paste. Just a fine tip soldering iron, some solder from radioshack, and some tweezers. Also, how can I removed the "residue" left behind from the soldering?

Thirdy: While this particular board will never see the "field," does anybody think I should be wary of it even though it seems to have survived the reversed polarity without a scratch? It seems to work, but I'm still worried it might come back to bite me later. Any thoughts? I would also love a suggestion for reverse voltage protection.


Regards,
Harrison



Schematic and PCB can be found: http://a-i.co.cc/max1555breakout.zip
By Philba
#49590
congrats.

testing to see if it's working - if you have a discharged LiIon, you should see the voltage across the battery terminals rise as it charges. Read the datasheet - there is a graph of dc charge current vs Vbat. Stick a DMM on the battery and see what it does as it charges. In particular it shouldn't exceed the max charge voltage. If you can get a second DMM, put it in series with the battery to measure the current and compare against Vbat. That's the best way to know if it is really working. I believe there is a maintainence charge that goes on but don't recall the specifics. You probably want to look at CHG as well.

Your soldering method is fine but you want to use magnification to inspect to make sure you don't have bad joints or bridges. IsoPropyl Alcohol gets the flux out. Solder wick gets the excess solder.

should you be wary of this board? LiIons are quirky beasts - yes, I'd be wary. While the max chip is probably going to give you good performance, you never know. I wouldn't leave it unattended. You are very lucky you caught the polarity reversal - that could have started a fire.
User avatar
By bigglez
#49595
Greetings Harrison,
Congrats on your project, and thanks for posting a detailed
report!
HarrisonHJones wrote:Firstly: How do I check to see if the charger is actually charging? The LED is on, but is that good enough? Is there another way to check?
As Phil suggested, the best way is to monitor the Li-Ion
voltage and current with two DMM (or other meters).
HarrisonHJones wrote:Secondly: Does anybody see a problem with my way of soldering SMD? I don't use solder paste. Just a fine tip soldering iron, some solder from radioshack, and some tweezers. Also, how can I removed the "residue" left behind from the soldering?
That's a good technique, I did something similar with
SMTs as first. One thing I found that surprised me was
that SMTs require paractically no solder to make strong
connections. In fact one can swipe the PCB pads with a
flux pen and just heat the component pads with a hot
soldering iron. There is enough solder plating on the
PCB to bond the component. So if you don't have one,
buy a flux pen.

To remove excess solder use solder wick (aka braid),
and to remove excess flux I use M.E.K. on a Q-tip swab.
HarrisonHJones wrote:Thirdy: While this particular board will never see the "field," does anybody think I should be wary of it even though it seems to have survived the reversed polarity without a scratch? It seems to work, but I'm still worried it might come back to bite me later. Any thoughts? I would also love a suggestion for reverse voltage protection.
Be very careful around storage batteries! The
newer types (Li-Ion) have much greater energy density
and they will explode and or catch fire if abused.

The MAX1555 is a good choice for charging, but you
didn't mention discharge control. It's important not
to drain the cell below the rated cut-off voltage (varies
slightly by brand and capacity). Doing so can damage
the cell. Luckily, the MAX1555 has a low charge
current mode if the cell is heavily discharged.

There is an ASSP chip from Texas Instruments to
monitor the Li-Ion battery, take a look at UCC3911

The best (as in simple, reliable) reverse battery protection
is a series diode. Unfortunately it drops about 700mV,
and wastes energy. Another method is to place the
diode in shunt with the device, so as to cause a
short under reverse polarity. This proects the
device but can damage the battery unless a fuse
is also added in series. Finally, a zener diode in
shunt can give reverse polarity and over-voltage
protection. I used this to head off my own stupidity
as I have both 5V and 12V wal-warts which each
have the same 2.1mm size barrel plug. A 5.6V
1.5W zener is cheap insurance to stop 12V from
reaching the five volt only projects.

Without the PCB silk screen you can mark your
PCB with a Sharpie to indicate "Pin 1" (I usually
do this for all connectors to indicate pin 1, and
match up with the stripe on ribbon cables or a
second Sharpie strip on the free connector shell.

Comments Welcome!
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