car remote draing battery 20uA

[boo9]

I have put week or so new battery and it stopped working,  no LED after press, works after putting new battery in.

I checked and the remote is using 20uA when not pressed.

With battery removed, I have 15K resistance on battery terminals.
C1 (upper right corner) shows aroud 20K resistance.

Is C1 kaput ?

What should be the value for C1 ? 90nF ?

[wraper]

Is that battery CR2032? Considering 220 mA/h capacity, with 20uA current it will last more than 400 days. Even if it have 10x lower capacity, it would last much longer than a week.

[SeanB]

I would go for any value from 1uF to 47uF ceramic 16/ 25V capacitor there, whatever you have in that size or with that spacing that will physically fit in the space. Scrub the board with IPA and dry thoroughly as well, to get rid of the fluc residues. Check the switches are not leaking as well. The higher value cap will help prolong battery life as it will better decouple the supply as the battery ages.

It likely is the capacitor leaking, and as many of these remotes use a CR2016 to save thickness they often do not have much spare capacity to begin with. A CR2032 works better, but might be too thick.

[boo9]

CR2032 it is.

54uA (micro) shows now. with my $5 meter

The reason I suspect C1 leak because couple weeks ago I had garage door remote with a leaking cap but then the LED was lit up slighly all the time,   the cap at the bottom, I replaced it with 90nF and problem (LED lit) went away. Should I be replacing the 90nF with something larger ?

[SeanB]

100n, 220n or 470n that will fit, with a 50v rating will work there. it is there for bypassing, not anything else, and in this application the exact value is not critical. I have added 10uF 25V tantalums in some to get better battery life, they work till the battery gets totally flat and no longer has an open circuit voltage above 5V.

[boo9]

> I have added 10uF 25V tantalums in some to get better battery life,
these would be THT not SMD, would they not ?

and why higher caps would prolong battery life ? Me thinks that if the circuit needs x Watt to function then no tricks will make it work with x/2 energy.

[tautech]

I have recently fixed one that was doing similar.
All components tested OK.
This remote had never been right and owner was using the spare one.

Found a "solder paste ball" of solder stuck to the overlay that was causing the current drain.
It was tucked very close to a device and easily missed but should have been found with post manufacturing testing, no doubt there was none.
Good magnification and a scalpel blade fixed it.

[wagon]

Remote controls get a fair bit of abuse and contaminants can build up on the pcb.  As was mentioned, clean the device with IPA.

[boo9]

No change after Cleaning with IPA.

Door garage remote I fixed with transformer soldering iron, can I use transformer soldering iron on that C1 cap or wait till the proper ESD safe soldering iron arrives ?

[David Hess]

> I have added 10uF 25V tantalums in some to get better battery life,
these would be THT not SMD, would they not ?

and why higher caps would prolong battery life ? Me thinks that if the circuit needs x Watt to function then no tricks will make it work with x/2 energy.

Higher voltage capacitors generally have lower leakage when used at low voltages.

[wagon]

No change after Cleaning with IPA.

Door garage remote I fixed with transformer soldering iron, can I use transformer soldering iron on that C1 cap or wait till the proper ESD safe soldering iron arrives ?

Just change the cap with what you've got.

[boo9]

I lifted the cap and with the cap removed I still have 20K on battery terminals. Cap measures open on 2000k range. Looks like cap is OK.

As far as I know tHis remote is not only for car door but is has immobilizer inside, may be 20uA is normal for idle immobilizer circuit ?
Anybody sees the immobilizer ciruciut on the pictures ?

[SeanB]

Clean the board and then apply flux and resolder the MCU pins, then clean again with IPA and try to get the last of the flux out from under the MCU. That likely will cure the problem of the flux residue being slightly conductive.

[jlmoon]

No change after Cleaning with IPA.

Door garage remote I fixed with transformer soldering iron, can I use transformer soldering iron on that C1 cap or wait till the proper ESD safe soldering iron arrives ?

If you don't have a return path to Vdd for that battery powered remote, there is a very slim chance (like nill) you can ESD that device.  I suspect the parts employed on that remote are not your typical ESD sensitive LSI parts anyway.  Just remove the battery and solder until your hearts content! 

[boo9]

> MCU
which one is the MCU ?  marked IC1 or SAW1 ?
should I lift the MCU then resolder o just reflow the joints ?

> Clean the board
any suggestions how to clean the board properly ? I have flooded the board profusely with  the IPA and brushed with toothbrush several times already.

[David Hess]

Water with a little bit of dish soap will be better than IPA by itself.

[SeanB]

IC1, you can remove it, clean it top and bottom along with the board and then resolder it. As well check the small transistor and the SAW resonator for flux residue. Dry the board thoroughly after scrubbing with the IPA, I use the top of a CRt Tv for ones at work, or just place it on a surface and aim a 12V PC fan at it for a hour or two.