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Stupid question but.....help!

Discussion in 'General Electronics Discussion' started by rockrockmcrock, Dec 19, 2011.

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  1. rockrockmcrock

    rockrockmcrock

    41
    3
    Dec 19, 2011
    Hi guys,

    I'm hoping someone on here can answer this simple question - I usually work with classic cars and the "electronics" are somewhat more robust than PCBs!

    OK - I am also a radio control modeller and have messed about with simple PCBs for my transmitters. Now I have a fairly old 'high end' transmitter, whose internal clock/memory is driven by a lithium cell. If the cell is removed the memory settings for all my models are lost. After about 5 years these BR2032 batteries tend to die, so they need replacing. A simple job right?

    But what if I want to retain my memory settings in the PCB chip? As soon as I remove the battery to replace it they're gone.

    So could I either:
    1. simply wire another 2032 cell in parallel with the old original cell and not replace the original.
    2 simply wire another 2032 cell in parallel with the old original cell before pulling the original from the circuit....

    .....or am I going to screw up a chip in some way I don't understand. Everything I know about electronics says it should work and do no damage - but what I know about PCB electronics is very little. :rolleyes:

    All help gratefully received! Thanks guys.
     
  2. daddles

    daddles

    443
    3
    Jun 10, 2011
    One answer is to put a capacitor across the battery terminals. This cap will store and supply charge to the circuit while you're changing the batteries. To pick the voltage of the capacitor, look for something around twice the fully-charged battery rating. To pick the capacitance needed, you need to know the current draw of the device and how low the battery voltage can drop before the device quits working. Since you may not want to spend the effort in finding such things out, you then have to use an educated guess. Since this thing works for multiple years on one battery, I'm going to assume the current draw is pretty small (in the uA range). Thus, my intuition says I'd grab a cap of around 100 uF and that would probably be OK. But the only way to know for sure is to test.

    I would not recommend putting a fresh battery in parallel with a discharged battery because the discharged battery will become a load for the new battery. Unless the batteries are rechargeable, this can lead to bad things. The way around it is to use a switch (with form C contacts (SPDT)) so that only one battery can be in the circuit. To avoid power loss during the switching, include a capacitor as above.
     
  3. rockrockmcrock

    rockrockmcrock

    41
    3
    Dec 19, 2011
    Thanks mate for such a swift reply - PCB's still make me nervous and it all seems like a bit of a black art at the lower currents/voltages/weird chemistries ;)

    Good idea - I understand the principle of capacitors and at some point in the distant past even some of the chemistry from my degree. Some of the 'practicalities of use' I don't really get but hey that's what the web's there for!

    This seems a stupid question but wouldn't the capacitor load the circuit whilst charging and possibly cause a voltage drop on the old 2032 cell below the level needed to maintain memory persistence in the chip? Is this a risk? Can capacitors be 'pre-charged'? I've always thought of them as the same as a battery really...sad I know....

    I think your intuition seems spot on - quick calcs based on a standard 2032 of 190mAh and min lifespan of 5yrs:

    5*365*24 = 43800 hrs in 5 years
    190mAh / 43800 = 0.00433789954 mA = 4.34 uA current draw
    BR2032 chemistry is a little different from CR - the nominal current stays very close to 3v before a sudden precipitous drop at end of life - hence min voltage for persistence must be close to 3v (2.8v?) at a guess.

    So from what you're saying 2032's are 3v so I'd be looking for a capacitor at about 6v, 100uF rating at a guess?

    If I remember my electronic symbols rightly it'd suggest capacitors aren't 'handed' like a diode and can be fitted any which way round?

    Ah! This did cross my mind if it was left in parallel permanently. I was thinking more of the following procedure as an option:

    1. fit new 2032 cell in parallel with old.
    2. once fitted, desolder old 2032 leaving only the new 2032 cell connected

    Is this feasible or when you say "bad things can happen" do those bad things happen fairly immediately!! :D
     
    Last edited: Dec 19, 2011
  4. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Hi there. rockrockmcrock.
    I would opt for the parallel battery, the time you spend popping one cell out and replacing another is hardly going to run in to minutes, so bad battery scenario wont come in to it.

    Find a coin cell holder and some thin leads and a couple of crocodile clips, place the cell in parallel pop out the old cell, put the new cell in, remove CC clips job done, as you dont know the current drain the capacitor idea is a gamble, 100uf might buy you the time, but i would opt for a bigger value capacitor if your going down that route, at least 1000uf, even then you might have to be quick, parallel batterys of different strengths only become an issue if permanent, and yours wont be, the battery idea is fail safe, ie, time is on your side. :)
     
  5. rockrockmcrock

    rockrockmcrock

    41
    3
    Dec 19, 2011
    Thanks mate - for me it might run into minutes as it's a tricky desolder from the board but only 5mins or so, which I guess shouldn't be an issue? Mind you I could always just cut the pins. And I got all excited and researchy about the capacitor idea too :) I even found the discharge/load curves on a tech sheet for BR2032's which suggest a min persistent memory voltage of approx 2.8v

    As an aside - I really do have to get serious about electronics sometime and learn it - it's right up my street! The problem is all the little in's and out's you're unsure of - a good primer is probably what I need if anyone has any 'old school' made out of paper book recommendations :D
     
    Last edited: Dec 19, 2011
  6. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    As i say the capacitor idea will work, but the battery idea will not leave you with time constraints.
    I live in the UK, my modest knowlege came from choosing a hobby due to life events meaning i had to hang up my busy out door hobby life style, Anyway ive always been interested in electrical electronic stuff since i was a kid, but only took it up more seriously about 7 years ago, most of my interest lay in analogue electronics and circuit building, at the time i trawled through every book my local library had to offer, as i could take them home for 3 weeks at a time, then return them and grab more material.

    Ive yet to branch out in to more digital control systems, i would say a couple of good reference books, and use a library if you can access one, it will broaden the topics you study and not dent your wallet, plus of course the online stuff.

    I saw your battery plight as disarming a bomb, to many films ha ha. :)
     
  7. rockrockmcrock

    rockrockmcrock

    41
    3
    Dec 19, 2011
    For me electronics has always been the missing link.
    My life has been about hacking stuff to get it to do what you want.
    80s - hacking code
    90s - hacking networks
    00s - hacking companies/business processes (I'm a 'specialist' management consultant of a sort!)

    I've retained many of my early career skills in coding and often think "I could write some code to do that, but have no idea how to design the hardware...damn!"

    As I say - digital electronics is the missing link for me that I've never got round to 'forging'

    Analogue - that's an art form - constantly glad of it every time I play my old Ibanez guitar and wonder at the rich sound of my real amp compared to my PC's 'software based' virtual amplifier

    Strangely enough so did I - but worried about the subtleties of micro chips
     
  8. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Well your circuit is important, you really dont want to kill it.
    Ive not to much problems in the hardware side of things, before becoming ill most of my working career was in metal work and timber construction, and the GRP molding industry.

    My PC/ IT out set was a push from my kids, ive been on a PC for about 2 years, first time embarrassing my eldest daughter, shouting over the library floor, how do you work one of these things, bloody thing wont work, funny now. I lack the software skills, programing and writing in PC jargon, or what the system understands, ha ha i dont totally yet, i am getting there be it slowly.

    Anyway good luck with the battery transition, it should be fine. :)
     
  9. rockrockmcrock

    rockrockmcrock

    41
    3
    Dec 19, 2011
    Sounds like we're on opposite sides of the same coin - coding's easy and logical, you'll have no issues and it's universally useful. Now cryptography & Steganography.....those are challenges that just suck you in!

    Thanks for all your help mate and good luck!
     
  10. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    I would opt for some quick disconnects. Solder two in parallel and then when you switch the batteries, the couple of seconds the new one is plugged in and the old one is removed would negate any bad effects, as even a battery needs time to heat up before it explodes. :)

    How long can it possible take to unplug a battery after you have the new one plugged in? You could still do this with a cap in parallel as well. But I would get rid of the sloppy soldering job, who does that these days every time they need to install a new battery?
     
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