3v Charging and Power Circuit

Discussion in 'The Projects Forum' started by bluline, Sep 13, 2015.

  1. bluline

    Thread Starter New Member

    Aug 3, 2012
    8
    0
    I searched the threads, but, because I am new, I am not sure what I am looking for, so I apologize if this is already covered.

    Here is my situation. I have a slot machine whose motherboard requires constant power. In casinos, the slot machines are never turned off, but for home use, I want to be able to turn the slot on an off. The MB was not designed for this use, and the battery drains, erasing all settings.

    The motherboard is set to take 2 AA batteries to keep the charge, but these drain quickly. Inside the slot machine is an outlet with constant power, regardless of the on/off state of the machine.

    What I would like to do is...

    Insert 2 rechargeable AA batteries, connected to an AC power supply, plugged into the constant power. I'd like these to charge to full when power is available (trickle charge?), as well as provide power to the MB. In the event of a power failure, i'd like the MB to draw power from the batteries.

    I have created small LED circuits, so I think I can do it, but I have no idea what the circuitry required is. Can anyone help?

    - dc
     
  2. ScottWang

    Moderator

    Aug 23, 2012
    4,858
    767
    Could you draw a block diagram and take two or three photos, it might help members understand it quickly.
     
  3. bluline

    Thread Starter New Member

    Aug 3, 2012
    8
    0
    Thanks for the reply. Attached is a picture of the Motherboard with batteries, and a diagram (not sure what a block diagram is) showing what I am looking for.

    Any 3v rechargeable would work, it does not need to be AA rechargeables. Not sure how many volts the DC adapter should be for this. The Motherboard takes 3v.

    Thanks for the help.

    - dan Slot Power.png Slot MB.jpeg .
     
  4. bluline

    Thread Starter New Member

    Aug 3, 2012
    8
    0
    Does anyone have any thoughts on this? Is there more information I could provide?
     
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
    1,252
    Do you have any idea of which devices are powered by the battery, and what the actual acceptable "battery" voltage is? A classic way of doing what you want is to have a DC supply that is slightly greater than the battery terminal voltage, and simply combine the two through two diodes. When the DC supply is awake, the diode to the battery is reverse biased. When AC power and the DC supply output go away, the battery's diode forward biases instantly to provide holdup power.

    Things get more complicated with rechargeable batteries because the terminal voltage under float conditions is higher than the output voltage when the charger goes away and the battery is supplying power. Linear Tech and others make battery changeover devices, but how good are you at hand soldering surface mount devices?

    Knowing the max limits of the power system will help rein in the possibilities.

    ak
     
  6. ScottWang

    Moderator

    Aug 23, 2012
    4,858
    767
    This is an idea:
    Using LM393 comparator to detect the voltage of adaptor, when the voltage great than 2.8V, it means that the voltage of motherboard was came from adaptor, so the bjt drive a relay to select the power come from adaptor or battery, because the battery power can't keep too long, so the contact of n.c. of relay should be connect to battery and the n.o. contact should be connect to the power from adaptor, and the com is connect to motherboard.

    adaptor → lm393 → npn bjt → relay → n.c. to battery, n.o. to adaptor, com to motherboard

    Another problem is power, maybe you can choose a 9V adaptor and using two lm317 to adjust the voltage to suit motherboard and battery needed.
     
  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
    1,252
    IF (big if) the rechargeable batteries are NiCad, then this is a possible power path.

    AC/DC adapter = 5 Vdc > diode > trickle charge current limiting resistor > batteries > motherboard.

    NOTE (big note) - The batteries are acting as a voltage regulator. They must be in place or the output from the AC adapter will kill parts.

    ak
     
  8. bluline

    Thread Starter New Member

    Aug 3, 2012
    8
    0
    Thank you both for the replies. I am beginning to think I am in over my head.

    I am comfortable soldering, but not as comfortable understanding what you both describe. I honestly thought I was missing a simple way to do this (expected something along the lines of the diagram found in this link: http://www.planetvb.com/modules/new...mode=compact&order=DESC&type=&mode=0&start=10), but my inexperience is shining through. I do not know what the max power is, not what is getting powered.

    There is a kit available that does this for $150 (just splice red and black wires, then plug in), but my (incorrect) thinking that this would be simple and not worth $150, led me here.

    I will read up on your suggestions to get a better understanding. Thanks again!
     
  9. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
    1,252
    The schematic in response #2 on your link captures the basic idea. Your application changes the resistor size, but this is a good plan.

    battery_backup.jpg
    Because your battery is only 3 V, D2 should be a Shottkey diode to minimize voltage drop. I asked about the chips being powered because if the battery-powered load is small enough, you could eliminate D2 and connect the load ("beacon") directly to the battery. Besides lowering the overall circuit complexity, it lets the batteries act as a voltage regulator to prevent wall wart variations from possibly damaging the motherboard components with overvoltage. That is the real concern here. The "right" way is to have a small voltage regulator circuit in the place of D1 to ensure that everything downstream is protected.

    Let's try another angle. With AC power off and the motherboard running on a fresh set of batteries, how long do the batteries last?

    AND, what is the AC power source (wall wart) you intend to use?

    ak
     
  10. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,813
    1,105
    You could replace D2 with a pnp transistor to reduce voltage drop even further :-
    BatteryBackup.PNG
     
  11. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
    1,252
    Alec, I don't see a path that charges the battery.

    ak
     
  12. ScottWang

    Moderator

    Aug 23, 2012
    4,858
    767
    Adjust the HVR1 to make sure the output of LM317 set to 3.7V, assuming that the battery drop to 1V need to charge, calculating the charge current,
    I = 3.7V-(D4)-(AAx2)/100mA
    = 3.7V-(0.7V-(1V+1V))/100mA
    = 3V-2.7V/100mA
    = 1V/100mA
    = 10Ω
    P = V*I
    = 1V*100mA
    = 0.1W
    So Choose 10Ω/0.5W.

    The relay will turn on when the 9V adaptor is working, when the 9V lost the city power, the relay is not works, so during this time the power of battery was connected to the motherboard, all this just assuming and calculation, you need to according to the real situation to do some adjustment.



    3VChargeAndBatteryKeepingPower_ScottWang.gif
     
  13. InspectorGadget

    Active Member

    Nov 5, 2010
    211
    42
    A couple of questions:

    1. How long do the batteries keep the motherboard powered?

    2. When does the "constant power" outlet get interrupted, if ever? Does it stay on when you "turn off" the slot machine, but goes off when you unplug it to move it? Or does the "constant power"outlet get turned off when you turn off the slot machine?


    By the way, in the schematic above, the 3V to the Motherboard will be interrupted for a fraction of a second when the relay switches. There should be capacitors on the Motherboard to smooth this out but it wouldn't be a bad idea to look at the voltage level with a 'scope as it switches, and if it sags, test the current draw and put in a suitable capacitor to keep it from sagging too much.
     
    ScottWang likes this.
  14. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,813
    1,105
    There isn't one. Long life non-rechargeable cells assumed.
     
  15. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
    1,252
    Two rechargeable AA cells in post #1.

    ak
     
  16. bluline

    Thread Starter New Member

    Aug 3, 2012
    8
    0
    1) The batteries last about 3-4 weeks.

    2). The constant power turns off only when the machine is unplugged or during a power failure.

    I'd like to either use a low voltage AC adapter or power regulator, to avoid even the chance of killing parts. A capacitor may also be needed to avoid the voltage drop as you mentioned.

    The diagram from my link used 9v, but that was just an example. I don't understand the diagrams but will keep reading to get me up to speed.
     
  17. InspectorGadget

    Active Member

    Nov 5, 2010
    211
    42
    How long do they last when the power is off? How long will they keep the machine from losing its memory in one straight period of being unplugged? Is the answer still 3-4 weeks?

    You said you want to turn the machine off when not in use. Does this turn off the constant power?

    ScottWang's circuit includes a 3V voltage regulator (3.7V - 0.7V in either diode), so it can't fry the parts the way a standard trickle-charger can. It's more complicated, but safer.

    Keep learning, and keep asking questions.
     
  18. ScottWang

    Moderator

    Aug 23, 2012
    4,858
    767
    Thanks for pointed out the delay time that I forgot to think, the original I want is to use the mosfet, but I concerned about the Vgs, so after I uploaded the schematic then I tested the Vgs, it seems ok, but it still has some other problems need to solve.
     
    InspectorGadget likes this.
  19. ScottWang

    Moderator

    Aug 23, 2012
    4,858
    767
    The new circuit used two p mosfets to be the switches, that was the beginning I want to do, you can check does it has any problem, and suggest the p mosfet thanks.

    3VChargeAndBatteryKeepingPowerMosfet_ScottWang.gif

    P mosfet:
    APM4435_Pch_30V8A_Vgs4.5V24mΩ_10V16mΩ.pdf
    APM4953K_TwoPch_30V_4.9A_53mΩ.pdf
    APM9435_Pch_30V4.6A_Vgs4.5V_80mΩ.pdf
    IRF7204__Pch_20V5.3A_Vgs10V_60mΩ_ICtype.pdf
     
    InspectorGadget likes this.
  20. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,813
    1,105
    Oops, I was going by the pic in post #3.
     
Loading...