Battery supplying different voltages

Discussion in 'The Projects Forum' started by denman_100, Nov 3, 2007.

  1. denman_100

    Thread Starter Member

    Oct 27, 2007
    13
    0
    Hi,
    I am trying to run some circuits off of different voltages levels from a Battery. One devices requires a 9V source, the other a 5V source from the battery. What is the best way to do this running off of a single 9V alkaline?

    How should I implement this if I were to build my own rectifier/regulator off of a transformer?

    Thanks
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Get a regulated 9 volt wall transformer capable of supplying more than the current requirement of both circuits. Then add a 7805 regulator for the 5 volt part.
     
  3. denman_100

    Thread Starter Member

    Oct 27, 2007
    13
    0
    Would that be done for running it off batteries as well? Using a regulator?

    Also, here is what my power supply circuit would look like with your suggestions:

    AC -> Bridge Rectifier -> Smoothing Capacitor -> lm317 for 9V -> output of lm317 is paralleled to 7805 for 5V
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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    What is the nominal battery voltage you have to work with?

    hgmjr
     
  5. denman_100

    Thread Starter Member

    Oct 27, 2007
    13
    0
    Well here is what is actually happening. We have a device that uses 4.5V batteries. We will hook up an RS232-Zigbee modem to it that runs off of 5-14VDC power supply or battery. We want to run both devices off of a power supply powered off a wireless induction charger. So we were planning on using a 7.2V NiMH battery as a backup when there isn't enough power. So I will design the power supply to output 8 volts to the modem via an LM317, then off of that another LM317 set at 4.5 volts. (As per BeenThere's suggestion).

    Whenever the power supply output voltage drops below the battery though the battery will kick in. 7.2V is enough to power the modem, but it's too much for the existing device. So I'll branch off the battery to something like a AN77L045M which regulates to 4.5V to power the device. Does this make sense?

    The Zigbee modem while asleep uses 6mA and the original device while running uses 5mA.
     
  6. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    It would help tremendously if you could sketch out and post what you have in mind.

    hgmjr
     
  7. GS3

    Senior Member

    Sep 21, 2007
    408
    35
    All you need is a linear or switching regulator, depending on power requirements, to step down from 9 to 5 V. Really simple.
     
  8. denman_100

    Thread Starter Member

    Oct 27, 2007
    13
    0
    Thanks for the replies. I've attached a schematic of what I'm thinking about doing. So as long as I have enough VDC from my supply (it receives power from transmitter) it will use 9 volts from LM317 to power modem and 4.5 volts from AN77L045 to power the device.

    If I am not receiving power, the 7.2V 150mAh battery kicks in and supplies to modem directly, while it is also being regulated down to 4.5V for the device. I just had a question about power consumption if I do this. I chose the AN77L045 because of low Vin requirements. Are there better ICs for this?

    The device uses 5mA @ 4.5VDC and modem uses 6mA @ supplied voltage during sleep, 80mA during transmission. I will be using a 9V (7.2V nom) battery as backup. How much current will the circuit actually draw? Will the extra regulator down to 4.5VDC drain the battery a lot? I am planning for the unit to run for about 8 hours a day on battery power alone so I can't have the regulator draining excessive current.

    Thanks
     
  9. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    I don't see anything of concern with the circuit as you have drawn it.

    It is possible that you will need a heatsink on the LM317 just to keep it from getting hot. Normally the LM317 should only be warm to the touch. The power dissipation will be at a max during those times when it is powering the load and recharging the battery when it has been discharge. This is when the heatsink will provide some heat relief.

    What is the amp-hour rating of the battery you are using?

    hgmjr
     
  10. denman_100

    Thread Starter Member

    Oct 27, 2007
    13
    0
    The battery is 150mAh, but I will probably use 2 in parallel to get more capacity out of it. How would I calculate power dissipation? I'm using (12V input - 9V output)*(load current), hopefully this is good. Also, I'm not familiar with heat sinking. Is there a special part to buy for that?
     
  11. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Digi-key has a number of clip-on-heatsinks for to-220 packages such as the digi-key part number HS365-ND.

    You may want to check mouser as well since I believe they do not have a minimum order requirement.

    hgmjr
     
  12. denman_100

    Thread Starter Member

    Oct 27, 2007
    13
    0
    Thanks,

    I'm also looking for a simple over voltage protection circuit. Would an RC circuit across the inputs to the rectifier suffice? If I used an SCR crowbar is a fuse necessary? The voltage source in my schematic is provided via inductive charging so it can't draw that much current anyways. Are there any other alternatives?

    Thanks
     
  13. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    The simplest over-voltage protection would be to place a transzorb in parallel with the load that is sized for a voltage slightly less than the maximum safe voltage the load can withstand. Then add a fast-blow fuse in the positive power feed just just prior to the transzorb and size it for about 125% of the maximum current you expect the load to consume. This is a conservative margin. If you find that the fuse is blowing too quickly you can either use a slow-blow version of the fuse or increase the fuse to 140%.


    hgmjr
     
  14. denman_100

    Thread Starter Member

    Oct 27, 2007
    13
    0
    Are there alternatives to using a fuse? These devices are in remote locations collecting data and will be checked upon once in a while. Once a fuse is blown, it'll only run until the battery dies. Is there a less permanent solution? I guess by over voltage I'm mainly referring to surge protection. I know it won't have a sustained voltage beyond what my circuit can handle.
     
  15. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Fuse are generally provided as a means of protecting a circuit from sustaining irreversible damage. If a fuse blows in a circuit then there is a strong possibility that a condition exists that needs technical attention. Since you invision this fuse as a protection against overvoltage, if it blows then in all likelihood the input voltage excceeded the set point. To prevent the fuse blowing thereafter then some attention would need to be given to preventing the power source from exceeding the design limit.

    Since this is a remote unattended piece of equipment then you may need to consider a more drastic measure. That measure would involve some sort of redundancy scheme. Such a scheme would entail providing an identical alternate backup circuit that would be switched in in the event of loss of the primary circuit.

    hgmjr
     
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