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Voltage splitter blues

  1. May 24, 2017 #1

    c d

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    Hi

    I have a lipo 14.8 V battery to power a motor. I want to use same battery to power a video transmitter, a TS832.

    The tx takes a 11.1V input.

    Made a voltage splitter off 14.8 battery that produces 11.4 V with respect to ground.

    Does not power tx when plugged in.

    When I plug a 11.1 V lipo into tx, tx works.

    Works defined as leds light up.

    Don't believe this is contact issue.

    What am I not understanding about voltage splitters?

    Thanks.
     
  2. jcsd
  3. May 24, 2017 #2

    Charles Link

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    Homework Helper

    The tx you are powering most likely has a somewhat low resistance/input impedance. The result is if you put two resistors in series to generate 11.1 V, this is only the case if you attach a large resistor(instead of the tx) in parallel to your second resistor. ## \\ ## It is possible the resistance of the tx is so low, and the internal resistance of your 14.8 V source might be large enough that if you connect it directly, your 14.8 V battery might only put 11.1 Volts into the tx. If the tx has a very low resistance, your 14.8 V battery might only deliver 8 or 9 volts straight into the tx. You need to be a little careful not to overdrive the 11.1 V tx input that is required, but it could be that your 14.8 V source is a good match straight on.
     
  4. May 24, 2017 #3
    The operative word there is MIGHT. Are you willing to bet the Transmitter that it will tolerate an overvoltage battery?

    So far there just isn't enough information to make any moderately safe suggestions.
    No guarantees, but some information that would help:
    • Do you have any information about how much current the transmitter draws? Maybe in the user manual or spec. sheet.
      • If not, can you measure the current it normally draws?
    • What value resistors are you using to drop the voltage?
    • What power source does the Tx normally use?
    • What is the make and model of the Tx?
     
  5. May 25, 2017 #4

    Charles Link

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    I like the answer @Tom.G gives, also for a second reason: If the tx requires a high current such as 1.0 Amps at 11.1 Volt, I think it would be possible to burn out the 14.8 Volt supply if it was only designed to supply lower current amounts like 0.1 Amps. It is important to have a power supply that meets the requirements of the device.
     
  6. May 25, 2017 #5

    davenn

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    that isn't really the issue

    lipo's are capable of respectable current



    you cannot use resistors for voltage dividers for this purpose
    the available current capacity is severely limited and that is why your TX isn't working

    you should be using a linear or preferably a buck converter switching regulator
    this is the only way it will work correctly

    there's plenty of buck converter regulators on ebay and they are dirt cheap

    https://www.ebay.com.au/sch/i.html?_nkw=buck+converter

    Dave
     
  7. May 25, 2017 #6

    c d

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    I'll go with the low current answer. Unfortunately, most of the m-meters I own do not measure current, also don't see anything in the tx specs. BTW, the wires in the splitter are about .01 inch in diameter.

    7812 regulator would give me 12 volts out; just ordered several.

    Let me add this to the question: I have several devices -- tx, osd, camera, all 11 volts -- i wish to power off the 14.8 battery. Could I wire several branches/runs off one voltage regulator, or would I be advised to use three voltage regulators, one for each device? Three meaning i would make three additional branches off the battery, and wire them into each of the three voltage regulators and respectively, into each of the three devices.

    The object here is: One battery is easier to manage.
     
  8. May 25, 2017 #7

    davenn

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    the 12V, 7812 will work, just, once your LiPO drops 0.5 - 1 V, it will stop working as it wont be able to maintain the minimum input / output voltage different required by the 7812 to maintain regulation

    THIS is why I gave you a link to a buck converter as they don't have such a high Vdrop across the regulator circuit
    and they are also so much more efficient than linear reg's meaning less power wasted and the battery lasts longer

    Dave
     
  9. May 25, 2017 #8

    c d

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    i see the bucks are efficient and cheap! I'll give those a try.
     
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