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using voltage divider to power propeller — Parallax Forums

using voltage divider to power propeller

shmowshmow Posts: 109
edited 2009-09-22 20:18 in Propeller 1
Hello All,
I have a propeller running a few components:
one - 912Mhz transceiver (3.3VDC)
three - 7" tall seven segment LED display (24VDC)
The project receives a count down clock update from a master clock and displays the time on a large digital clock - three digits at this time.
I would like to use the power supply for the 7 segment displays (24VDC - 3A) to power the entire circuit (instead of two different power supplies - 24VDC and 7.5VDC).
I thought I could use the voltage divider. I set up the divider to get an 8.5VDC output.
I tried just powering the propeller alone with a couple of LED outputs as a test.
The voltage divider output was measured at around 8.5VDC but there didn't seem to be enough current (an uneducated guess) to power this simple prop circuit.
Is there something I'm missing?
What may be the problem?
Cheers,
Shmow

Comments

  • SeariderSearider Posts: 290
    edited 2009-09-22 01:13
    Fundamentally there is no reason that a properly designed voltage divider will not work. In practice, it will probably be easier to add a second regulator. If you want to design a proper voltage divider, then you need to use ohms law to calculate the voltage and available current based on the resistor values that you choose. I expect that you will find that a lot of power will be wasted driving the voltage divider.
    Is there a reason that you are driving the LEDs with 24 volts? LEDs by themselves are current driven devices and don’t care too much about voltage. At least within a wide range.
    If there is some reason that you cannot use 7.5 volts for your LEDs, how about a regulator like the LM7808 single chip regulator. It can take an input voltage up to 24 volts and will output 8v

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    Searider
  • Mike GreenMike Green Posts: 23,101
    edited 2009-09-22 01:18
    You can't use a voltage divider this way.

    You need a switching regulator that produces the voltage needed for the transceiver and the Propeller. TI and others make switching regulators that can take 24VDC and produce regulated 3.3V with little heat produced. If you can't use a switching regulator, then you could use something like an LM7812 to drop the 24V to 12V and another regulator after that to produce the 3.3V needed. You'll need a heat sink for the LM7812 and for the 2nd regulator because the voltage drops are so large ... there'll be a lot of heat dissipated. (21V x 100mA = 2W).
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2009-09-22 01:23
    Yes, you could use a voltage divider but the values would be so low that it would be very inefficient and hot as. Primitive regulators normally would use a current limit resistor and a zener but what you really want a switching regulator to switch from 24V down to 7.5 or 5 or even 3.3. These regulators are very small and efficient (cool). If you just want to attach something rather than building a board then you could do this with a 3 terminal linear regulator in similar fashion but you would need a large heatsink.

    *Peter*

    P.S. ditto Mike
  • shmowshmow Posts: 109
    edited 2009-09-22 01:26
    Searider,
    thanks for your input; I will try the LM7808 that you recommended.
    The large LED displays require 24VDC (digi-key part # 160-1022-ND).
    Cheers,
    Shmow
  • shmowshmow Posts: 109
    edited 2009-09-22 01:36
    Thanks Mike and Peter;
    is there a heat sink you would recommend? I am designing a PCB to put this circuit together so I can fit in the regulators and the heat sink. I'm a bit of a novice with heat sinks as I have never done them before.
    What if I install a basic fan next the PCB; would that be enough without the heat sink (just thinking out loud here.)
    Regards,
    Shmow
  • shmowshmow Posts: 109
    edited 2009-09-22 01:48
    The switching regulator looks like a better option.
    Thanks for all your help folks.
    Cheers,
    Shmow
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2009-09-22 18:45
    Watch the max volts that the 3.3V reg can take. Some have a max in the region of 7 Volts.

    Heatsinks can be as big as you can afford space for, fans always annoy, eventually.

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    Style and grace : Nil point
  • whiteoxewhiteoxe Posts: 794
    edited 2009-09-22 20:01
    As I just received my first prop , may I ask is the max voltage for the powering of the prop to kept at 3.3V?

    I am following the diagram for the PE Educartion kit which uses 3.3v for the propellor and 5V for powering other components I bought the two 'regulators or are they a type of transistor at dick smith electronics here in Queensland.

    Sorry bout the big type , I was just playing [noparse]:)[/noparse]

    Mike.

    Post Edited (greenspinner) : 9/22/2009 8:11:42 PM GMT
  • hover1hover1 Posts: 1,929
    edited 2009-09-22 20:08
    Here is the data sheet:

    http://www.parallax.com/Portals/0/Downloads/docs/prod/prop/PropellerDatasheet-v1.2.pdf

    Page 26 has the supply voltage requirements.
    greenspinner said...
    As I just recieved my first prop , may I ask is the max voltage for the powering of the prop to kept at 3.3V?
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2009-09-22 20:18
    Nominally, yes. (I think the spec allows the max to be 3.6V.)

    The interfacing to higher voltage parts is usually via resistors to limit the pin protection current to 500uA or less (although most use 1K, which gives a higher current to the protection diodes). There are also all sorts of official interfacing chips.

    I have ran a Prop on about 4.2Volts for a project, to allow a Z80 to run, low volts, direct to the Prop. nothing bad has happened (yet) but this is very much outside of the original specs. Others run them on higher clocks, than the specs.

    They do seem to be resilient little beasties but I would get to know him a bit better, before any punishment gets metered out.

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    Style and grace : Nil point
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