Need to drop voltage to 1.5v from either 3,5, or 12v source

Discussion in 'The Projects Forum' started by live4soccer7, May 3, 2010.

  1. live4soccer7

    Thread Starter Well-Known Member

    Jun 7, 2008
    88
    0
    I've got a digital counter that runs off of a 1.5v battery and I need to hard wire power for it. I have source voltage from a supply of 3,5, and 12v. What would be a real easy way to drop the voltage in this instance?

    I do not know the currently consumption of the device, but I do know that it is small. If this is something that must be known I could find out. If not then that would be great!! If there is any other info that is needed I will do my best to get it.

    This is all DC.

    Any advice/help is very much appreciated.
     
  2. Bychon

    Member

    Mar 12, 2010
    469
    41
    There are adjustable voltage regulators by the dozen that start at 1.2 or 1.25 volts and go up. I think 317 is one of them. Go to www.mouser.com and use their search engine to find voltage regulators, adjustable. You'd probably start from the 12 volt supply because most regulators need more than 8 tenths of a volt to work with. Be sure to download the data sheet. It will tell you how to build the circuit and wire it.
     
  3. live4soccer7

    Thread Starter Well-Known Member

    Jun 7, 2008
    88
    0
    I've got about 50 of the LM317LZ's from mouser.

    I'm still a little confused as to which one I should use. It seems there are several different ones.

    I believe this is the datasheet for it:

    http://cache.national.com/ds/LM/LM317L.pdf
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If the load is light, you can use an LM317L with a 0.1uF cap on the input and output terminals, connect a 120 Ohm resistor from OUT to ADJ, and wire the ADJ terminal to GND. That'll give you right at 1.25v out. If the load is really light, you could probably even get away with using the 3v DC input.

    If you want 1.5v out, you will need to add a 24 Ohm resistor from ADJ to GND. Anywhere from 22 to 27 Ohms should get you in the ballpark.
     
  5. live4soccer7

    Thread Starter Well-Known Member

    Jun 7, 2008
    88
    0
    what do you consider "light" for a load?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    About 15mA total. The 120 Ohm resistor will take about 10.4mA, so you have 4.6mA left.

    If you know what your load current will be, you can always increase R1 (the 120 Ohm resistor) to make more current available; that means you'll also have to adjust R2 (from ADJ to GND).

    Unless you can give me a range for your load current, I can't give you a better idea for resistors to use.
     
  7. live4soccer7

    Thread Starter Well-Known Member

    Jun 7, 2008
    88
    0
    Thanks, I will do some more testing tomorrow on the part and get that info. Thanks.
     
  8. live4soccer7

    Thread Starter Well-Known Member

    Jun 7, 2008
    88
    0
    Is this the circuit that you are referring to?
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Yep, that's basically it.

    I forgot, the LM317L only requires 5mA output current to provide guaranteed regulation. Therefore, you can use a 240 Ohm resistor from OUT to ADJ, and about 47 Ohms from ADJ to GND to get 1.5v.

    Iadj is nominally 50uA, and with these relatively low-value resistors, it doesn't have much affect on the output voltage.

    50uA x 47 Ohms is only 2.35mV difference - so instead of 1.5v out, you'd have 1.50235v out.
     
  10. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    And that would be if you had perfect resistors.

    I dont have any, so you can count on a variance in just about every case. (a few %)
     
  11. live4soccer7

    Thread Starter Well-Known Member

    Jun 7, 2008
    88
    0
    last few questions and I will have this thing down. Sorry for my lack of knowledge in electronics, but I'm trying to learn as I go.

    Is that an electrolytic cap that I need on the right hand side of the diagram?
    -The footnotes on the diagram state I don't need the cap on the left hand side of the diagram
    -Should I use a cap in both places? If so, what type in each place? Also, you stated a .1uf cap in both spots, but in the diagram it shows a .1uf on the left and a 1uf on the right.

    Also, what determines the Iadj?

    Thank you very much for all the help. Definitely makes small projects enjoyable with others assisting.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    That's what AAC is all about ;)

    Yes. You can also use a 0.1uF (100nF) poly metal or ceramic cap in parallel with a larger electrolytic if you wish.

    The thing about aluminum electrolytic capacitors is that they tend to have a higher ESR than other types. If you put a smaller value (metal poly film or ceramic) in parallel with a larger-value aluminum electrolytic, the metal poly film/ceramic cap will take care of the high frequency transients, and the electrolytic will take care of the low frequency transients.

    If it's very close to the filter cap, you may get away without using it. Otherwise, inductance in wiring will slow down the response time of the regulator.
    yes, it's good practice to do so.
    I'd use 0.1uF poly metal or ceramic on either side of the regulator.

    I regard .1uF caps on either side as a minimum requirement for stability. Even though various manufacturers may claim that you don't need caps on input or output for stability, a very small amount of capacitance can set up the regulator to be an oscillator. Having a small cap on either side of the regulator swamps that effect; and the likelihood of the regulator becoming an oscillator is then virtually nil.

    The regulator internals. You can't adjust it. It is typically 50uA, but may be as high as 100uA and still be in specifications.
     
  13. live4soccer7

    Thread Starter Well-Known Member

    Jun 7, 2008
    88
    0
    dang... you are freakin awesome man!! Thanks for all the help. I will be building this tomorrow and will help finish up our project.
     
Loading...