Step Down from 12v to 7.5/8v

Discussion in 'The Projects Forum' started by noxin, Apr 4, 2008.

  1. noxin

    Thread Starter New Member

    Jan 18, 2008
    3
    0
    Hey all,
    I'm a relatively new person to electronics and I was hoping someone could help me out with a simple circuit to step from 12v DC to 7.5 or 8V for about 1A.

    From my previous readings on the forum, I'd be looking at a LM317 or a KA7812 (to handle up to a 2A Load), but there was mention of needing a resistor or two in there some place, but I'm not sure how to figure out which resistor(s) I need.

    Thanks for any help!
     
  2. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    See attached for a simple solution, note that you can parallel these devices for higher currents.
     
  3. noxin

    Thread Starter New Member

    Jan 18, 2008
    3
    0
    Okay, that looks do-able.

    I would hook up my +12V to pins 7 & 8, wire out from pins 1 2 or 3 to my load and hook up a resistor between +12V and Pin 5, which is the VControl. It looks like I'll need a 1W
    2.4k ohm resistor.

    Sound right?
     
  4. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    No, VControl goes to the input supply.

    Place a resistor (Rset) from pin-4 to GND, based upon the following:

    Rset = Vout / 10uA

    Connect both pins 7,8 (with 5) to input and use all three pins 1,2,3 for output, and don't forget to read the datasheet about input/output bypass caps.

    If you don't really understand this part, then you may want to go with the LM317A adjustable in the TO-3 package. See page 8 of attached for selecting resistors, use a 124 ohm as R1, then calculate R2.

    Vout = 1.25 * [1 + (R2/R1)] + (Iadj * R2)
    where: Iadj = 100uA max.

    R2 = [(Vout / 1.25V) - 1] * 124 ohms

    You can ignore IadjR2, or not, it doesn't add much to the output voltage.
     
  5. noxin

    Thread Starter New Member

    Jan 18, 2008
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    0
    okay, so the LM317 (easier is better for a n00b!)

    Pin1 - +12V
    Pin2 - Goes to the load, but in between R1 (124Ohms) and R2 (670 Ohms (or a number reasonably close to that))
    Pin3 - Goes to the load but in front of R1.

    and R2 connect to ground.


    One more follow up question... The diagram on page 8 shows two leads/wires going to Vout, do I cross connect those and put them to the load or something else?

    Thanks for all your help with this, I really appreciate it.
     
  6. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    I don't see two wires to Vout. I see Vout defined with two wires, one to Vout and the other to GND.

    670 ohms is the correct answer. However, the closest is a 665 1% (see attached). Choosing 124 ohms for the R1 is handy, because it sets the min. load for ~10mA, but you can adjust R1 & R2 for better values that will fit the desired output voltage.

    According to the datasheet, the min. load spec is 5mA, so you can set R1 for 100 - 249 ohms or so. And then, recalculate R2. You could choose completely different values in the 1K - 10K range (for instance R1 could be 1.24K), but then you should add a load resistor from the output to GND that will maintain that min. load requirement (or instead, perhaps have an "on" LED that runs at ~10mA).
     
  7. Norfindel

    Active Member

    Mar 6, 2008
    235
    9
    Hello
    If you're using the TO-220 package, pin 1 seems to be ADJust, pin 2 is Vout, and pin 3 is Vin (the pins are numbered left to right, the device being hold like it can be seen on page 1 in the datasheet).
    In the low part of page 1 in that datasheet, you can see the typical circuit. Vin goes to the input, unregulated suppy. Vout goes to the output, here would be the regulated output voltage. From this point, you need to connect the 124 ohm resistor to the ADJ terminal (strange number, you could use a 120 ohm resistor and recalculate the other resistor). From the ADJ terminal to ground, you need to connect the correct resistor to get the desired voltage.
    Use a heatsink and thermal grease, or other interface compound. 1A at a 4V difference between input to output means 4 watts of power dissipation. That would mean at least 200 degrees (celsius) junction temperature for the TO-220 package, and that's too much. You need a heatsink to make it work. It doesn't needs to be too large, say, 10 ºc/w, if i'm not doing something wrong.
     
  8. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    I updated the datasheet above to show pin numbers on page 2.

    Vin = +12V input
    Vadj = resistors (Vout & GND)
    Vout = +8V
     
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