variable power suply using lm338 wattage question

Discussion in 'The Projects Forum' started by julienrl, Apr 12, 2011.

  1. julienrl

    Thread Starter Member

    Aug 10, 2009

    I recently built a small variable current \ voltage power supply using 2 lm317's and a few resistors\caps (based on a variant of what is in the first 2 posts here). Now I want to move on to version 2 and use LM338's in order to increase my output (I don't see myself using more than 2 amps at most, and probably usually using under 12v and under 200mA, but I liked the idea of having the extra possibilities). For that matter my DC supply is only 3.7 amps, but I figured going with a bit of overkill on the transistor was the least of my worries.

    Now my question is how many watts will my resistors have to be? For my amperage controller I currently use a 5 watt 1 ohm, but I want to use a variable resistor and will be using higher voltages and current. More importantly, I currently use low wattage resistors to vary my voltage and I am scared that as my change gets more significant (going from 30 to 3v for example, which I know is very inefficient.. I do plan on having a choice of inputs with time) the heat dissipation will be greater not only in the transistor, but also in the resistors... am I wrong here?

    Anyways, to sum it up,
    1-do my voltage regulating resistances (including the 120ohm base one as specified in the datasheet) need a higher wattage as my power supply's current and voltage go up, and if so, how much more powerful do they need to be with my power control resistors going from 24 to 2700 ohms?
    2-how many watts does my current regulating resistor need to be (or how do I calculate it) as I increase voltage and output amperage, considering that it will range form 0.1ohm to 100ohm ?
    3- (contingent on 1 and 2), if the watt rating of my resistor becomes high, are there solutions that would allow me to have 2 15-turn resistors in series (one for coarse and one for fine tuning).

    While I am at it, I plan on putting a voltage and current readout on my custom VPS. what kind of effect would having my ammeter in series with my output have? I plan on using one of these puppies.

    thank you in advance.
    Last edited: Apr 12, 2011
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    Power is simple to calculate. It is the product of the voltage times the current (P = I * E).
  3. julienrl

    Thread Starter Member

    Aug 10, 2009
    It is true that P = v * I, but what v am I taking into account. The output or output - drop (in which case, do devices that require x voltage truly use that much?... since it is a variable voltage power supply, the voltage change as lithium batteries get charged for example, but i am not quite sure where or how).

    That would mean that at 12v 1amp the power would be of 12w. This does not seem like an unlikely setup (12v 1 amp) and yet I have a hard time finding any resistor over 5W, let alone a 10 or 15-turn potentiometer over 1w. How would a 30v 5amp power supply ever be possible?
    Last edited: Apr 14, 2011