Discussion in 'General Electronics Chat' started by curry87, Jun 26, 2010.

  1. curry87

    Thread Starter Member

    May 30, 2010
    I have a potentiometer with a 100mw power rating how do i calculate what resistor i need in order to protect it from too much current at 9v dc ?

    Whats the difference between log power rating and linear power rating ?

    Whats the difference between these two layouts ?

    9v->limiting resistor->potentiometer->0v
    9v->potentiometer->limiting resistor->0v
  2. retched

    AAC Fanatic!

    Dec 5, 2009
    Logarithmic follows an audio like boost pattern.
    Linear follows a slope so the resistance changes evenly throughout the pot.

    Logarithmic may go 1/3 of the way then have most of the resistance through that middle third to follow human hearing more closely.

    Log pots shouldn't be used where you need more accurate calculations, as they can jump suddenly rather than a smooth even transition.

    As for you limiting, the pot is a resistor, but if you feel you need more, OHMS LAW is what you want to use.
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    Power ratings have nothing to do with the resistance taper of the pot.

    Assume the pot and resistor are the same value. In the first case, the pot can vary from 0 volts to 4.5 volts. In the second, the pot can vary from 9 volts to 4.5 volts.

    Look up "voltage dividers".
  4. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    A variable resistor RHEOSTAT TYPE running at very low resistance settings might be burned out if you allowed too much power through it. If you put too powerful a signal into an Amplifier (Design Problem Dependent) you might burn the control contact surface for Volume and create scratchy noise at low volume settings.

    What size of series resistor you need does NOT depend on the minimum and maximum resistance across the Variable resistance. You need to also allow that you can lose some low end control when used as a current control. As a Voltage control you should not have this problem. With a Volume control for example adding a series resistor you could lose the ability to turn the sound off if it is working as a current control, but using the resistor on the wiper avoids this problem if you put the full signal across the ends of your variable and take a voltage level instead with current control from a wiper series resistor.

    For your (OP) question it seems like you might not be using it as a control Voltage Divider across the 9V. If you are using it as a purely current control rheostat variable resistor, this can create the burnout problem that needs a series resistor.

    9V 100mW max you would need a current limit of around 10mA which would need 100 Ohms.

    The limiting resistor normally needs to be on the High side (+9V or Signal In) OR on the wiper.

    Having a resistor on the low side or the wiper is needed when sinking current.

    In many cases a design includes resistors at both ends and on the wiper and this is good if you prefer to avoid problems and want to be able to use a less expensive but fragile control piece.

    For the anyone still learning the difference between current controls and voltage controls the current limiting series resistor with a Variable resistor is a good habit.
    Last edited: Jun 26, 2010
  5. SgtWookie


    Jul 17, 2007
    Ohm's Law says:
    R=E^2/P or Resistance = Voltage * Voltage / Power (in Watts)
    9^2 / 100mW = 81/0.1 = 810 Ohms.

    810 Ohms is the smallest resistor that you can use, if you wish to risk the possibility of operating the pot at it's maximum rating. Generally, you wouldn't want to go higher than 50% of the rating, or your pots' service life will suffer.
  6. curry87

    Thread Starter Member

    May 30, 2010
    If i want to use the potentiometer as a current limiting control then out of the two ratings 0.2w linear and 0.1w log should i obey the linear rating or in between log rating and linear rating ?
  7. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    Sgt Wookie is correct. Apparent Power calculation and the 810 Ohm resistor will avoid problems.

    I estimated a value of series resistance as 100 Ohms based on the DC formula for True(Purely Resistive) power P=EI.

    Using a low 100 Ohms of a series resistance in AC circuits where reactance needs to be included in the circuits could cause problems because you need to expect much higher momentary voltages and currents.

    Staying underrated is good practice as a habit. Performance stability can sometimes mean using only 10% of the devices power rating. Reliability data and performance characteristics can easily be as much as 100 x worse when you increase power to the area between 50% and full rated power compared to devices staying in the low power region. You go from can go from no component problems to increasingly frequent problems because low power will also help you compensate for the unexpected.

    This is the kind of design technique that you will be able to identify if you ever compare a very good piece of equipment with a very inexpensive piece of equipment.
    Keep in mind that the component costs are even less important compared to the time invested for a homebuilt project compared to the automated production lines of a major manufacturer. If you see two parts one for $0.25 and the other for $0.50, learn to look at it as twenty five cents and not as twice the cost, then pick the better part. Unless you are buying 100 parts at a time the differences will most often favor the better part.

    At the same time don't overdue it. Some parts are actually insensitive when used at very low power. 10% of rating is normally a sweet spot but if some devices are too far under 1% of power rating they won't work properly. Some signal transformers for example will not couple input to output effectively unless they are fed with 30 to 70% of their rated power! That is just one example.

    For your Potentiometer - I would try to work with the 0.1 Log rating.