How to select resistance of a potentiometer

Discussion in 'General Electronics Chat' started by mpangrekar, Jun 11, 2009.

  1. mpangrekar

    Thread Starter Member

    May 16, 2009
    How do I figure out what resistance to select for a pot.

    I have 12VDC i/p supply and load is dimmable LED bulb (4Watts, 0.33Amp).

    Just as a trial & error, I tried a 1/4 Watts pot with 1M Ohm resistance (audio taper) and there was almost no dimming, I got 0% to 100% dimming in 10 degrees rotation of the knob.

  2. bertus


    Apr 5, 2008
  3. PRS

    Well-Known Member

    Aug 24, 2008
    What kind of LED are you using? What is its voltage rating? Also, can you draw a picture of how you're hooking it up? Use Paint if you need to.
  4. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    The pot you tried is likely a taper pot. You want a linear.

    At 12V, your going to drop ~.6 v across the diode, so 11.4v/.33amp gives you about 34 ohm, which will be your current limiting resistor. Add in series to this a 1K ohm pot, and you should get a good adjustable range.
  5. flat5

    Active Member

    Nov 13, 2008
    GetDeviceInfo, the voltage drop of a regular silicon diode is ~ 0.6 volt but not so for an LED. We need more info on the LED.
    The type (manufacturer part number) would be nice to know.
  6. mpangrekar

    Thread Starter Member

    May 16, 2009
    Thank you for your replies. Attached is datasheet for the MR16 dimmable LED bulb. The bulb runs on 12 VAC or DC & rated for 4 Watts. The mfgr Ushio claims that the bulb has a PWM circuit built into it (they cannot give out any more info than that since it is proprietery) and it is dimmable with voltage 7V to 12V.

    The pot I am thinking about would have 12V DC as i/p and would give variable o/p to LED bulb.

    Attached also find test data by Ushio (dimming with variable voltage).

  7. AdrianN

    Active Member

    Apr 27, 2009
    Do not use a pot. Use a PWM circuit as you were advised in the other thread. Or use a MR16 12V dimmer. On the USHIO website there is no mentioning about a PWM circuit being built into this lamp. Just curious: Did you guess or you got this info from some place?

    A pot at 0.33 Amps will bring high power losses. Just calculate the power dissipated per pot: Power_lost = (0.33 Amps)^2 * R, where by R I noted the pot in series with the adjacent resistor value.
  8. Wendy


    Mar 24, 2008
    There is a reason potientiometers are not generally used as dimmers for LEDs, they work, but the big reason to use LEDs is their efficiency, and a straight pot nulifies that because it works by dumping the current as waste heat.

    Why are you so set on using this part, if I might ask? Length of life?

    The one spec I do not see, not directly at least, is current. The data sheet mentions 4W, and 12V, so going through the math I assume ¼A.

    You can not get a linear response from this lamp with a variable voltage or current. It is the reason PWM is used for LEDs, it works where what you are trying doesn't very well.

    The pot you are using is 1MΩ. Since the power supply is 12V, and the equivalent resistance of your LED (this is not accurate, but for comparison purposes only) is 48Ω (Ohms Law,12V/0.25A).

    LEDs are not light bulbs, they are active devices, and require a minimum of electronics to perform to spec. You need a much smaller range of pot, and most of the range will be in one end (extreme nonlinearity). Figure a 1KΩ pot it will not go dark, but close (minimum current 11ma), a 10KΩ will take it down to 1ma.

    I mentioned it before, but a voltage regulator will also get you close. It will get hot (unlike a PWM circuit), but the pot will do something over it's entire range (with proper design). A simple PWM circuit will not get got (and the LED lamp will be cool when it is dim), and will be extremely linear.

    Have you read this?

    LEDs, 555s, Flashers, and Light Chasers
  9. mpangrekar

    Thread Starter Member

    May 16, 2009
    I spoke with Ushio, their Engineer told me that they used PWM inside the bulb.
  10. Audioguru

    New Member

    Dec 20, 2007
    Ask their engineer how to dim their LED light because the datasheet does not show how.
  11. Wendy


    Mar 24, 2008
    There is an old saying, Believe half what you read, None what you hear.

    The Datasheet makes no mention of PWM, in any form. This is a pretty critical piece of information to leave out, given how it affects its use.

    I suspect what the engineer said is to use PWM to dim their bulb, not that it had it. But that is the problem of verbal vs written. BTW, I know something about these circuits. Even the light bulbs don't use a straight pot, but use drive electronics that involve an SCR (with won't work with LEDs). The pot is to control said drive circuitry, whether PWM or SCR. Maybe you should get a recommended drive circuitry schematic when you talk to the manufacturer, something in a written format.

    Note to Bertus, there is a 3rd thread...

    You ask the same questions, you get the same answers.
    Last edited: Jun 19, 2009
  12. mpangrekar

    Thread Starter Member

    May 16, 2009
    I already spoke with Ushio, they said, they don't have lot of experience with this and said, a pot may work for a i/p supply of 12V, they recommended a transformer with pot for high voltage (120V) but nothing specific for low voltage (12V).

    Thank You for all your help !

  13. BobaMosfet

    Active Member

    Jul 1, 2009
    We are assuming it does not use PWM.

    The sheet the provide is NOT datasheet. No current specification is given. All the graph shows, that you provided is that it will dim based on voltage supply change from 7 to 12 volts (that is, off at <7 and on full at >12). Nor does it mention PWM (which they should tell you in the spec if it does).

    If we use the standard power equation (PIE), we can derive how much current it draws max.

    I = P/E
    I = 4W / 12V
    I = .333 (or 333mA)

    Once we know current, and voltage, we can calulate Resistance using Ohm's Law for both 7 and 12 volts.

    R = E/I
    R = 7/.333
    R = 21.021 Ohms

    R = 12/.333
    R = 36.036 Ohms

    In short, you need a trim pot with a range of 15 (that is 36.036 - 21.021)Ohms. And the way you start your low end at the proper resistance is by putting a 21 Ohm (more likely a 22 is what you can get) resistor on the ground side of the trim pot.


    I cannot imagine why they would put a PWM circuit in a light assembly. Maybe in a controller, but not in a light assembly. That doesn't make any sense.
    Last edited: Jul 22, 2009
  14. millwood


    it is a mr16 bulb so you need to find a dimmer for 12v light bulbs. your local lighting supply store is your best bet.
  15. beenthere

    Retired Moderator

    Apr 20, 2004
  16. russ_hensel

    Well-Known Member

    Jan 11, 2009
    It may be that there is pwm inside and this voltage is just a control voltage into a high z input. How many pins are there on the led? Where is the full data sheet?
  17. BobaMosfet

    Active Member

    Jul 1, 2009
  18. KL7AJ

    Senior Member

    Nov 4, 2008
    This brings up an interesting point about selecting pots in general. If you are drawing ANY current at all from the wiper terminal, it's not really a potentiometer, it's a rheostat. Many people use a "potentiometer connected rheostat" with various degrees of success. The main reason for doing this, instead of using a normal rheostat is that it allows you to turn the device in question completely off.

    However, this is never an ideal solution. Any pot-connected rheostat is going to exhibit very non-linear behavior, unless its value is very low relative to the which case, it's very inefficient!

    The ideal solution for driving a load like an LED is to use a true rheostat with an "off" or open position at the end. Or use a solid state driver.

    Compared to most other devices in your typical solid state circuit, LED's draw the most power....they need to be treated as power devices, not merely voltage devices.