Resistance to chose for my potentiometer

Discussion in 'General Electronics Chat' started by Filou, Feb 9, 2014.

what resistance do I need for my potentiometer ?

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  1. Filou

    Thread Starter New Member

    Feb 8, 2014
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    Hi,

    I try to make a simple electronic circuit.
    So I have an instrument (medical ophtalmoscope) that contains a light bulb 10w, 6v. The adapter is noted 6v, 1.7A . I want to bypass the small rechargeable battery pack that isn’t working well. So I want to dim my bulb using a potentiometer. I tried a 100kΩ that was smoking (I chose that resistance because that's what is noted on the potentiometer of my original batt pack . So I guess there was a more complex assembly of resistors connected to it.)

    My question: what is the ideal resistance needed for my potentiometer in this circuit (and does it have to be linear or logarithmic) ? I want the brightness to vary ok but not necessarily perfect. (A range of acceptable resistances would be fine ). I want it to work without burning it this time.
    Thanks a lot,
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Any potentiometer will bur, as they are not rated for the high current.
    Better use a PWM circuit.
    Here is an example:
    Project: Simple PWM circuit

    When you use a logic gate mosfet, the circuit will work on the mentioned 6 Volts.

    Bertus
     
  3. inwo

    Well-Known Member

    Nov 7, 2013
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    A rheostat connection will be better than pot.

    Look at the size and decide if it's what you really want to do.

    "(RWA) M22/03-0052
    Rheostat, 6 ohm, 25 watt, 2.04 amps. 1/4" slotted shaft. 1-5/8" diameter body, 1-3/8" behind panel depth. Made by Ohmite."


    Could be $25-$50 on the surplus market if you can find one.

    This post explains why you got the (good) answer that you did.
     
    #12 likes this.
  4. Filou

    Thread Starter New Member

    Feb 8, 2014
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    Thank you folks ! I'll think about all this...
     
  5. #12

    Expert

    Nov 30, 2010
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    Been there, done that. inwo has the right idea.

    The otoscope I fixed had a 3.5V center tapped transformer and a 5 ohm rheostat. The center tap was only used as a ground point to avoid static electricity build up or leakage from the 120 volt side. The strange part is that the transformer was one of those ferro-resonant types with an oil filled capacitor to kind of regulate the voltage. What can I say? You have to put something complicated in it to charge a doctor $300 for a flashlight.:rolleyes:

    Probably an incandescent bulb for a (2) D cell flashlight.

    The microscope used a Radio Shack multi tapped supply transformer. 3V, 4.5V, 6V, 7.5V.
    I put a 6.3V bulb in it so he could over-drive it if he needed dazzling white light.
    The doctor liked it.
     
    Last edited: Feb 10, 2014
  6. Filou

    Thread Starter New Member

    Feb 8, 2014
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    Can someone give me acceptable ranges (ohm, watt, amp...) for my rheostat ? I have problems finding the exact model suggested by Inwo.
    Thanks
     
  7. inwo

    Well-Known Member

    Nov 7, 2013
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    Metalmann likes this.
  8. inwo

    Well-Known Member

    Nov 7, 2013
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    How about a 6v wall wart on a light dimmer?

    Would have to be an old style with a transformer. ac or dc if unregulated.
     
  9. #12

    Expert

    Nov 30, 2010
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    6 ohms..not in stock
    Mouser: $35 to $40:eek:

    I have to go to the bank within 80 minutes. Be back later.
     
  10. Metalmann

    Active Member

    Dec 8, 2012
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    Just a thank you, INWO for the link to rheostats!:cool:
     
  11. #12

    Expert

    Nov 30, 2010
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    You're going too high on the watts. Basically, the math says that when the pot resistance matches the lamp resistance, that is the highest wattage on the pot. For 6 volts into a 10W bulb, Rbulb = 3.6 ohms. As long as the voltage is not increased, any pot that can get to 3.6 ohms will be going halvsies with the bulb for power dissipation.

    6V, 7.2 ohm, 5 watts, and the pot gets half of that. Double the wattage for the safety factor and a 5 watt pot will work all day. Then again, sometimes you can buy a 10 watt pot for less than a 5 watt pot. Go figure.:confused:
     
  12. #12

    Expert

    Nov 30, 2010
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    Filou and Metalmann like this.
  13. inwo

    Well-Known Member

    Nov 7, 2013
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    That's what I thought at first.

    Then when I did the math it came out to 22 watts.

    I squared R

    If it gets set to the last turn, meaning near lowest resistance, the full lamp current will flow thru that turn.

    That turn however is only a small percentage of the total resistor.
    Ergo it can handle only that same small percentage of the total watt rating.

    In practice the heat may be distributed somewhat more than that.
    I believe this to be the reason rheostats are rated in amps, not watts.

    You may notice that the 25 watt ones are only rated at a couple amps.
     
  14. #12

    Expert

    Nov 30, 2010
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    I see your point.
    It might be too much to expect a 5 ohm pot to be set at 1/2 ohm, in series with 3.6 ohms, and dissipate 1.07 watts (20% of its rating) with 10% of its internal resistance wire.
     
  15. inwo

    Well-Known Member

    Nov 7, 2013
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    Exactly!

    How this plays out in the real world I don't know.:confused:

    That's why my conservative guess.:)

    My first post. " decide if it's what you really want to do."

    If this is his choice, I will actually try it before recommending less watt rating.

    It may be fine!
     
  16. Filou

    Thread Starter New Member

    Feb 8, 2014
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    I thought a rheostat would be better than a pot.
    As long as you’re pretty sure the potentiometers you are suggesting won’t burn, as I already experimented it with a higher resistance (100kΩ), I will take both: 5Ω-5w and 5Ω-10w and test them. I’d just like to have your comments about the potential overheating of the pots before I order.
     
  17. Filou

    Thread Starter New Member

    Feb 8, 2014
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    I mean : 5Ω-5w and 10Ω-5w
     
  18. inwo

    Well-Known Member

    Nov 7, 2013
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    My opinion is that they will overheat.:)

    My calculations show greater than 15 watts needed.

    I'm not sure I have #12 convinced. Or myself for that matter.
     
    Last edited: Feb 11, 2014
    Metalmann likes this.
  19. inwo

    Well-Known Member

    Nov 7, 2013
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    For an example using a 5 ohm pot and 1.75 amps.

    When adjusted to 10% (.5ohms) draws 1.5 amps.

    10% of the 5watt rating (where the heat must be dissipated) is .5 watts.

    Actual watts to be dissipated is 1.5A X 1.5A X .5R= 1.25 watts

    Too small by a factor of over 2. 3 would be safe = 15 watt pot minimum.

    My math is probably not worst case. I'd still go 25 watt.

    It may be that the dissipation percent may not follow the pot setting (percent) exactly. May be wrong by a factor of two or more.
    I don't know how to predict other than field testing or specific pot data.

    I welcome other opinions or experience.
     
    Last edited: Feb 11, 2014
  20. #12

    Expert

    Nov 30, 2010
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    That link I put up has 5 ohms 10 watts, and 10 ohms 10 watts, too.
    My math says 1/2 ohm in series with a 3.6 ohm lamp puts 1.07 watts on the 1/2 ohm.
    A 5 ohm pot set at 1/2 ohm is at 10% of its resistance and 10% of a 10 watt pot is 1 watt.

    That's my story and I'm sticking to it. :D
     
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