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Sales vs. Design....Ohm's Law vs. Power

  1. Aug 18, 2015 #1
    We manufacture heaters. Simple heaters that are designed for a specific voltage, for a specific wattage, thus a fixed resistance and the corresponding amperage. However, sales guys preach that it is OK to hook up the heater to a lower voltage without damaging it. They claim the voltage is lower thus the amperage is lower so although the heater will not function optimally, it will not be damaged.

    My head says that is correct for Ohm's law but if you look at the Power equation the amperage would be more at the lower voltage thus a problem. Our design uses the power equation when building the heater circuit thus the higher wattage produces the lower amperage. But when designing dual circuit heaters we always design to the higher voltage which does not jive with the power equation but makes sense for Ohm's Law.

    I see a fixed resistance, although producing heat, the resistance is what it is.

    I have two groups of people living under two separate equations and I am looking for clarity on which one I should follow for our product.
  2. jcsd
  3. Aug 18, 2015 #2


    Staff: Mentor

    At fixed resistance, lower voltage means lower current and lower power. The power equation doesn't apply because the power isn't constant.
  4. Aug 18, 2015 #3
    Safety before power.

    With a fixed resistance the power will drop with voltage. That's safe, but potentially a little cool.

    Power = Voltage times Current. (P=VI) Since we know from Ohm's law that I=V/R; P=VV/R or

    P = V2/R

    So both equations are correct. But the power level is only for the rated voltage.
  5. Aug 19, 2015 #4


    User Avatar
    Gold Member

    They are correct, although the resistance is also "a bit" lower since the wire is cooler.

    As I said above, the resistance is "a bit" less when the element is cooler.

    But, realize the resistance does not become low enough to get to the original "hot" amperage because that amperage would produce the same temperature as it did initially.

    You need to think of the power equation as simply expressing what current you would need at a particular voltage to dissipate a certain power.

    Ohm's law rules the day,
    in that it tells you the relationship between the voltage and current for a fixed resistance.
  6. Aug 19, 2015 #5
    I think part of this misunderstanding is that people think the device will always use the same POWER, as if the POWER is constant. -- but that is not the case. The devices resistance is ( essentially) constant in this case, and the voltage of supply is constant - those two parameters define all of that then happens. The POWER equation is the resulting power ... we do not start wiht power and back calculate the other values.
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