How many volts can you put through 1/2 watt resistor?

Discussion in 'General Electronics Chat' started by stanman11, Oct 12, 2012.

  1. stanman11

    Thread Starter Member

    Nov 23, 2010
    I forgot all the calculations needed to determine things like this.
    I have ADHD and I have troubles bringing the information i need to the front of my mind. thinking about getting back on my meds and taking them how and when I want to. Not what this low income only job he can get as a doctor doctor tells me.

    Any way with out going back off topic again.

    I need to find out how E, W can handle.

    Example: if W is 0.5 watts. R is 120 ohms. Whats the max capacity it can handle in volts E?
    Do I have to Add (I) amps to this?

    E X I= W right?
  2. crutschow


    Mar 14, 2008
    Yes P (in Watts) = E (V) X I. So there are a couple arrangements of this to get resistor power. One is P = V^{2} / R if you have the voltage across the resistor and P = I^{2} X R if you know the current through the resistor.

    Edit: The maximum voltage allowed across a resistor would then be V = \sqrt{P X R}.
    Note that in practice you should dissipate no more than about 75% of the maximum resistor rating for good reliability.
    Last edited: Oct 12, 2012
  3. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    DC resistive circuits - yes, volts x amps = watts

    (someone answered first)
  4. stanman11

    Thread Starter Member

    Nov 23, 2010
    Kind of confused about the symbols. My book says E is volts. Can it be either E or V?
    And another book says E= electric field or volts per meter
  5. Wendy


    Mar 24, 2008
    Old school uses E for Voltage, many people use V. Does it really matter? With algebra the variable designation is not important, as long as you understand what it is.
  6. takao21203

    Distinguished Member

    Apr 28, 2012
    An 1/2W resistor can handle many volts. No matter what you obtain from the maths, small carbon resistors really tend to become high-Z over time.

    For an experiment you can put 20kV through 1/2W resistor.
    But from what the OP disclaimed I would not really recommend it.

    As it looks, Ohms law is fundamentally not understood here.

    Definition of voltage, Amps, resistance, and electric power.

    Electric power is important in the end.
    But also high voltage brings with it various effects.

    Electrical wires have a certain Amps capacity.

    To me it looks OP does not (yet) understand the meaning of "capacity", and actually measured "dimension".

    "capacity" and "dimension" don't apply here with their literal meaning. I have read ADHD etc. can include difficulties to understand or to abstract non-literal meanings.

    Let's say it this way, an electric wire can have a carry capacity of 10 Amps, but the actual current is only 0.1 Amps.

    It is required to understand how these abstract concepts relate to each other.
  7. stanman11

    Thread Starter Member

    Nov 23, 2010
    Thank you for clarifying and thanks for the answers. I can see how to some people it doesnt matter but to some one new like me. And the book the person has using E and others using V. its easy to get confused if you dont know that there both used as the same term.
    if that makes sense.

    Thank you for being my teachers
  8. WBahn


    Mar 31, 2012
    The use of 'E' for voltage stems from when voltage was referred to as "electromotive force" or EMF. To the best of my knowledge, it was coined by Volta himself well before the term 'voltage' was adopted in his honor.

    Voltage isn't "put through" a device. For devices like resistors it is either applied across the device and current flows through the device as a result or, equivalently, voltage appears across a device as a result of current flowing through it.
  9. takao21203

    Distinguished Member

    Apr 28, 2012
    Batteries normally only have voltage specification (at least small batteries). This is where the idea eventually comes from. I saw only one case of a non-rechargeable battery with the capacity in mAh printed on it.

    "putting through" some voltage would rather mean to figure out how much voltage the device can withstand. In a sense the resistance could be very high, the current very low, nonetheless the device is destroyed by high voltage effects.
  10. bountyhunter

    Well-Known Member

    Sep 7, 2009
    Actually, different resistor types like carbon composition, carbon film, metal oxide do have max voltage ratings based on construction. But the term "1/2 Watt" defines maximum power, not max voltage.
  11. MrChips


    Oct 2, 2009
    The OP said 120 ohms 0.5W as an example.
    Without using the formula, let us take some example voltages.

    Suppose the voltage is 12V
    The current I = V/R = 12/120 = 0.1A
    The power P = V x I = 12 x 0.1 = 1.2W
    That's more that twice 0.5W

    Suppose the voltage is 6V
    The current I = V/R = 6/120 = 0.05A
    The power P = V x I = 6 x 0.05 = 0.3W
    That's nicely under 0.5W
    Hence, in this example 6V would be a safe limit for 120Ω 0.5W resistor.
  12. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    Resistors have a max voltage rating just like caps and other parts. Small 1/4W and 1/2W resistors are usually rated about 200v. Even running then at 200v will cause breakdown of the resistor, this is very common in CRT TV repair where one of the first places you look is for resistors that are exposed to >80v or so.

    Start resistors used to initiate oscillation in mains SMPS are usually paired in series to better survive the 200-350v they see, and even then, start resistors are a very common cause of breakdown (they eventually go open circuit).

    Philips make very large special resistors rated for 2kV to be used in the focus circuits of CRT TVs, these still break down very frequently even with just a few hundred volts across them and I have drawers full of replacement parts. Even the paint fails on them...

    You should not go talking about 20kV like it is "normal" voltage, things start to go very different any time there are more than a couple hundred volts present on a component.

    As a good design rule I would never subject a small resistor to >80v for any length of time.