Maximum Current a Resistor can handle - Page 1
Forum section: electronics / beginners

author: VanillaIceCream
author_rank: Contributor
timestamp: August 05, 2021, 03:26:58 am
content: Maybe a dumb question:Can you use resistors for a really big amount of current (+1Amps for example) as long the input voltage is very, very low and the power dissipation of the resistor doesn't exceed the maximum rated power dissipation value or is there a maximum current limit for each resistor type?I know that the maximum current allowed is the square root of the ratio between the resistors power rating and the resistance butthere were no maximum current values in any datasheets of different 0.25 W resistors I found so that's why I'm asking.

author: BlackICE
author_rank: Regular Contributor
timestamp: August 05, 2021, 05:47:16 am
content: I x I x R = max rated power. Since the R is fixed and non-zero I can never be infinite.

author: gcewing
author_rank: Regular Contributor
timestamp: August 05, 2021, 11:25:06 am
content: Since V=IR, if V is very very low, then I will also be very very low. There's no way you can get "unlimited current" through a resistor without unlimited voltage, and therefore unlimited power.There is no need for maximum currents to be listed in resistor data sheets, since, as you pointed out yourself, it can be calculated from the resistance and the power rating.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 05, 2021, 03:14:05 pm
content: RTFDS.Typically, resistors have a rectangular SOA (safe operating area), with maximum voltage and current across the family regardless of value, and a power limit cutting across that rectangle. So you get a truncated hyperbola region. (Hyperbola being, for constant power, V and I are inversely proportional, so it's a section of the 1/x curve, scaled appropriately.)This is true even for "zero ohm" jumper types. The limit might simply be convenience, but might also be limited by materials ("zero ohm" jumpers are typically some 10s of mohm; a full solid layer of resistive material, not an actual short), construction (typically resistors are laser-trimmed, blasting away a slot of material; the inside corners of that slot can get VERY hot at ratings), leads (THT resistors typically use tin-plated steel leads) and so on.How much, depends on the type. There are typical ratings for typical parts/styles (e.g., axial 1/4W carbon film, usually 300V or so), but safer not to make assumptions. Also, if you're buying eBay/Ali/Amazon specials, all bets are off...Interestingly, I've seen chip resistors as small as 0603 with multi-kV ratings -- sounds unbelievable, right? You're right to think so; they only achieve that rating when encapsulated in potting. Read the datasheet carefully.Tim

author: TimFox
author_rank: Super Contributor
timestamp: August 05, 2021, 03:32:28 pm
content: You can't control both the voltage and current across a given resistor: even for a non-linear resistor, the current is a monotonic function of the voltage.For a given model that is available in a wide range of resistance values, there may also be a manufacturer's absolute limit on voltage and another on current that are imposed over the power limit. Usually, for resistors, this only involves the voltage limit for high-resistance parts.

author: Zero999
author_rank: Super Contributor
timestamp: August 05, 2021, 08:31:27 pm
content: Solve Ohm's law for current, using the resistor's value and power rating.

author: VanillaIceCream
author_rank: Contributor
timestamp: August 06, 2021, 05:49:41 am
content: Quote from: gcewing on August 05, 2021, 11:25:06 amSorry, I should have been more specific.What I meant is what would happen if the input voltage would go down and the resistor value would go down too so that there's a high current on the resistor while the maximum power dissipation on the resistor doesn't exceed the given maximum power dissipation value.Just to be sure:You're saying that one 0.25 W resistor with a value of 0.25 Ohm can at worse case definitely handle a current of 1 Ampere if the Input Voltage is 0.25 V so that thepower dissipation is 0.25 W right?(P=V x I0.25W /1A =0.25V0.25V / 1A =0.25 ohms)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 06, 2021, 05:57:45 am
content: 1A probably, but you'll have a harder time justifying the same reasoning at say 0.1 ohm, or 0.01. Again, read the "friendly" data sheet; the manufacturer knows their product best.Tim

author: Zero999
author_rank: Super Contributor
timestamp: August 06, 2021, 07:35:14 am
content: Quote from: VanillaIceCream on August 06, 2021, 05:49:41 amThere's a limit to the lower resistance, for a given package size. There are zero Ohm links, in resistor packages, which of course don't have zero resistance and have a finite current rating.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 09, 2021, 01:29:36 am
content: Quote from: VanillaIceCream on August 06, 2021, 05:49:41 am- we don't say "an input voltage" if we are talking about a simple two-pin part (but not about some sophisticated device). We use the terms 'input voltage, output voltage' if we are talking about a quadrupole (e.g. an amplifier). But for a resistor or a diode, we have a voltage across a resistor (or across a diode).

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 09, 2021, 01:33:00 am
content: Quote from: VanillaIceCream on August 06, 2021, 05:49:41 amI would rather say 'at best case' but not 'at worst case' because we have to take a temperature into consideration. I mean ambient temperature (or resistor average temperature).

author: IanB
author_rank: Super Contributor
timestamp: August 09, 2021, 02:01:12 am
content: Quote from: VanillaIceCream on August 06, 2021, 05:49:41 amWhen analyzing any given situation, it's often helpful to consider the limiting case. In this example, you might consider the limiting case to be a length of ordinary copper wire.Take any average piece of hookup wire, say 24 gauge. If you put 1 amp through it, it will be fine. If you put 10 amps through it, it will (at a guess) get pretty hot. If you put 100 amps through it, I am fairly sure it will get white hot and melt.Also note that power dissipation depends heavily on the surroundings. If you take a typical 15 amp rated mains extension cable, leave it coiled up on a drum, and put 15 amps through it, it may get hot enough to catch fire.In a similar way, no resistor has a single "maximum power dissipation" value. It all depends on how the resistor is installed, and how the heat sinking is arranged to keep it cool. Cooling could be arranged by having a lot of copper near it on the circuit board, or by having good air circulation around it. If you expect a resistor to get hot, you need to consider where the heat is going to go, and don't put heat sensitive items (like capacitors) near it.