# I am adding 1 ohm resistor with a coil in series and an connecting to a 25 W DC source. How to choose the correct resistor of 1 ohm?

I am adding 1 ohm resistor with a coil in series and connecting to a 25 W DC source. How to choose the correct resistor of 1 ohm? Is 1 ohm, 25W resistor is OK for this circuit? Please help.

The supplied voltage is 5 volts and current in the circuit is 5 amps.

• Do you mean '25 V DC source'? Jan 9, 2022 at 11:01
• Your question can't be answered because coil resistance and the voltage of the supply is required to calculate how much power is dissipated in the 1 ohm resistor. Jan 9, 2022 at 11:02
• Since it is connected to the DC supply so coil's resistance is zero. The supplied voltage is 5 volts. and current in the circuit is 5 Amp. Jan 9, 2022 at 11:06
• What kind of coil have you got that has a resistance of zero? Jan 9, 2022 at 11:12
• @ShivamKumar I highly doubt your coil has zero resistance, unless it is a superconductor. Jan 9, 2022 at 11:37

If you have a 5 V DC power supply connected to a 1Ω resistor and a coil with negligible resistance in series, then you will draw 5 A, and dissipate 25 W in the resistor.

You will need a 1Ω resistor that's capable of dissipating 25 watts. Such a resistor will be physically much, much larger than the typical 1/4 watt resistors you use on circuit boards. In catalogues, they will be called 'Power Resistors'. There are several ways to lose this power. Some types are designed to be used in free air and run very hot, cooled by natural convection. Some must be bolted to a heatsink. By Dmitry G - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=9806443

For instance, the above resistor is designed for 12 watts in free air, the hand gives you a reference for the size.

• So 1 ohm 1/4 watt resistor is ok to use by keeping it in air? Jan 9, 2022 at 11:27
• @ShivamKumar NO! you misunderstood the answer. You need a resistor rated at 25W, not 1/4W. The 25W resistor still needs cooling by some means. A 1/4W resistor would burn up instantly at 25W Jan 9, 2022 at 11:30
• @ShivamKumar for reference, a resistor that can handle 25W in free air without a heatsink will be pretty big. How big? This one (digikey.com/en/products/detail/nte-electronics-inc/25W1D0/…) is 2.5in x 0.5in x 0.5in. It could heat up to anywhere from 50C to 100C, which is pretty dangerous to touch. Jan 9, 2022 at 11:41