One or Multiple Resistors for High Power? - Page 1
Forum section: electronics / beginners

author: willindsey
author_rank: Newbie
timestamp: April 17, 2020, 02:11:35 am
content: Hi everyone!I am working on a project to controlling multiple fans. Its a personal project and I am trying to save a few buck. I found a single high watt low ohms resistor option and I also found 4 6 Watt resistors that I could use in parallel as an alternative. The multiple resistors option saves a few bucks. I haven't had to make a design decision like this before and I was wondering what the pros and cons of each option are and if I am making a beginner mistake somewhere in this logic?Thanks and stay safe out there!

author: MarkF
author_rank: Super Contributor
timestamp: April 17, 2020, 02:27:04 am
content: Neither.No resistors.When the MOSFET is turned ON, the current through the resistors will raise the voltage at the MOSFET source pin and possibly turn if OFF again.You will only want a small resistor on the MOSFET gate.

author: willindsey
author_rank: Newbie
timestamp: April 17, 2020, 03:03:45 am
content: Oh that makes total sense and is something I didn't even know about. Thank you for pointing it out.The power source is 1A 12V DC and each fan draws 0.18A at 12V. Are you suggesting to remove the resistors from the schematic or should I relocate them above the drain? I don't follow how the current in the circuit would be regulated if the resistors are removed. The 12V DC power source is also needed to power the 555 timer for pwm while the Mosfet is "on". Thank you for your help

author: digsys
author_rank: Unknown
timestamp: April 17, 2020, 03:33:46 am
content: A 12V Fan on a 12V supply should need NO resistor at all ! especially for that low current. IF you are concerned with Inrush - even though, fans being mostly Inductive - ie current at start-up is minimal, then you can add say 2-5R resistor in series with each fan - high or low side doesn't matter, but ONLY needed if Inrush is an issue. Calc based on 12V supply / 0.18A current draw ~ 66R

author: MarkF
author_rank: Super Contributor
timestamp: April 17, 2020, 04:10:31 am
content: Quote from: willindsey on April 17, 2020, 03:03:45 amI will answer the question with a question...How is the current regulated if the fan is plugged directly into a 12V power source?You may however want diodes across each motor for back-emf when the PWM switches OFF.

author: WattsThat
author_rank: Frequent Contributor
timestamp: April 17, 2020, 04:24:52 am
content: You’re probably conflating the need for resistors when using LED’s which are controlled by the applied current. Your fans are just dc motors which are voltage controlled devices, not current. When you limit the current to a dc motor, it just slows down.However it was that you calculated the resistance and power for the resistor(s) had to be quite convoluted to arrive at a power requirement of 24 watts

author: willindsey
author_rank: Newbie
timestamp: April 17, 2020, 04:47:06 pm
content: Its apparent now that I was miss-understanding the need for the resistors and was treating the problem as if I was using LED. Thank you for the clarification. I calculated power as V*I * 4 (#fans), but sourcing the components on Mouser, the 4 watt resistors were cheaper hence the 24 watts. Hey it gave a safety factor lol

author: willindsey
author_rank: Newbie
timestamp: April 17, 2020, 04:53:40 pm
content: Thanks Mark! Your question prompted me to dig into more research on DC motors. Initially, I was thinking that the resistance in the DC motor was primary from the coil winding and assumed it was relatively small (between 1 and maybe 4 ohms). Under this assumption I thought the motors would max out the 1A power supply. I have learned that the current rating of the DC motors is essentially derived from the operational voltage and resistance of the motor. Additionally, I learned that the DC motors are able to self regulate due to the back emf they generate. The highest current demands is when the motors are not spinning, like what digsys was mentioning regarding Inrush. Thank you for your help and great reply! Cheers