Ohm’s Law – Resistors for Fan Speed Part 2 - Page 1
Forum section: electronics / beginners

author: the offset voltage
author_rank: Regular Contributor
timestamp: August 04, 2025, 08:28:00 pm
content: Hi,I'm the OP of this topic/project:https://www.eevblog.com/forum/beginners/ohms-law-resistors-for-fan-speed/Short into before my question: I am working on this application where I have to design a simple switch speed controller for a ventilator where I have to use a 12V ventilator, a rotary switch and I have to calculate the values of the resistors (I have to use 3 resistors to have 3 selectable speeds for the ventilator) - i have calculated the resistance as this was covered onhttps://www.eevblog.com/forum/beginners/ohms-law-resistors-for-fan-speed/.My new question: Should i connect a diode in parallel with the fan for protection?I saw in other schematics, that they have a diode in parallel with a motor, and i don't know if should i add one, i see many circuits uses 1n4007.As my fan is 12V/80mA and will be powered by a power supply, how should i select my diode, if i need a diode in parallel with the fan. I don't want to select a diode if is the case without knowing how that diode is selected.Thank you,

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 04, 2025, 08:45:03 pm
content: No.

author: the offset voltage
author_rank: Regular Contributor
timestamp: August 04, 2025, 08:57:01 pm
content: Thank you,

author: fourfathom
author_rank: Super Contributor
timestamp: August 04, 2025, 09:36:50 pm
content: And the reason for the diode is to protect a transistor or relay from a "back EMF" voltage spike caused when the fan is switched off or cycled on/off by PWM duty-cycle speed control, or caused by motor commutator make/break. Since you are using a resistor and (?) variable power supply voltage for speed control these spikes should not be an issue.

author: TimFox
author_rank: Super Contributor
timestamp: August 04, 2025, 09:48:58 pm
content: On the other hand, if the motor uses brushes ("DC" or "universal" motor), it is common to put a reasonable capacitor across the voltage terminals to minimize noise from the brushes arcing during commutation.

author: CaptDon
author_rank: Super Contributor
timestamp: August 06, 2025, 09:38:24 pm
content: First off, a three speed fan switch would normally have one position directly wired for 'full speed' meaning you may only need two resistors. Second off, this project sounds like the class instructor knows less than the students. You can't make all calculations based on 80 milliamps because as you lower the voltage at the fan through a resistor the current 'should' also drop so the bottom line is you could calculate a guess but trial and error will be the determining factor. Not wise to double post the same topic line either.

author: BeBuLamar
author_rank: Super Contributor
timestamp: August 07, 2025, 08:28:25 pm
content: I keep wondering. If this is only an excercise in Ohm law does the instructor make you buy a motor and resistores and switches to do it? Do you already have a motor? I didn't see any where that you did any test with the motor before going ahead and buy all the parts.

author: the offset voltage
author_rank: Regular Contributor
timestamp: August 07, 2025, 10:14:38 pm
content: I keep wondering. If this is only an excercise in Ohm law does the instructor make you buy a motor and resistores and switches to do it?- It's not an exercise, this is an Application Activity after Ohm's Law section. Each section from this book (Principals of electric circuits by Thomas Floyd) has at the end an Application Activity where you have to design a PCB for a specific project. Now, in my case, i try to reproduce it with the aim to apply what i have learned and also with the objective to design (learn how to design in my case) a PCB.Do you already have a motor? I didn't see any where that you did any test with the motor before going ahead and buy all the parts.- yes, i mentioned on:https://www.eevblog.com/forum/beginners/ohms-law-resistors-for-fan-speed/

author: TimFox
author_rank: Super Contributor
timestamp: August 07, 2025, 10:16:01 pm
content: However, controlling a motor isnotcalculated using Ohm’s Law itself.

author: the offset voltage
author_rank: Regular Contributor
timestamp: August 07, 2025, 10:18:54 pm
content: Well, don't blame me, i only reproduce the application project from Ohm's law section..And to be honest, if from the first application project i have headaches, i can only imagine what i will meet at the middle of the book as i will advance

author: TimFox
author_rank: Super Contributor
timestamp: August 07, 2025, 10:35:28 pm
content: Quote from: the offset voltage on August 07, 2025, 10:18:54 pmI'm not blaming you--but I find it hard to understand why this was posed to you as an example of Ohm's Law, rather than the behavior of motors.