Ohm´s Law Question

Judas Priest

Super Member
I used to be able to do this in my sleep, but I´m out of practice.

If I want to use a 12v fan running on 24v, what resistor do I need?

The 12v fan draws 40 mA.

Do I need 300 ohms? Is that correct?

edit: Oops, I thought I was in the DIY forum.
 
voltage over current gives resistance, so 12/0.04=300, the effective resistance of the fan is 300 ohms. No further calcs necessary since 24 volts is 2X 12. use a 300 ohm resistor, just as you already figured out! Dissipates near a half a watt, so best use 1W.
 
I made this my desktop background on my workshop computer. I use it very frequently, saves me from having to think too much about it.

https://commons.wikimedia.org/wiki/File:Ohm's_Law_Pie_chart.svg

the value you want is towards the center, pick the formula from the outer ring that has the two values you have. Just have to remember that E is volts.

That "E" is a thing with me, as that's the way I was taught. Current (I) is measured in amperes. They couldn't use C because the capacitors got it first. Resistance (R) is measured in ohms. Electromotive Force (EMF) is measured in volts. Formulas should use I, R and E, not V. But I'm old. And usually grumpy. May the (electromotive) force be with you.
 
OHMS_LAW_VIPR-Relationship.jpg

No dishonor in putting a printed copy up on the wall (or have a jpg very close at hand).
 
For measuring logs?
No... For going to the moon.:rolleyes:
slide-rule.jpg
 
That "E" is a thing with me, as that's the way I was taught.
Same, but it can produce some confusion if one doesn't know that going into it. PIE is all well and fine, I like apple, but watts that got to do with ohms?
 
Speaking of Ohm's law and apples. From the label, it might be quite electrifying and shockingly good with a powerful flavor.


upload_2018-2-1_21-41-12.png

BTW, there is no potential to resistance to this cider and to do so is futile.
 
Love the moon poster but in actual fact it was a LOT of slide rules...and an onboard navigational computer. A very small one. :p
 
ah the AGC core memory. That was actually a damned impressive computer given the tech of the time and the constraints of weight and such it had to work within.
 
ah the AGC core memory. That was actually a damned impressive computer given the tech of the time and the constraints of weight and such it had to work within.

Thus my understanding of and admiration for it.
 
That "E" is a thing with me, as that's the way I was taught.
Same here. The "V" just doesn't register with me. It's one of those things pounded into us in class, and decades later I still can't relate to "V" in the formulas. :D
 
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