Resistor from 17v to 13v for 40mm fan

[JeffEmbracedDC]

Hey, guys. My mamba max esc is getting a little hot for my liking, so I bought a 40mm 12v fan (from radioshack (http://www.radioshack.com/product/index.jsp?productId=2102824&cp)). It has a voltage range of 10.8v-13.2v. I am running 12 cells on my mamba max which is about 17v peak. I don't know all that much about electronics, so I'm sure what size resistor I would need. Do I need to measure the ohms of the fan or something? Thanks, guys.

-Jeff

[glassdoctor]

You could figure a resistor to drop the voltage, but you don't need to.... just hook it up. I run "12V" fans on my 4s lipo packs all the time... which come off the charger at 16.8V. IMO it won't hurt the fan at all.

Besides, the first time (and every time) you yank full throttle the voltage will drop to about 12v anyway. So the voltage will often be well below 12v with a resistor.
If you want to try a resistor, to drop the voltage by 2v to the fan, assuming it's rated at 150ma... you need a resistor around 12-15 ohms. 2 (amount of voltage drop) / .150 (current for the fan) = 13 (ohms)

And if you want a 5v drop.... 5/.150= 33ohm

That could be messed up but I think it's correct.

Google "ohms law" and "ohms law led" for some good reading....

[JeffEmbracedDC]

Hey, I appreciate it. I might try running it straight off of the battery. The only reason I wouldn't is because the fan I bought was WAY over priced ($14 something with tax) and I won't want to have to buy another one in a few days after mine fries. I'll probably do a few test runs and measure the voltage of the batteries every minute or so and see what the average is. Thanks for your input!

-Jeff

[eVaDeR85]

Id say 17v for a 12v fan will not hurt it at all. Its also common for manufacturers to use components rated for twice the voltage and current handling for which the device is intended to be used for. Ive seen 12v case fans for computers take 24v for extneded periods before without problems. The only thing is they do not last near as long as you can guess but if you factor in your fan will not be running constantly nor is anywhere near twice rated voltage I think your in the clear. However I will say this..your going to have one nice loud fan that blows TONS of air lol.

[JeffEmbracedDC]

I've tested it on the the 12 cells and it ain't too loud. And with the car running you'd never ever hear it :) hah. It's not too bad, and it does move quite a bit of air. 7.7 CFM @ 12V (6500 rpm) It should mount perfectly on the ESC. Only thing, is that I'd like it to turn on with my ESC switch, although I think I'll have to settle for it turning on when I plug in my batteries. That's fine with me though. Thanks for the second opinion!

-J

[eVaDeR85]

Well if your really good with electronics you can design a transistor device that will turn the fan on when you switch on your reciever. The voltage used to power your reciever and servos can be used to power the gate of a high enough rated npn transistor with the appropriate resistor on the gate of course to keep from burning out the transistor. It would be fairly efficient as well if you designed the circuit to completely saturate the transistor so there is little or no voltage drop across the emitter and collector.

[Demon-TC3]

no i would have it plugged in, in case the esc thermals (doubt it will) you can shut that off without stopping cooling!

[JeffEmbracedDC]

Aye, good point. I decided yesterday that I'm going to solder the fan straight in to the battery circut. I've mounted it pretty solidly with cable ties and i really think it's gonna keep it a lot cooler. Thanks.

-J

[Demon-TC3]

no worries man, just get me a vid asap! :wave: :D

[Rufus]

Aye, good point. I decided yesterday that I'm going to solder the fan straight in to the battery circut. I've mounted it pretty solidly with cable ties and i really think it's gonna keep it a lot cooler. Thanks.

-J

That should work great, just be careful if you plan to use lipos... you don't want to forget disconnecting the fan after you're done. If left On for a long period of time it will deplete and kill your pack...

[Demon-TC3]

na hes using NIMH cells as far as im aware...

[eVaDeR85]

Ok ive got to explain this better. The Fan's positive terminal would be connected to battery input. Gate of transistor connected with proper resistance to 6v lead from BEC. Fan negative connected to emitter of transistor. Collector connected to battery negative. What this would do is when the ESC is turn on and the BEC power up the reciever, the 6v from the BEC would power the gate of the transistor allowing current to flow through fan directly from battery. Now whether or not the ESC thermals the BEC still provides power to the reciever so power to the fan will not be interupted either way. Because I really dont think ESC's actually kill power to the reciever and servos when they thermal correct?

[Demon-TC3]

no they dont but the BEC itself will heat up using 12 cells so it is better to shut off all power from the esc and cool it down...just solder a switch into the fan if your really that picky but i dont think its much to worry about.

[tcolesen]

Just FYI, if you want to drop voltage, diodes work well. Each diode will drop approximately 0.7v, so you can connect them in series to drop 0.7v, 1.4v, etc.

EDIT:
I just looked at the pics of your buggy, and it appears that the Mamba Max doesn't get any cooling air running over the heatsinks. This would definitely cause it to get hot. If you could mount it to the center diff support, you might not need a fan.

[JeffEmbracedDC]

I have a hole in the body directly above it that was previously used as a nitro-refil hole. I will probably add a piece of lexan inside and outside to scoop air directly towards the esc. I'm gonna stick with the fan for now mainly to keep it simple and keep the layout clean. Thanks for your help. I'll post some more photos of it when I set it yop later today or tomorrow. Later, guys.

-Jeff