Resister help please

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original45
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Resister help please

Post by original45 »

Trying to reduce cooling fan speed to make it quieter.
I have a couple 12 v 1 watt cooling fans. I have already run them from 8 v which is ideal speed.

So I thought I would add inline a1/4 watt resister 10 k.
Fans won't even run yet it will on 6 v even without resister.
Tested the resisters both my 5 k And 10 k resister on ohms and their fine.

So why won't the fans run ?.I even tested inline on a car Bulb .
A guess here is their too big maybe not enough juice to run the fans ? I don't know.
Trying get these to run around 8 v ish .
Advice appreciated.

Previously posted in wrong section sorry.
Thanks again.
wa10
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Re: Resister help please

Post by wa10 »

10k is much too high value, you want to be in the low hundreds of ohms, not 10 or 500 thousand ohms,

start at 200ohm & see if its slow enough, if you go too low in current the fans won't start without some help flicking them,

current = voltage divided by resistance 12 /10000 = .0012 amps
watts = volts x amps 12v x .0012a = .0144w

try running the 2 6v fans connected in series in series first, that often works to undervolt them.
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Buick Mackane
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Re: Resister help please

Post by Buick Mackane »

According to a quick calculation you'll need a 40 ohm half watt resistor to run the fan on 8 volts from a 12 volt supply, Or better still try a thermistor their resistance drops as they heat up

EDIT just noticed you want to run two fans, So make it a one watt resistor
Last edited by Buick Mackane on 27 Oct 2020, 21:30, edited 2 times in total.
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wa10
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Re: Resister help please

Post by wa10 »

47ohm worked spot on for my yaesu fan when all i could get was 6v motor but thats one fan.
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original45
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Re: Resister help please

Post by original45 »

Yup it's confused the hell out of me.
Yes I am presently running 2 x 12v cpu fans 1watt each side by side.
At 12 v their too fadt.if I run them 8 v their perfect.

But that required 2 power supplies one for radio other for fans.I'm not going to wire inside radio because I'm not confident.so intent to wire them via the 12 v power cable to radio but somehow add the resisters to slow fans .

Finding it hard to think of a simpler better safer way.
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Buick Mackane
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Re: Resister help please

Post by Buick Mackane »

original45 wrote: 27 Oct 2020, 22:28 Yup it's confused the hell out of me.
Wire them in series as suggested above, Each fan will then be supplied with 6 volts
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Re: Resister help please

Post by original45 »

Thanks guys, will try that tomorrow. Sounds ideal. Done this with speakers but didn't think possible with fans.
Learnt something new.
wa10
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Re: Resister help please

Post by wa10 »

i was answering your first post & thought you wanted about 6v & hit submit & it was gone :D ,

you could always use a 7808 8v regulator if 8v is ideal,

1w fans are very low power,
100ohms works ok in my ft100d but 2 fans are more than 1w each,
47ohms worked in my base radio but that was 1 fan & more power,

3 options
wire the fans in series, + of one fan connected to - of the other leaving one positive & one negative wire to connect to the 12v supply
use a 8v regulator
or experiment with resistors in the range of what's been suggeted earlier,

good luck.
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original45
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Re: Resister help please

Post by original45 »

Omg , early rise this a.m so set up a pair of my fans.wired in series as you guys mentioned.
Going to explore the other options too but like with all I do I prefer straight and simplr.

The old fans I had on were slightly bigger and faster so the sinks were ice cold 99% time .But alas were noise .
This is so simple in series.
I just wish now I had confidence to solder it inside radio to come on on switching on.

Always admire those who fit temp sensors or fans come on on tx even and clever mods but this will surely help to keep my radios and amps cool runnings without voodoo magic.
Thanks you guys, made my day 😁
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Buick Mackane
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Re: Resister help please

Post by Buick Mackane »

original45 wrote: 28 Oct 2020, 08:57
Always admire those who fit temp sensors
Thanks you guys, made my day 😁
Fitting a temp sensor is as easy as wiring fans in series,

Glad you got the desired results :thumbup:
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Re: Resister help please

Post by 26mb04 »

I've probably said this before, but it's always worth checking if small 12V fans will run from a USB (5V) supply. Many will, and they'll be much quieter than running at full whack. Obviously they'll be slower, so you'll need to make sure they are capable of providing enough air flow for the task. In my limited experience, a slow constant fan can be just as effective as an intermittent fast one. Make sure, though, that you test first.
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original45
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Re: Resister help please

Post by original45 »

Will look at this too for future use.presently trying to fit anything that gets warm with fans .mobile set with amp next.
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Re: Resister help please

Post by Robert19901 »

I had a similar problem with a fan, read the correspondence, did as advised, everything works, thanks a lot.
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