how to work out resistance of a bulb? (1 Viewer)

[seanoo]

hi all, the reason i need to know is that the fan on my sog has busted . sog want £46 for a new fan and i have to post the old one to them before they will send out a new one as they only supply on an exchange basis . so i have been to a computer shop and bought a few old 12v 80mm cooling fans. the first one i installed only lasted 2 days and then burnt out so i replaced with a 2nd which burnt out again after 1 day. the last fan which is quite a bit beefier although the same size runs so fast it sounds like a mini jet engine so i have put a 12v 5 watt bulb in the positive lead and this has slowed it down by about half and it works well . its a bank holiday here in spain so i can't get to a shop to get a resistor to replace the bulb but i need to know which resistor to get or is there a better way to lower the fan speed maybe with a pot? all the best sean.

 

[jonandshell]

5watts divided by by 12volts = 0.416 amps.

Therefore, 12 volts divided by 0.416 = 28.84 ohms.

A 30 ohm resistor should do it!

 

[seanoo]

lightning fast reply jon cheers! i will get a few of them on tuesday and replace the bulb.

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[jonandshell]

Of course you could fit a 24 volt fan and this will run much more slowly!

 

[seanoo]

i think i'll stick with the 12v fan i've got as it seems good quality. what i'll probably do is get 3 x 10 ohm resistors and solder them in series to keep any heat down and shrink tube them.

 

[jonandshell]

i think i'll stick with the 12v fan i've got as it seems good quality. what i'll probably do is get 3 x 10 ohm resistors and solder them in series to keep any heat down and shrink tube them.

Three 30 ohm resistors in series will make them 90 ohms!

It you meant parallel, that will make them 3 ohms!

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[jonandshell]

You need a 30 ohm resistor capable of taking the current!

 

[34127]

You need a 30 ohm resistor capable of taking the current!

Yes, make sure you get the correct wattage rated resistor

 

[34127]

5watts divided by by 12volts = 0.416 amps.

Therefore, 12 volts divided by 0.416 = 28.84 ohms.

A 30 ohm resistor should do it!

That's assuming there is 12 volt across the bulb but the above calculation should be a good starting point. You could try a few different values of resistor to get the desired effect. If you have a voltage meter you can measure the voltage across the fan.

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[Terry]

Are you sure you have a 12v fan ? I only ask as quite a few comp ones are 5v -you could be sticking 12v into a 5v one making it buzz around twice the speed -Personally I don't mind my fan whirring away -at least I know it's working
terry

 

[jonandshell]

That's assuming there is 12 volt across the bulb but the above calculation should be a good starting point. You could try a few different values of resistor to get the desired effect. If you have a voltage meter you can measure the voltage across the fan.

Agreed!

Measured resistance of a bulb is always zilch!
It only becomes resistant once the filament warms up. Therefore resistance can only be calculated based on known voltage and current values using Mr Ohm's well proven law!

 

[seanoo]

Three 30 ohm resistors in series will make them 90 ohms!

It you meant parallel, that will make them 3 ohms!

hi jon, no in my post i said 3 x 10 ohm resistors. not 3 x30 ohm ones.

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[seanoo]

Are you sure you have a 12v fan ? I only ask as quite a few comp ones are 5v -you could be sticking 12v into a 5v one making it buzz around twice the speed -Personally I don't mind my fan whirring away -at least I know it's working
terry

definately a 12v fan but just hooked up to 12 v it goes too fast and noisy to do the job.

 

[pappajohn]

i think i'll stick with the 12v fan i've got as it seems good quality. what i'll probably do is get 3 x 10 ohm resistors and solder them in series to keep any heat down and shrink tube them.

Three 30 ohm resistors in series will make them 90 ohms!

It you meant parallel, that will make them 3 ohms!

 

[seanoo]

i will get a pack of different resistors and try those and if it fails i will just leave the bulb there! it doesnt glow very much so there isnt much heat produced.

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[jonandshell]

hi jon, no in my post i said 3 x 10 ohm resistors. not 3 x30 ohm ones.

Sorry, will read before hitting the keyboard next time!!!!

 

[DBK]

I think it would be much more fun if you used a bulb and mounted it inside the cassette. This would improve aim significantly when used at night.

 

[Gizmouk]

Wouldn't it be best to start at step 1 ?
1. Measure the voltage across the original fubar fan.

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[seanoo]

Wouldn't it be best to start at step 1 ?
1. Measure the voltage across the original fubar fan.

hi gizmouk , i dont need to now as i have the fan working as it should at the speed i want albeit with a bulb as a resistor. now i just need to play with a few resistors to get the same affect.

 

[Gizmouk]

Couple of resistors as a potential divider should do the trick

 

[Eddystone]

Surely if you are using a 5w bulb you will need at least a 5w resistor. You still have to dissipate 5w of power as heat.
Brian

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