Brushless DC fan - power question

Stuey · 12th October 2009, 01:22 PM

Hi all,

Just a quick one. I've Googled for a while for no joy.

I'm running a 12v 0.22A brushless DC computer fan to cool a drill charger I built. It's powered from a spare 12v tap on the main transformer, separately rectified with a small integrated bridge rectifier and no filtering, and seems to work fine.

It'll be run for up to ten hours straight.

Should I be using some smoothing capacitance on the motor circuit? I don't know much about these motors.

Cheers

Stuey

Steerpike · 12th October 2009, 06:21 PM

A smoothing cap might do more harm than good. A transformer winding rated at 12Vac means 12V RMS. When that is rectified and smoothed, it gives around 15V dc. But without smoothing, it remains at 12V RMS - or marginally less - taking the voltage drop of the rectifier into account.
So adding a smoothing cap might also require a voltage dropper resistor to ensure you dont exceed the motor's voltage limit.

The motor probably won't mind a rippley Dc at all.

Stuey · 13th October 2009, 03:00 AM

Quote:

Originally Posted by Steerpike

A smoothing cap might do more harm than good. A transformer winding rated at 12Vac means 12V RMS. When that is rectified and smoothed, it gives around 15V dc. But without smoothing, it remains at 12V RMS - or marginally less - taking the voltage drop of the rectifier into account.
So adding a smoothing cap might also require a voltage dropper resistor to ensure you dont exceed the motor's voltage limit.

The motor probably won't mind a rippley Dc at all.

Yes, I had thought about the voltage peak with a cap, but as you suggest my main concern was the 'jagged' DC ripple.

Thanks; I'll leave it.

Stu

DigitalJunkie · 13th October 2009, 05:18 AM

I'd be kinda concerned that the circuit in the fan might not be so happy with rippley DC,but if it seems to be working,meh. If you decide to add the cap,you can make it a CRC,to drop some voltage and help 'filter' some of the noise from the fan. I've noticed that some fan's put the switching noise back onto the rail,but a (C)RC on the fan usually cures it for me,as well as making the fan run a bit slower and quieter.
Just a thought.

star882 · 13th October 2009, 05:51 AM

Fan motors are very forgiving of power.

12th October 2009, 01:22 PM	#1
Stuey diyAudio Member Join Date: Mar 2006 Location: Perth, Western Australia	Brushless DC fan - power question Hi all, Just a quick one. I've Googled for a while for no joy. I'm running a 12v 0.22A brushless DC computer fan to cool a drill charger I built. It's powered from a spare 12v tap on the main transformer, separately rectified with a small integrated bridge rectifier and no filtering, and seems to work fine. It'll be run for up to ten hours straight. Should I be using some smoothing capacitance on the motor circuit? I don't know much about these motors. Cheers Stuey

12th October 2009, 06:21 PM	#2
Steerpike diyAudio Member Join Date: Jan 2006 Location: Johannesburg, South Africa	A smoothing cap might do more harm than good. A transformer winding rated at 12Vac means 12V RMS. When that is rectified and smoothed, it gives around 15V dc. But without smoothing, it remains at 12V RMS - or marginally less - taking the voltage drop of the rectifier into account. So adding a smoothing cap might also require a voltage dropper resistor to ensure you dont exceed the motor's voltage limit. The motor probably won't mind a rippley Dc at all. __________________ Steerpike's Toybox

13th October 2009, 05:51 AM	#5
star882 diyAudio Member Join Date: Mar 2007	Fan motors are very forgiving of power. __________________ "Fully on MOSFET = closed switch, Fully off MOSFET = open switch, Half on MOSFET = poor imitation of Tiffany Yep." - also applies to IGBTs!

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13th October 2009, 05:18 AM	#4
DigitalJunkie diyAudio Member Join Date: Apr 2003 Location: Portland,Oregon Blog Entries: 3	I'd be kinda concerned that the circuit in the fan might not be so happy with rippley DC,but if it seems to be working,meh. If you decide to add the cap,you can make it a CRC,to drop some voltage and help 'filter' some of the noise from the fan. I've noticed that some fan's put the switching noise back onto the rail,but a (C)RC on the fan usually cures it for me,as well as making the fan run a bit slower and quieter. Just a thought.

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