Best way to power a heatsink fan for a power supply project - Page 1
Forum section: electronics / projects

author: microbug
author_rank: Frequent Contributor
timestamp: April 30, 2013, 05:54:29 pm
content: Hi,I am making abasic bench power supply, and as it uses a LM317 (TO-220) it gives off a lot of waste heat, which is dispersed with a heatsink. In the video, it suggests a large, 'passive' heatsink - without a fan, but I already have a functioning CPU cooler with 12v fan from a few-year-old desktop (Google RI5-8IDSF-X1-GP) which I think would work fine. The problem, however is how to power it. The project runs from the mains, going through a transformer, decoupling capacitor, bridge rectifier and large (10mf) smoothing capacitor before reaching the regulator. I was thinking of powering it with the pre-regulator current and using a larger transformer, but the amount of power going through the fan would likely kill it. Any suggestions?

author: mariush
author_rank: Super Contributor
timestamp: April 30, 2013, 06:23:49 pm
content: The easiest method would be to just replace that fan with a 24v fan. Then you can just use a diode to do a half wave rectification and a small capacitor to smooth out the DC a bit and the fan will get about 12-15v directly from the transformer through the diode that rectifies the ac voltage, so your fan will cool down the heatsink and yet will be silent as it won't run at the highest rpm.Otherwise, you'd have to use a regulator to set the fan to a particular voltage, I don't see other easy way. At low loads, the transformer will give a peak voltage of over 12v, let's say up to 15-17v, and that's too much for a fan, not to mention it will be too noisy.

author: microbug
author_rank: Frequent Contributor
timestamp: April 30, 2013, 06:25:13 pm
content: What sort of size capacitor do you think would be appropriate?

author: mariush
author_rank: Super Contributor
timestamp: April 30, 2013, 07:03:11 pm
content: Well, with a single diode for half the time you'll get 0v and for the other half of time, the voltage will go up from 0 to about 15v peak and then from 15v down to 0.. so you'd need a big capacitor to keep the voltage all the time above a threshold.You'd probably have to tap into the voltage coming out of the bridge rectifier you already use.From there, you can reuse that formula that's shown in the video about sizing the capacitor,but adjust for half wave rectification and your new Vripple ...you want a minimum of about 10v and you have a peak of about 15v , your frequency is your mains frequency, the fan would probably use about 0.1-0.2A.. so your capacitor should probably be aboutfor 10% ripple ( let's say 13-15v) C = 5 *0.2 / 15*100 = 6.66 e-4 ... i think that's about 660uF, so if you're going to use a 470uF 25v capacitor, the voltage is probably going to fluctuate between 10-15v.

author: microbug
author_rank: Frequent Contributor
timestamp: April 30, 2013, 07:04:58 pm
content: OK, thanks. I'll look at that formula again, it'll probably be useful some other time.

author: SeanB
author_rank: Super Contributor
timestamp: May 01, 2013, 06:12:03 am
content: Simplest will be to make a small board to take the unregulated main supply and have a 7812 on it, or another 317 with appropriate resistors, to make a 12V supply for the fan. Most of these fans use around 300mA max, so the regulator will only need a small heatsink, preferable in a line so air is blown over it by the main fan. Small board with the regulators, and 2 470n ceramic caps and possibly 2 resistors will be easy to fit next to the main heatsink.

author: microbug
author_rank: Frequent Contributor
timestamp: May 01, 2013, 06:30:53 am
content: Do you mean that I should put the 7812 / 317 directly onto the mains or onto the AC coming out of the transformer? Won't the regulator that powers the fan fry because there is so much excess power running through it?

author: Jon86
author_rank: Frequent Contributor
timestamp: May 01, 2013, 06:31:52 am
content: 7812 is probably going to be your best bet. If you want to go with something more simple, try using a power resistor and hook it up to the output of the rectifier. The fan speed will vary, but it's pretty simple.

author: microbug
author_rank: Frequent Contributor
timestamp: May 01, 2013, 06:33:40 am
content: So, should I attach the 7812 to the output of the bridge rectifier or the transformer?

author: Jon86
author_rank: Frequent Contributor
timestamp: May 01, 2013, 06:45:38 am
content: Yes.

author: microbug
author_rank: Frequent Contributor
timestamp: May 01, 2013, 06:51:42 am
content: Yes to which?!

author: Jon86
author_rank: Frequent Contributor
timestamp: May 01, 2013, 06:53:06 am
content: Sorry, I read it wrong. Connect it to the output of the rectifier.Also, (24 - 12) * 0.3 = 3.6WYou probably want a small heatsink on that too, just in case you didn't think about it

author: microbug
author_rank: Frequent Contributor
timestamp: May 01, 2013, 06:55:13 am
content: Thanks!

author: SeanB
author_rank: Super Contributor
timestamp: May 01, 2013, 09:52:22 am
content: Yes, to C1, like the other 317. It will only need a small heatsink on it, a clip on one only a little bigger than the TO220 package will do, easily found.

author: Jon86
author_rank: Frequent Contributor
timestamp: May 01, 2013, 10:52:03 am
content: If you're in the Uk then bitsbox.co.uk do great TO220 Heatsinks for less than a quid.