Thermal design for cc load - the fan. - Page 1
Forum section: electronics / projects

author: conducteur
author_rank: Regular Contributor
timestamp: May 10, 2014, 11:22:33 pm
content: * I don't have soldered al the components yet on the pcb.I just started this project, a dummy load to test batteries and power supply's and heatsinks. It's based on dave's schematic in eevblog nr 102.The opamp is a lm2904, i use 12 12 ohm resistors (so should be 1 ohm) to measure the current, and a buz11 mosfet. This works just fine.I've added a pic microcontroller to control a 16*2 lcd. With an lm35 i want to measure the temperature of the heatsink i want to use, using the internal ADC in the pic, another adc channel is used to measure the current an display that info on the lcd (= voltage across my 1 ohm resistor). I want to use the pwm module in the pic to control a fan if the heatsink gets too hot.I want to put the circuit in a box, with a plexiglass cover on it, so you can still read the data on the lcd display. The backside will be open, and the heatsink will extend out of the box. (i think i'm going to make this box with my cnc mill).I have 12*0,25W resistors for shunt resistor, that's 3W. So the max current in my cc load is limited to +/- 1,6 a 1,7 Amps. The maximum power will depend on the heatsink and fan. Where is the most efficient place to place the fan? I think i'ts best to cool the ribs of the heatsink, wich will extend out of the case, so its not easy to mount a fan on that side of the heatsink.The two TO220 at the left side of the lcd screen are the voltage-regulator ic's for 12 and 5 volt for the circuit. If i place the fan there, in the left upper corner, wil it help enough? If you blow fresh air along the lm35 mounted on the heatsink, will it affect the measurement of the actual heatsink temperature, is that a problem? My load doesn't need to be able to dissipate a certain amount of power, but i just want to maximize the maximum power that i can dissipate in my load, and if i do that correct, i think it will be sufficient for the things i want to do wit it.If you have any suggestions for me, don't hesitate to write it down here! Thanks a lot!

author: digsys
author_rank: Unknown
timestamp: May 10, 2014, 11:41:03 pm
content: First off - mount the heatsink outside the box, which is preferably metal? Diecast? Al? That alone will avoid a few problems,especially heating up everything inside !! Make sure the fins are perpendicular = airflow. In the loads I've made (and others),we chose heatsinks that were designed to have a standard fan fitted to the ends, sure is a lot less work.

author: conducteur
author_rank: Regular Contributor
timestamp: May 11, 2014, 09:26:39 am
content: Yes, the heatsink will extend outside my box. So, i need a different shape of heatsink?

author: SeanB
author_rank: Super Contributor
timestamp: May 11, 2014, 09:39:02 am
content: Preferably go get yourself a pair of old Intel P4 or i7 heatsinks, as these are both compact and come with a fan. They also can dissipate 100W with the fan running, in a compact package. If you use one without a heat pipe you can also tap the mounting base M3 to easily mount the devices. With a heat pipe you need to take care not to hit one when drilling. Without the fan it will do about 20W of heat easily with only free air cooling.

author: Jeroen3
author_rank: Super Contributor
timestamp: May 11, 2014, 11:35:18 am
content: I've looked into this recently. With an BT141 I could put 50 Watts of power in an intel lga775 stock heatsink with fan while keeping it touchable-hot.Not including electrical isolation pad!Datasheets of the intel lga775 heatsinks specifiy at least 0.85 C/W at 40C amibient. However, the heatsink uses too much board space, so now I'm looking for active north-bridge coolers.

author: conducteur
author_rank: Regular Contributor
timestamp: May 11, 2014, 03:09:01 pm
content: I did a test with the heatsink on the picture and a 40mm fan on 12V in the corner next to it (not controlled by the pwm, i still have to write the software for the pic), and got almost 40 Watts dissipated without a problem... That's more than enough for my purposes. I'll think i do it like i thought to do it. Thanks for your help!

author: Jeroen3
author_rank: Super Contributor
timestamp: May 18, 2014, 11:23:17 am
content: I just did some tests and was able to put 25 Watts in this thing while keeping the to220's tab temperature at 60C. (about 1.3 C/W)http://www.xilence.de/en/products/more/more/northbridge-heatsink-fan.htmlWithout fan 5 watts makes 80 C.So a fan has massive impact on the convection.