DC load using a CPU cooler

[microbug]

I'm going to make myself a simple electronic load with optional dynamic load (pulsing) capability, based on Jay_Diddy_B's dynamic load (driving the current input with a DAC). I'm just wondering why I haven't seen any loads around that have CPU coolers (active heatsinks) instead of a lump of aluminium. From my limited research, they're fairly cheap and can have thermal characteristics of less than 1C/W (easy to dissipate 75W per cooler). Comments?

[bktemp]

I have done that, works great. Especially the better ones with heat pipes can easily dissipate >100W. The only problem: Either it is big an quiet or small and noisy.

[mrflibble]

I have done that, works great. Especially the better ones with heat pipes can easily dissipate >100W.

Ditto.

The only problem: Either it is big an quiet or small and noisy.

Yup. IMO best go for big and quiet. And to keep it cheap you can buy after market coolers for an outdated socket from the bargain bin.

[microbug]

Thanks for the bargain bin idea! You can get coolers fairly cheap on aliexpress as well. Good to know it works!

[rob77]

why to buy a new cpu cooler ? i built my DC load with a old pentium II passive cooler with a second old amd sempron cooler on top of it it takes ~ 75W continuous and 100W short term... doesn't have to be noisy either - 2 PTCs (one glued near the MOSFETs and one near the power-resistor) and comparators with hysteresis controlling the fan.

[n45048]

I have an entire drawer full of various heat sinks/fans from Socket 3 upwards. Let me know if you're after something in particular.

[microbug]

I would, but shipping a fairly heavy heatsink from Australia to the UK would cost a fortune! I imagine there are plenty of cheap heatsinks for outdated sockets on eBay as well.

[n45048]

Anything up to 500g from AU to UK costs AUD$10.80 via Sea Mail and 500g - 1000g is AUD$20.45 (roughly double it for air mail).

[microbug]

Wow! That's not bad... I'll let you know if I need something, but the point of this is that I can choose some outdated cooler which is cheap and design the PCB to fit it.

[max_torque]

Thing is, a CPU cooler is doing a specific job, which is to MOVE a high heat flux heat from a very small area, and then reject it to the ambient.  A typical DC load, to handle a high power dissipation will need multiple parallel power devices, and so you don't have such a small area / concentrated heat source.  Also, in modern computer design, the issue is packaging the heat transfer device into an ever smaller physical space, again, unlikely to be an issue with a typical bench top DC load.

And ultimately, you have to reject that heat to the ambient atmosphere, which for a high power DC load requires a lot of surface area or a lot of fast moving air, as it is (should be) continuously rated.  This means either a large heatsink or a powerful fan.

I suspect that while CPU coolers have nice low thermal transfer co-efficient, they actually don't have amazing total heat rejection  values (because the CPU is still a relatively low power device (it gets "hot" because it is so compact!)

For my DC load i used a nice big heatsink, placed the power devices nicely spread out on it, and used two  powerful speed controlled server cooling fans to blow a LOT of air past it.  This manages pretty much 300W continuous in an 30degC ambient.

[microbug]

I can see that CPU coolers might not be ideal, but price (at least for me) plays a big part in it. Unless you have any suggestions for a place to get large heatsinks (I'm in the UK, so shipping plays a big part unless it's local) then I'll probably use a CPU cooler or two. It doesn't need to be hugely powerful; 100W would be fine.

[SeanB]

CPU coolers have a big advantage of coming with a built in fan, or if you need more than the 100W typical rating you can use old water cooling units as well, which can go up to 300W or more with the right orientation and care on airflow. If you really want high power a car radiator with electric assist fans and a washing machine drain pump will do multi kilowatt power levels.

[timb]

I was looking at using IRF'a DirectFET packaging for my load. Tiny metal surface mount packages that are as close as you can get to a bare die.

You can place then fairly close together, which eliminates the "small area" problem mentioned above.

There's some nice heat pipe based CPU coolers on Amazon that are specced for 250+ Watts of dissipation. Hell, they've got a tiny $15 one that does 100W!

My current prototype uses an extruded aluminum case from Hammond. I found an old Thermaltake P4 Heatsink in my fan box; it mounts perfectly onto the top cover of the case. I've got it glued on with Arctic Silver Thermal Epoxy. I then drilled and tapped two mounting holes through the inside top cover of the case (into the heatsink), for attaching two TO-220 MOSFETs.

So effectively the entire case becomes part of the heatsink.

Works great!


Sent from my Smartphone

[jmole]

or if you need more than the 100W typical rating you can use old water cooling units as well, which can go up to 300W or more with the right orientation and care on airflow.

the newer ones are nice too, you can get a closed loop watercooling system (i.e. low-hassle, runs on 12V) with dual 120mm fans, a radiator, and a water block for pretty cheap. then all you need is some decent thermal epoxy or even thermal tape.

[poorchava]

I used cpu coolers in my linear 24V/2.5A power supplies. No problem whatsoever even at dead short and maximum current.

[void_error]

I initially wanted to use a CPU cooler for my electronic load but then I remembered I had these in my junkbox (80x80mm fan for reference):



A modest CPU cooler can handle up to 100W if you use enough MOSFETs to improve heat transfer.

[SeanB]

Patrick Norton did a water cooler using an old truck radiator as the thermal dissipater. Basically did not need a fan to blow air through it, it was so large compared to the heat input that convection alone worked.

I did a machine conversion to cool water using a condensing unit I bought as a spare part for a display counter, the base plate, accumulator and condenser coil on a plate. Just used the condenser and made some fittings to go to 12mm nylon pipe on one end and the other end to dump back into the tank. Has about 2C temperature drop across it water wise and works well.

[Spikee]

I currently have a board under way from china that i designed to be a programmable dc load.
It uses IXYS L2 fets that are laying flat (metal tab up). A normal intel775 heatsink bracket is designed to fit.
In my design i used the Artic freezer 7 pro rev 2 cpu cooler. It is verry quiet and can handle around 175W using the stock fan. One needs to but an additional fan or a faster one on it to handle more heat.

My project is called Megaload. Some details are somewhere on this forum. Will post more details when the board comes in.

[Balaur]

The CBA uses a rather modestly-sized cooler with a fan. Have one, works very well.

[microbug]

Spikee, your project sounds pretty interesting... I just selected the Arctic Cooler 13 (200W) for my load; I'm using 3 D2PAK 100W rated FETs and I'll add a thermal cutoff on the heatsink. How did you design in the bracket? Is it just a set of holes or is there a part you can buy?

With the 3 100W FETs, I could get a 300W load with the Arctic Cooler 13 Pro, but I don't really need the extra power. It would be quite easy to upgrade if I did though, just stick in the larger cooler.

[Spikee]

Spikee, your project sounds pretty interesting... I just selected the Arctic Cooler 13 (200W) for my load; I'm using 3 D2PAK 100W rated FETs and I'll add a thermal cutoff on the heatsink. How did you design in the bracket? Is it just a set of holes or is there a part you can buy?

With the 3 100W FETs, I could get a 300W load with the Arctic Cooler 13 Pro, but I don't really need the extra power. It would be quite easy to upgrade if I did though, just stick in the larger cooler.

It uses the intel 775 socket mounting holes. Theoretically all heatsinks that support this socket should fit.
The heatsink i use now can handle stock 150w and costs around 15euro. The A13pro can handle 300w but costs 35euro.
If i needed more power i would just use another board (it is modular isolated i2c control) or put an additional or faster fan on it.

[rob77]

With the 3 100W FETs, I could get a 300W load ...

sorry, but you simply can't you always have to consider the SOA (safe operating area) and also have to count with the thermal de-rating of the mosfet. for my load i paralleled 4 mosfets to get 100W dissipation and keeping the mosfets within SOA - i used IRFZ34N , "68W" each according to datasheet, but considering the derating and SOA, it's only 20-25W each.

[blackdog]

Hi,

If you want a electronic load that has a good ac respons, look @ my schamatic on this dutch forum.
Please use google translate, and read howe to wire is, this is of great importance.

There are 2 types of electronic load's, one is the "stupid" type, only DC, and my schematic is a controled AC type.
But, the wiring must be done neatly, NO SPAGHETTI.
Read the hole Article, there is a lot of useful info in there.

Can you use different MOSFET's NO!
Can i use more MOSFET's, not in this schematic, the NE5534 cant drive more than 2.
Can i switch the BD139-16 for a 2n3904, NO!
Can i use a LM324 as a opamp NO,NO,NO

If you build it, folowing my derections, its better than Agilent enz.
why would you want to modulate the dummy load, only then watchin you scope you can see the real impedance of youre power supply.
And there are other neat tricks to do with a modulated Current Souce :-) (Class A Audio Amp)

The schematic,


Test setup on a PC heatpipe cooler



Article link
http://www.circuitsonline.net/forum/view/121338/1


Kind regarts,
Blackdog

[poorchava]

If you don't need this to run at full power you can just bolt the mosfets to some random large heatsink and dump it into a tub of mineral oil.  Ice water is even better but then you need to make sure that only fins of the heatsink are immersed. I made quick and dirty 24V/35A load this way using 12 SUP90N03 mosfets.

[macboy]

A typical higher-end CPU heatsink is asked to dump 125 to 140 W of power into ambient (internal PC temperature) of 35 deg C while keeping the DIE, not heatsink, not case, but DIE temperature of the CPU below 60 deg C.  Add to that the fact that it is compact and inexpensive, and it is a winner all around.