Heatsinking your mosfets on an iw02 chassis….

[ph2t]

This is something I ve been meaning to do for a while, I wasn t really sure if this would make that much of a performance difference but after a mega hot day at our recent meet at templestowe and a bit a research I ve come to the conclusion that heatsinking mosfet may provide a more noticable difference than before.

This mod has been done on the IW-02 chassis namely because the mosfets are located on the top of the PCB and with the top cover cutout become a quick and easy candidate for putting a heatsink on.

But first, some theory. MOSFETS have what you call a positive temperature co-efficient when talking resistance. This means that as the mosfets get hotter, their internal resistance increases and therefore limits the amount of current passing to the motor. Keeping mosfets cool enables you to maintain a nice and low “on resistance” and ensure the maximum current going to the motor.

The graph below is for the n-chan mosfet in IRF7317(used in my IW02 thread) , so I m talking real data here….

Given this graph is what we called normalised, 1.0 = the standard value of resistance. 1.5 = 150% of the original value. 0.5 = 50% of that original value. This should help you understand the graph.

So if we look at normal operating termperature of 20deg, the resistance of the mosfet is normal, ie: 100% it s orginal value.

Now if we increase this value to, say, 60 deg (this is pretty hot I know, but damn, you should of felt the fet stack that day!!!!!! bloody hot!!!) then the resistance on the MOSFETS will increase by approx 15%. This doesn t sound like much but it does make a difference! Going to the hotter side of the scale shows that the increase can be a great as 20%. This can have an effect on the amount of current going to your motor since the mosfets are now in a higher resistive state than before…..

As I ve said time and time again, increase your current and you therefore increase torque. Top speed is a fucntion of torque vs. your rolling resistances (drivetrain, gear mesh, bearings, racing surface, etc, etc, etc….).

So with all that crap out of the way, lol, here s some pics of my butchery….. 🙂

Here s the original scenario, with my 3×2 stack of IRF7317 mosfets just poking out the top of the PCB cover.

Notice the mosfets come up just enough to be clear of the cover, enabling the possibility of a constant connection with the heatsink.

I got a small,90 cent heatsink from my local eletronics store, Jaycar. I chopped off one side to stop it from screwing around with my roll shock setup and all is sweet.

So far so good!

SO I did some testing. Punching the throttle really hard into fwd, then reverse and repeating this will always result in hot fets. After doing this about 4 times the heatsink started to get warm to very warm.

I need to go and get some heatsink compound and layer it between the mosfets and heatsink which should improve the heat transfer even better.

Time will tell on this mod, I need to take it down to the track and get some serious time in to ensure that the heatsink is doing its job in lowering the mosfets temperature during races..

ph2t.

[micro_Amps]

Its gotta help ph2t, good idea.

[chino]

cool, u can cut a vent hole thru ur wind shield like a nitro car haha. good thing it gets warm, that way u kno theheat sink is doing sumthing

[betty.k]

i remember what got this started.

few weeks back ph2t came over with his xmod. he assumed it will have a nelly in it so blowing the fets didn’t really matter and would even be fun! so he dropped one of his “p dash arms in an atomic can” motors in it and we set to work.
the fets got hot, very hot in fact! but they didn’t blow. why?
because there’s a great big bit of aluminium that sits under the pcb, inside the chassis and makes contact with the fet casings, ie a heatsink! i’m no expert but i’m guessing that i waver, mini z and xmod fets are pretty similar in performance. so you could theoretically add heatsinks to stock fets and enjoy more performance and even run a motor hotter than an x speed

i cringe at the idea of keeping the batts, esc and motor sealed up with no airflow, no matter what the scale i always try to ventilate the body and i find the before and after comparisons to be quite favourable:8ball:

Edited by – betty.k on 26 December 2004 17:47:29

[jamiekulhanek]

heres another idea I discovered.

I used the high motor mount position and removed the rear window, the heatsink gets plenty of air and keeps the motor cool enough to touch, even on 6 cells with a plasma dash!

The heatsink was about 50c at Dick Smith Electronics…

Edited by – jamiekulhanek on 26 December 2004 23:05:11

[TallduDe]

this is a very good idea, chino’s idea is a good one aswell but it would ruin a good shell.

[Z-Nagual]

I’ve got heatsinks on both my nelly and my motor.
Little black square ones that I got from jaycar and they work quite well. I nearly got the guy at the local hobby shop to sell me a heatsink used in a 1/10 speed control. It came with a little heat transfering adhesive strip to stick it on. It also came with a big power capacitor that I didnt want $$$$$$$$:angry:. If this sort of adhesive is strong enough (It should be) it would be much better than screws.:smiley2:

[Z-Nagual]

I only just noticed your pic Jamie and its the same heatsink as mine. Brilliant installation. I might have to mount mine like that. My motor doesn’t heat up half as much as the nelly does though. Atomic bb, plasma arm, neo mag. How did you attach it to your motor? Mine is crudely attached with rubber bands.

[betty.k]

i think jamies was professionally attached with a zip tie!:p probably the best way to hold your motor in place i reckon.:8ball:

[jamiekulhanek]

Yeah, the bracket that goes between the motor holder and the H plate needed a piece filed out so I could feed a zip tie through, i found it to be a very reliable way to mount it all together.

Also i used the high mount for this mod…

[jamiekulhanek]

Quote:

I only just noticed your pic Jamie and its the same heatsink as mine. Brilliant installation. I might have to mount mine like that. My motor doesn’t heat up half as much as the nelly does though. Atomic bb, plasma arm, neo mag. How did you attach it to your motor? Mine is crudely attached with rubber bands.

Try running 6 cells or 2 lithium based cells and you will start to realise what heat is, though these heatsinks do a fantastic job of keeping the motors cool, I found that I got longer life out of the plasma dash’s.

[Author]

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