Cooling fan wiring?

[jhautz]

I am using a MM in my Xray XB8 on 5s and 4s depending on the situation and have no problems, it runs great, but I do run a fan on it in to help keep the little guy cool. Obviously I pull the red wire on the rx plug to disable the internal BEC and run the servo and Rx off of an external UBEC like everyone does. I am looking at ways to simplify wiring on this thing as the MM is at the back of the buggy and UBEC ans Rx is at the front so I'm trying to reduce the number of wires I need to run front to back on it.

I know the MM brains still run off of the MM internal BEC even when the red wire is pulled so the internal BEC is still functioning, but with just a very tiny load on it. My question is what would happen if I ran just the little 25mm fan on the MM internal BEC. The fan is a 5v fan that says .9W so that means it will pull around a .18A load at 5V. I'm sure running it on 6v would make that Watt rating a little higher and the amp rating a little higher, but its still a fairly small load of maybe .25A.

How much heat would this generate on the internal BEC if i ran just the cooling fan on the internal BEC power? Everything else would run of of the external BEC. Would I be canceling the effect of the fan cooling out by generating extra heat with the additional load on the internal BEC on these higher voltages?

I'm thinking about trying it so I only need to run a little wire around the side of the MM to the wire solder points rather than all the way to the front of the buggy just to plug th fan in.

What do ya think?

[BrianG]

For the sake of argument, let's assume the ESC brains draw 0.05A. Add a worse-case fan current of 0.25A for 0.3A total. At 4s, the power would be 3.24W ( (14.8v - 6v) X 0.3A). A little high. I did a test a while back with the MM BEC with 3s and a 0.25A load and the BEC got quite hot very quickly, and that was on only 3s.

[jhautz]

Thanks Brian. I just did a test of my own to see what would happen. I attached the fan to the MM output and didnt assemble the case back onto it. I plugged it up with a 4s pack and used my temp gun to test the temperaures of the components. The 2 little chips that I assume are part of the internal BEC got up to 180 degrees in a matter of 30 seconds. So I guess that answered my question. NO... It doesnt work. I cant believe how useless that internal BEC is.

Crap. I thought I was gonna be smart for second there.

[BrianG]

lol, what, you didn't believe me? That was just a measly 250mA - imagine that with a ~2A servo load? And those chips aren't even on a heatsink. Even on 3s and 250mA load it gets toasty, which is why I say use a UBEC on anything over 2s or 7 cells.

It has a use, but only for 2s setups. But it is useless for anything over that

[sikeston34m]

I'm sure this is just about the #1 cause of ESC failure.

[jhautz]

Originally Posted by BrianG

lol, what, you didn't believe me? That was just a measly 250mA - imagine that with a ~2A servo load? And those chips aren't even on a heatsink. Even on 3s and 250mA load it gets toasty, which is why I say use a UBEC on anything over 2s or 7 cells.

It has a use, but only for 2s setups. But it is useless for anything over that

No I believed you, but I started doing it before you posted and I didnt read it until I had already figured out my own answer.

After the last pot I went back and tested it with 3s running only that little fan and it got up to 180 again. It just took a minute longer to get there. So I tested it on 2s and it still got up to 165 or so in a couple minutes. I really think that the thing isnt good for anything at all except for powering the esc brains, and if there were a way to bypass that I would do it to. The dang thing is just a little heater.

[BrianG]

It probably accounts for 99% of failures if the FETs aren't the issue.

[bdebde]

Use a 12 volt fan and wire it to the battery leads on the MM circuit board.

[jhautz]

Yea... i know. But all the fans I have that fit are 5v. I got it sorted out already. I have the wire routed so it looks pretty clean now.