View Full Version : Install of IRF7389 FETs

2002.08.19, 07:08 AM
Just a little curious as to the best process for the install of these little buggers. Say 2 stacks of 3 fets are going to be installed. Is it best to build the stacks, then solder the stacks onto the board, or is it best to solder them one by one onto the board, building as you go? Logic says it is best to do the latter( soldering them one at a time onto the board), but maybe some have built a stack off the board. Advice from those who have actually performed the procedure would be highly appreciated. Psyklops

2002.08.19, 09:49 AM
I've only got 2 FETS piggy backed on mine but even with just the 2 I found it easier to solder the 2 together, then solder onto the board. I also used some very very thin double sided tape to hold the top FET in place while soldering it to the bottom FET.

good luck

leigh :)

2002.08.19, 09:55 AM
Thanks Leigh!! I am going to enlist the help of someone who is extremely adept in soldering PCBs, so it SHOULD all go well.......I hope. Once I feel competent enough I will do it myself. Psyklops

2002.08.20, 12:09 AM
i think the guy who put the fets in my car said to take the original fets out, then solder the stacks together before putting it on the board...

2002.08.20, 12:17 AM
Herman ,

yep that is exactly the way to do it ... you must take your old ones out first. They no longer get used. And it's definatly easier to solder the new ones together before soldering them to the board.


2002.08.20, 05:44 AM
Thank you everyone, clearly the best way is to build/solder the stacks off the board, then solder the stacks onto the PCB. I am aware the hitachi 3004 FETs are to be removed....I just wasn't sure as to the best method for stack building. Leigh, do you leave the thin D-sided tape in between the FETs? Or do you slide it out after soldering? If so, I suppose this would leave a small gap that may assist in heat dissappation off the FETs that are on the bottom of the stack. Once again, cheers everyone!! Psyklops

2002.08.20, 11:10 AM
Well I was going to go about it slightly different... I was going to align propperly and super glue with very small drop of glue, each of the fets in the stack together... the feet have to be bent down on the upper fetts to reach the fett below it... after the stack is glued together, I was gona just solder them to the board and together at the same time... :D And I would say 2 is good, and 3 is the most worth stacking... any more is sort of too much.

Remember one side of the fett is all soldered into one circuit area (the motor wire area) and can be done sloppily, the other side, CAN NOT.. and each virticle columb of feet must be a unique circut...

I might even solder my motor wires (with plug) directly to the fetts... bypassing the stupid screws, wich just cause resistance. similar to two of the servo wires.

IF there is enough room, one could use a thin strip of aluminum in between each fet, that could then be bent in a /\/\/\/\ shape above the fett, wraped around the board, but it would have to be partialy insolated from touching the fett circut feet... so it dont cause a short... this would create a custom heat sink.. sort of how I would have made for my motor, but it dont get hot any more ;)

2002.08.20, 05:35 PM
Sounds like a plan Drac....even better would be a non conductive heat sink material, so there is no risk what-so-ever of a short. Hmmmmm let me have a fish 'round. Psyklops

OK, I have found in my trusty catalogue a silicone heat sink compound that would do the job of glue AND the aluminium /\/\/\ idea Drac has. I am yet to determine if it is electrically conductive...will keep you all posted soon.

The heat sink compound is not electrically conductive....I think i may try this stuff out

2002.08.21, 09:47 PM
Any one have install IRF7389 in a F1??My have no reverse but still have brake. Any one??It works fine in my street racer.

2002.08.21, 10:15 PM
kaz101 .... i got 4 of them in my f-1... they work really well (and it does have reverse)... did you know you have to trigger the throttle forward twice to go in reverse?

the problem might be in the soldering... there might be a partial short, or probably one of the fets might have been heated too much during soldering... what soldering iron did you use to put them on? i think the recommended one is the needle tip with 20 watts power...

2002.12.24, 07:22 AM
hey guys, i wanna y i cant stack my irf 7389's on top of my stock 3004's. my 3004's arent fryed. and can i use a 24 watt soldering iron and guy's wat do u'z rekon of my z.

2002.12.24, 07:25 AM
sorry about my pics guy i know their clear hopefully 1 day ill get a betta cam. heres a rear view.

2002.12.24, 09:28 AM
cmefly..how many 7389s have ya got? You can stack straight on top of the 3004s but I prefer to remove them. One mix is 2x3004 +1x7389 per stack, so 6 FETs total....I dunno, if you are comfortable soldering these little buggers, go for it;) It could be an expensive learning curve is all;).

2002.12.24, 09:49 AM
i got eight 7389 fets in total so thats 2 x 4 stackz on top of the 3004 fets

Ken Mifune
2002.12.24, 12:11 PM
I am using a total of six 7389s over the two stock 3004s. It works great although it might be limiting for really hot motors.

2002.12.24, 08:04 PM
I have a problem with my newly installed FETS.
I have removed both 3004. and installed 2 stack of 3 7389.
Tested out the shorts on the inner legs of the FETS(away from motor) and no shorts.

Then I installed the batteries and start driving. I ve already felt the improvement on the TOP SPEED BUT suffered very poor ACCELERATION on forward(sluggish, takes 3-4 sec to reach top speed). However the backwards acceleration and TOP speed is perfect. the backwards top speed is even faster than the forward.
Maybe poor solder joints? or Heat damaged FETS?
I remove the 3 7389 FOrward FETS and replaced with another 3 new 7389. now the Forward is Very reponsive. But the top speed is not as good as the backward from before. but the backward is very sluggish. takes like 5-10 seconds to reach 50% of the forward limit.

please does anyone knows whats wrong?

2002.12.24, 11:44 PM
most likely poor solder joints ;) use flux, it makes the solder flow nicely onto the pads the pins rest on. Also, ensure that all the pins are connected to each other properly.

Ken Mifune
2002.12.25, 03:04 AM
Are your FETs facing the right way?

btw: both stacks handle forward and reverse. (dual channel FETs)

2002.12.26, 02:29 AM
YEs. I have checked the orientation. The 4 legs on the top of the 7389 is facing the motor side.

I have resoldered the joint. and double checked the 8 legs on the input sides. no shorts.

Then I read on some forum here saying ppl are using 7389s on top of 3004. So I ended up putting the sotck 3004 on top of each 7389 stack. and solved the responsiveness problem.

Now what is needed is a powerful motor to torture test the FETS. :) Thanks for you guys input.

I cross check on the Specs of 7389 and 3004. and found the input (GS) capacitance on 7389 is higher than the 3004. maybe the 3004 on my mini-z is now handling the initial acceleration amp. and the 7389 is handling the top speed amp. i dont know. need to find a powerful motor to torture them FETS.

Which motor u suggest for 2 stack of 7389,7389,7389,3004?

2003.01.03, 02:06 AM
If you still have this problem try reversing the motor wires and flip the transmitter reversing switch on top for the gas. Should give you crappy reverse and good forward.

2003.01.10, 03:22 AM
just tried out doing a single 7389 upgrade myself (just to experience what it was like)... and hmm like myzracer... i've experienced the z faster in reverse compared to forward... and a slight 3-5 sec. lag when you give it some gas...

i'm more likely to think that it was my lousy soldering skills that was the culprit... aurgh...

2003.01.10, 03:33 AM
just tried out doing a single 7389 upgrade myself (just to experience what it was like)... and hmm like myzracer... i've experienced the z faster in reverse compared to forward... and a slight 3-5 sec. lag when you give it some gas...

i'm more likely to think that it was my lousy soldering skills that was the culprit... aurgh...

2003.01.26, 08:29 PM
I have fixed my reverse delay.... not by flipping the motor wires :)
If u are still have the reverse problem try this......

0) Straightening the FET legs. make sure its damn straight. and if u are doing it properly, the legs from the top fet should be touching the next fet below it. its easiler for the solder to fuse.

1) Fix those damn solder joints.... Use a voltmeter to test out the ohm between the top FET and the PCB pad. it has to be zero / the same as shorting out the probe leads. Do this for each FETS legs on the input side(G,S side). take ur time. keeping ur sandpaper and solder paste handy. I found out the delay is becos of the poor solder joint causing the delay problem..

2) try to find some way to shim between the FETS and the Body. so the FETS are PRESSED against the bottom of the chassic but not too much so it will break ur PCB.. it helps in holding the FETS and keep the FETS from dislocating when the solder joints melted.
look at the picture its easiler to see than to explain. I use a metal plate and some thermal conductive foam found on my old cdrom as shim.

BTW i have removed the stock 3004 cause it will cause problem if u are running hot motor. so my stacks are pure 7389 now.

2003.01.26, 08:32 PM
forgive my spelellgin and grammers mistakes. ....