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Old 2003.04.22, 09:23 PM   #1
PAULANGELO
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Question Power switch after FET upgrade

Has anyone run across this?? I have successfully (I think ) installed FETS to my Mini Z. It is now a 2x4 stack. When I put the batteries in, the motor starts running wildly until I turn the power on. Then it sits very quite until I give it the throttle and it will take off like I would want it too. It runs great, no smoking and the batteries don't get too hot.(they get warm but they did that before with just a turbo too). Then the funniest thing happens. When I turn the power off again, the Motor starts running really fast in forward . Any one else run into this and can it be fixed??
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Old 2003.04.22, 09:38 PM   #2
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the key word you used was "I think" its NOT supposed to do that, take em off and try again. desoldering braid is your friend


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Old 2003.04.22, 09:51 PM   #3
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That is just interferance dont worry about it. Everyone has it.
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Old 2003.04.23, 07:57 AM   #4
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OK, Now I am really confused. I know the 2 stacks are all right. I used a microscope to solder them and there is absolutly no bleeding or shorting out of the fet legs. could they be on backwards? if they were, wouldn't it not work at all? AND why is it that it is only the power switch?? When in the ON position, it is fine (actually really FINE ) but when it gets turned off, it seams to short out somewhere. The motor goes crazy as if it is full throttle. So far I have 2 conflicting responses. I really don't want to remove the fets if I can help it. I left the original fets on and just stacked on top. I have read numerous posts on this subject but never ran into this situation . ANY and ALL help is apprciated.
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Old 2003.04.23, 04:51 PM   #5
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Your cant mix 3004 (stock fets) with 7839 (or whatever the number is) fets. That is probley your problem. NOW if they are all the same fets its just interferance. (the rx on the pcb sucks)
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Old 2003.04.23, 07:46 PM   #6
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Well, chalk it up to experience!! You can NOT mix the FETS . No matter how hard you try. I brought the car to the track and it went great (for a while ) and then I lost forward. So I did what any lazy person would do, I switched the + / - wires to the motor and ran the rest of the night without reverse! I would get the revers occasionally when I let the car cool down for a while. Looks like the original fets still get burned (or affected by the heat) when they are the first in the stack. SO, I got 10 more FETS and and goign to try again! Thank you for any response.
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Old 2003.04.23, 08:59 PM   #7
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imho, you may have to redo your stacks from the ground up. Make sure your gear mesh is correct. This can be a contributing factor to overheating.
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Old 2003.04.24, 02:13 PM   #8
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Thank you NML. I plan on starting from scratch. I have 2 PCB's and 10 brand spanking new 7389's . Hoping to remove the "used" 6 fets from the first atempt for a total of 16and see if they can be used. Will keep ya'all updated on the progress.
PS, What is the minimum number of FETS needed for your Diablo series motor?
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Old 2003.04.24, 04:45 PM   #9
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I got your mail PAULANGELO and responded yesterday. I didn't get to see your post (this thread) 'til just right now (i've been real busy ) It seems like your original problem could be that somehow the poles on your switch are touching the legs of the FETs for some reason. I'm not really sure how but if you have a clear chassis then you'll see the problem better. Make sure loose wires (especially the white wire for the switch) is not stripped and touching other connection on the pcb as it may also cause same problem.

Okay I read on, it looks like my suggestion doesn't matter now as you've burned your stock fets. Also about mixing fets, i highly suggest putting more 3004's that 7389's as 3004's (stock ones) are very easily fried. They turn on earlier that 7389 (by nanoseconds) so they receive all the current first thatn 7389's.

Last edited by MysterE; 2003.04.24 at 04:50 PM.
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