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Old 2003.03.19, 03:03 PM   #16
Caffeinated
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Quote:
Originally posted by warnoffroad
I found it easyest to put a little bit of solder on the iron and rub it on the wanted spot.
That's considered bad practice by expierenced solderers. If anybody with military solder training saw you do that they'd ream ya!

The problem with "rubbing" the solder from the tip to the joint is by the time the tip gets to the joint all of the flux has burned off teh applied solder & 90% of the time you'll end up with a cold solder joint.

When your looking at small solder joints like these you usually wont notice a cold solder joint unless you peek through a lighted magnifier & inspect the reflection of the light on the surface of the solder joint.

Even cold solder joints work, they are just more prone to breaking with vibrations, & having some (although very small) DC resistance.

If you do a web search (google?) for solder techniques I'm sure you'll find more detailed info than I can provide...
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Old 2003.03.19, 08:07 PM   #17
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electra , dose that mean i can run Plasma with 2X2 fets???

has anyone tried it?

what's the min number of fets needed to run Plasma

palsma=4.2 ah

tahnks
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Old 2003.03.20, 05:27 AM   #18
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Quote:
Originally posted by rmyc
electra , dose that mean i can run Plasma with 2X2 fets???

has anyone tried it?

what's the min number of fets needed to run Plasma

palsma=4.2 ah

tahnks
Do a search for 'plasma'...it is a no-go from what I have read, and passed on countless times now...unless you like re-soldering your FET stacks
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Old 2003.03.20, 05:56 AM   #19
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Assuminng the 4.2 A spec on the plasma dash motor is correct IN THEORY a 2x2 should have no troubla at all, i dont know how many winds are on the motor or anthing like that so tamiya specs could be way off. From what i have heard it isnt a good motor for mini-z's anyway because it has 'low torque' - this to me suggests a motor with so few winds that the batteries cannot provide enough current to power it adequately. I take no responsibility if you blow the fets though - if you want to be on the safe side try it but take the pcb out, stick your finger on the fets and throttle up to full with the motor under load for 20 seconds, if you have to take your finger of to avoid getting burned then you know the motor is drawing too much current ; -)
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Old 2003.03.20, 06:04 AM   #20
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Oh yeah, and the correct way to solder SMT devices is with a fluxed solder past that is applied in a line to the pcb where the legs of the device go - you dont have to worry about separating the pads, if you do it right they sort themselves out. You then heat the whole thing either with a hot air gun or a radiant heater to reflow the solder. The part will float on the melted solder and align itself neatly on the pads. I do this with relatively primitive equipment, my only luxury being a binocular microscope to inspect the joints afterwards. I have also done it with ordinary solder and a fine solder tip but this takes practise, a steady hand and a good eye. The solder should be applied to the joint slightly after the iron to avoid burning off the flux as mentioned by Caffinated.
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Old 2003.03.20, 01:15 PM   #21
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the -4.2A rating on irf7389 for P-ch is at 70 degrees C (-5.3 at 25 degC). At that temperature you can guess that it will climb higher than once the fet is exposed to a motor that has that current rating, thereby toasting the fet (Pch at least). Therefore plasma dash are not recommended and it's been tested.

also, at first i thought that 14x2 will be the same as 10x2 or even 6x2. but after we've (me and NML) tried and tested it, the results are very clear that the higher stack gave the most optimal output. It's been proven by us and all the Zer's who tried this setup.

nice infos though electra
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Old 2003.03.21, 03:14 AM   #22
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Myster-E,

The rating for the p channel of a single irf7389 is -5.3 A at an ambient temperature of 25°C,

with a drain of 4.2A the resulting power dissipation of the device is

(4.2*4.2*0.058)+(4.2*4.2*0.029) = 1.53W

so case temp is 25 + (50*1.53) = 101°C

whilst this is within the 150°C rating of the device it much too hot

however a 2*2 stack...

power dissipation of...

(2.1*2.1*0.058)+(2.1*2.1*0.029) = 0.336 W per device

case temperature of

25+(0.336*50) = 41°C

which is a perfectly acceptable temperature

so as i see it, if the ratings on that motor are correct a 2*2 should be fine, like i said - i dont trust tamiyas ratings and i still dont recomend that motor
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Old 2003.03.21, 12:04 PM   #23
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the numbers you're saying are the ideal/theoretical calculations for a constant current drain rating of the motor. There are more factors that you need to take into account and could take all day to find all ideal numbers. Unless you tried the stacked fets personally, assumptions can only be proven after testing has been done. I've tried an 4x2 stack with a plasma and have melted the solder due to high temperatures that the fet had reached (well over beyond 300°C). Although with my previous original calculations I believed that stacked fets could handle motors above 4.0A but my experimantation proved otherwise. I also may have left out some factor but I don't say anything definite until after I tried it myself. Most people here in the forum have tried stacked fets (different levels) and will agree that the higher the stack the better (could be due to the fact that current flow is inversely proportional to the resistance in the circuit and directly proportional to the voltage apllied to the resistance)

I've taken electronics, calculus and physics so I have a general knowledge of what I'm doing . Most of what I'm offering are results of testing, experimenting and hardwork. There no gimmicks to these upgrades and it's been proven by many here in the forum.

Last edited by MysterE; 2003.03.21 at 12:07 PM.
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Old 2003.03.22, 08:04 AM   #24
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In that case i think the drive circuit for the mini z must be very poorly designed, either it isnt turning the FET's on hard enough or fast enough, or both - sadly i am away form home and cannot verify this but i would be really interested to see a scope trace of the gate drive signal - it may be that there is some effect other than simply reduced Rds at play - maybe the IRF fets have a much higher gate resistance or capacitance than the HAT4001's which i have been unable to find specs on - if anyon has the spec sheet i would love to see it. I trust IRF to publish accurate specs on their devices and i feel sure there is somehting that is being missed - it may be somehthing simple like lowering the value of the resistor on the gate designed to reduce slew rate that would bring in rewards equal to a much larger FET stack.
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Old 2003.03.22, 11:49 AM   #25
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Quote:
Originally posted by Electra
Oh yeah, and the correct way to solder SMT devices is with a fluxed solder past that is applied in a line to the pcb where the legs of the device go
Do you need to remore the flux afterwards ? What do you use for cleaning up then ?
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Old 2003.03.22, 02:51 PM   #26
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here is the link for the irf7389 datasheet
http://www.irf.com/product-info/data...ta/irf7389.pdf

i hope this'l help you
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Old 2003.03.22, 04:43 PM   #27
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correct solder procedures

smt solder paste has a non-acid flux that doesnt need to be cleaned off, altough a bit of meths works fine if it bothers you - a quick spray of clear laquer helps a lot of you drive you mini z in dirty areas which would otherwise cause servo glitches

have a look here

http://www.usbmicro.com/apps.html

and here

http://www.seattlerobotics.org/encod...6/oven_art.htm

for good DIY SMT techniques (that are far more complex than necessary for mini-z's but intersting nonetheless)
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Old 2003.03.22, 09:12 PM   #28
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can i use fine silver solder?? silver solder is the only solder i've tried ...it's very nice

thanks
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Old 2003.03.22, 11:36 PM   #29
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Re: correct solder procedures

Quote:
Originally posted by Electra
smt solder paste has a non-acid flux that doesnt need to be cleaned off, altough a bit of meths works fine if it bothers you - a quick spray of clear laquer helps a lot of you drive you mini z in dirty areas which would otherwise cause servo glitches
I couldn't find any smt solder paste at the local electronic parts store, only the ordiniary acidic solder paste. How do you apply the "ordinary" acidic solder paste during smt soldering ?

Thanks.
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Old 2003.03.25, 11:48 PM   #30
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Its all about maximum voltage at the motor.
More fets means less voltage drop. The more current a motor draws the higher the voltage drop across the Fets.
Current * RDS on = voltage drop
A motor drawing 5 amps will see a .8% increase in voltage if you double the number of fets.
A motor drawing 5 amps will see a 2% increase in voltage if it has 7 Fets instead of one.
the above based on a 6 volt battery pack and RDS of .029
Motor speed is proportional to the voltage (assuming constant load).
So is a 2% increase in voltage alot? Well that could be 1 lap every 50 laps or .2 seconds on a 10 second lap. Thats alot when racing. When racing even 1% is alot, Of course if you crash a lot then all the hop ups won't help.

the less current the motor draws the lower the voltage drop, so lowering resistance by stacking Fets will be less noticeable.

Another thing. Fets are NOT swithing at Full throttle there always on, so gate capacitance etc. shouldn't be an issue.
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