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Discussion Starter · #1 ·
Can I safely use a schottky diode with a futaba mc330cr speedy IF I disable reverse?
 

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Discussion Starter · #4 ·
not hard when you get the spelling right off the novak site
 

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Discussion Starter · #8 ·
no you don't have to run a schottky on your 330 nismo. if you do you can't use reverse or the schottky blows





you can mount a schottky on the + to - terminals of the end bellEdited by: jiggen5354
 

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nismospeed said:
how do you reverse the voltage???
also where do you put the schottky diode??

To get a DC motor to reverse you just reverse the voltage fed to it. That's just what your reversing ESC or MSC does.


Schottky diode is soldered across the motor's terminals. It is polarised so you must get polarity correct... otherwise they will blow up. Literally!!






Schottky is NOT a replacement for the 3 filter capacitors on the motorwhich are there to filter out RF spikes.
 

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Guys,

what does this diode do for your RC car?

I've just read about them and how they are able to activate or switch ultra fast. Also, since it's a diode it only allows current to flow in one direction - I imagine that it stops the reverse current flow from the motor when you are braking/not accelerating, but to what purpose???

Cheers, Oggy.
 

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Morphy said:
What about the back EMF?

Good question. Back EMF is the voltage generated by the motor when it acts like a generator. Its also what generates the braking action when you short out the motor's terminals -the motor quickly stops spinning.


If the motor is turning in the forwards direction (eg car is coasting), back EMF is same voltage polarity your usual input -- so it will not effect the diode or its operation. (But if you pushed your car backwards fast enough, I guess you *could* generate enough current to blow your diode.)


When you hit the brakes (ie ESC's brake FETs turn on), Schottky wakes up and conducts the reversed current. You should get better brakes. Your ESC's brake FETs should also be cooler as they now do less work.


btw "Schottky" is a special diode, can't just use any powerdiode. Yeah they all look the same as any IN4001 that sells for 20c. Had to go buy more on Sat and paid $10/2pk at LHS.





Some ESCs have a Schottky built in... but its still advisable to add one on the motor for hot mods.
 

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oh... and a big "btw"... all your highschool learnings on physics & electric/eletronic circuitry is all beaut and fine... but it don't help you much here.



Motors are a dynamic device. Its essentially a moving coil electromagnet in a permanent magnet field.


The commutator switches the current's polarity if the electromagnets... so you have a +ve armature pole attracted to the South magnet pole & viceversa. That's what happens when the motor receives power - so each of the arm's 3 poles switches 6 times every revolution.





But beneath the surface ... there will be timing overlaps and inefficiencies. Imagine the point when the pole is *leaving* the magnet field... its actually generating a little EMF in the reverse polarity.


This reverse current/voltage if unchecked will just feed back up the ESC's motor wire... and just to counter it you need more power from the battery.


But if you check it (short it out) with Schottky, our battery's got nothing to fight against and can use 100% of the power to do good. (Yes your FWD FETs will also run cooler.)





Need a Schottky (which is fast-switching) as you could have say 4,000 little spikes per second (6 spikes per rev X40,000 rpm).


A normal cheapo signal diode can handle 60Hz (bridge rectifying AC current) but does not switch fast enough to do 4MHz.
 

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zoomer said:
Had to go buy more on Sat and paid $10/2pk at LHS.

Dude you got ripped! They are $0.45 each at Jaycar and a fraction more at Dick


(Thats ifthey could even understand what a schottkydiode was. Asked a guy at Dick for a LED - didn't mention what size/shape/color but he told me that he just sold the last one... yeah right! I turned around and grabbed a bag of 20 - He thought LED's were twist ties with a plastic handle...)
 

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Oh and would'nt the sckottky blow if/when you slide around and roll for three meters backwards at full speed? The EMF generated by the motor could be enough for it to blow.... But then @ 45c who cares...


Wouldn't a cap across the motor have the same benifits as the diode, but will also allow reversing?
 
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i got the "caps" on my motor for the RF thingy zoomer mentioned...

but for just bashing around a car park a futaba 330 wont need a schottky diode....



but blocking the reverse on it will it do damage to your esc??.....im hopeing not.



Nismo
 

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You mean these?


1N5819 1A 600mV Schottky Diode
IN5819 Schottky. 1 A / 600mV. SOD-81 A


I think the rating on the RC ones are like 5A or 10A and they switch at much lower voltage thresholds. 600mV = 0.6V is pretty high, that's not much different than a standard signal diode (need 0.7V difference before they start conducting).


RC ones can be bought in bulk if you want 100pcs etc, could be as low as $2. But I don't buy enough to justify buying a whole bag.


(btw I'm actively boycotting Jaycar & the Jaycrap they sold me before, so I happily pay crazy $$ to my LHS so he can afford to go golfing.
)


ps putting that 1A Schottky on probably don't do any damage... but it won't be doing any work either. Don't forget your ESC will likely have a Schottky built-in already, that'll do all the work unless you've got something better on the motor.


Morphy said:
Oh and would'nt the sckottky blow if/when you slide around and roll for three meters backwards at full speed? The EMF generated by the motor could be enough for it to blow.... But then @ 45c who cares...

Current handling is your friend.
That's why RC uses 5A vs the pissy Jaycrap 1A.


You'd have to do quite a bit to generate 5A of current running backwards. Don't forget your motor+diode running backwards will act like its got brakes jammed full on too. Your car just will not have enough momentum to slide backwards enough to generate 5A+ of current.


Plus all that extended reverse voltage to the ESC will probably blow a few things too... like your Brake FETs!!
(whether they are on or not doesn't matter)





Wouldn't a cap across the motor have the same benifits as the diode, but will also allow reversing?

Caps filter out spikes, but they don't short out the reverse voltage EMFs. Also you need small caps to filter out high freqs but they don't have much capacity. Then you need big caps to filter low freqs.





If there's a cheaperequivalent to the Schottky I'm sure someone would have found it long ago. (I'm finding there's an overrepresentation of Engineers on the RC tracks... just something about RC cars that attracts the Engies!!)
 

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nismospeed said:
but blocking the reverse on it will it do damage to your esc??.....im hopeing not.

Nope. Futaba MCxxxRs can switch off their reverse function so you can go racing. The better reversing ESCs can usually switch out their reverse.


I tried it out when testing the one before MiG got it. From memory I think you just hold down the setup button then switch on - it toggles it and goes "bip bip" (and it uses your MOTOR as the 'speaker'!!).





There were some talk about wiring your 4-wire reversing ESC with some 3-wire format that makes it so it won't reverse... but frankly it goes against all common sense even for an Eng flunky. I wouldn't try it.
 

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zoomer said:
If there's a cheaperequivalent to the Schottky I'm sure someone would have found it long ago.

If it was me, I would find a manufacturer on the web and usually they have a request free sample button. Have used this feature many times to get componets that local shops don't stock or have never heard of.



Most places will simply send you 10 of 15 without any greif, it will cost them more in postage than parts!


zoomer said:
(I'm finding there's an overrepresentation of Engineers on the RC tracks... just something about RC cars that attracts the Engies!!)

Funny that, being that I'm an engineer. Actually almost everyone that has seen my Savage has been an engineer or an engineering student... Hmm engineers might not be able to spell but we have the best parties!
 

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yeah, haha... Engie (at heart) here too,
complete with the trade accounts at the usual useful suppliers. Not practising fellow though, as I'm pretty sure I'll end up killing someone with a wonky cantilever bridge or a multi-gigavolt zap from an uninsulated transformer that glows blue from simply ionising the atmospheric gases.


What I'd love is to visit a "Frys Electronics"... all my friends (Engies of course) who've gone to the US say that's like a Mecca must-go place. One day, one day... think I'd better leave my creditcard at home but!!
 
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