HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Feb 03, 2013, 01:40 PM
CNC Cutter
birdofplay's Avatar
United States, IN, Indianapolis
Joined Dec 2005
8,706 Posts
Discussion
FETs feeding Larger FETs

Got a dumb question.

These NEW quadcopter boards like the 1SQ or the Syema X1

a much more stable than any of the miserable expensive boards for larger
machines that I have tried.

I know they cant supply enough power for LARGER motors but

WHAT IF those outputs could be fed to larger FETs to drive larger motors ?

Not being into serious electronics design I'm looking for a reasons Why not
and possibly some pointers in the right direction.

I am NOT talking about Brushless or 3 phase motors just the BRUSHED types.
Brushless need the Feed back for timing making things a bit messy.

Just trying to do more with less,

Thanks

Bob
birdofplay is offline Find More Posts by birdofplay
Last edited by birdofplay; Feb 06, 2013 at 09:36 AM.
Reply With Quote
Sign up now
to remove ads between posts
Old Feb 04, 2013, 06:19 AM
Life begins at transition
Australia, VIC, Sale
Joined May 2007
3,593 Posts
Potentially could do.
Look out for feeding the FETs at a higher voltage than they're built for.

It would help to find out if the board you've got switches +ve or -ve. That will decide if a N- or P- fet is easier.
Odysis is offline Find More Posts by Odysis
Reply With Quote
Old Feb 06, 2013, 09:36 AM
CNC Cutter
birdofplay's Avatar
United States, IN, Indianapolis
Joined Dec 2005
8,706 Posts
There are various boards and replacement boards available.

I guess I'll have to get out the meter and check one of them.

The Little quad replacement boards are Sub $20 and work very well in their
small machines. like the Syma x1 or the 1SQ for example.

With a larger FET being fed by the little boards a larger home brew set up can be envisioned.
birdofplay is offline Find More Posts by birdofplay
Reply With Quote
Old Feb 06, 2013, 08:41 PM
Registered User
Joined Nov 2007
908 Posts
Yes it is fairly easy to make a booster to power large motors. Basically the output of a brushed motor controller is an open collector driver (except the output is a FET instead of a bi-polar transistor). The output capacitance of the FET is large compared to a transistor so you will need a low value pull-up resistor, maybe 500 to 1K ohm. Even something like an IRF3205 FET (110 amps) only has 800pf output capacitance so it is not too bad. Add a driver circuit and really big power FETs or monster IGBTs and you can drive both high voltage and high current.

Below is a circuit I have used with modifications for my application. The output FET shown is an IRFZ48N but it could be almost anything. The circuit also shows 5k pull up resistors which were OK for the controller I used it with, you may want to use 500 ohm or 1K resistors.
mjsas is offline Find More Posts by mjsas
Reply With Quote
Old Feb 07, 2013, 07:31 PM
CNC Cutter
birdofplay's Avatar
United States, IN, Indianapolis
Joined Dec 2005
8,706 Posts
WOW ! thanks for that Schematic and tutorial.

It was Just what I was looking for.

Mouser or Digikey will be hearing form me soon.

Best,

Bob
birdofplay is offline Find More Posts by birdofplay
Reply With Quote
Old Feb 07, 2013, 08:08 PM
Registered User
Joined Nov 2007
908 Posts
The schematic does not specify diode D. This diode can carry as much as the current Q4 does so it has to be sized correct. Actually at throttle it carries the same current as Q4.
mjsas is offline Find More Posts by mjsas
Reply With Quote
Old Feb 08, 2013, 10:41 AM
CNC Cutter
birdofplay's Avatar
United States, IN, Indianapolis
Joined Dec 2005
8,706 Posts
Warning noted, thanx, Bob
birdofplay is offline Find More Posts by birdofplay
Reply With Quote
Old Mar 03, 2013, 02:28 AM
Registered User
--Oz--'s Avatar
United States
Joined Mar 2006
2,144 Posts
Quote:
Originally Posted by mjsas View Post
Yes it is fairly easy to make a booster to power large motors. Basically the output of a brushed motor controller is an open collector driver (except the output is a FET instead of a bi-polar transistor). The output capacitance of the FET is large compared to a transistor so you will need a low value pull-up resistor, maybe 500 to 1K ohm. Even something like an IRF3205 FET (110 amps) only has 800pf output capacitance so it is not too bad. Add a driver circuit and really big power FETs or monster IGBTs and you can drive both high voltage and high current.

Below is a circuit I have used with modifications for my application. The output FET shown is an IRFZ48N but it could be almost anything. The circuit also shows 5k pull up resistors which were OK for the controller I used it with, you may want to use 500 ohm or 1K resistors.
Why not just replace the X1 fets with a larger fets that have low gate voltage (Vgs) requirements, there would be a lot less work?

I have the X1 and the only thing is the motors seem to wear out to quick IMO, was thinking of getting bigger brushed motors and directly connecting the props (after the fet upgrade), any motor/prop suggestions welcome.

http://www.rcgroups.com/forums/showt...4#post24125840
--Oz-- is online now Find More Posts by --Oz--
Last edited by --Oz--; Mar 03, 2013 at 02:33 AM. Reason: added link to X1 motor brush and comm picture
Reply With Quote
Old Mar 03, 2013, 11:05 AM
CNC Cutter
birdofplay's Avatar
United States, IN, Indianapolis
Joined Dec 2005
8,706 Posts
Interesting Idea !

Thats the spirit I was hoping for in this thread.

IF there is ROOM to get AT the FET connections and IF we knew what TYPE it is
then they could even be OFFboarded with heatsinks and larger connectors.
Some might call it a Shield PCB .

BUT per the diode warning above in #6 ...
More than just the FET might have to be taken off board and I wonder about the
"Little" boards' ability to handle the higher currents.
birdofplay is offline Find More Posts by birdofplay
Reply With Quote
Old Mar 03, 2013, 03:05 PM
Registered User
New Zealand, Wellington, Porirua
Joined Jan 2013
96 Posts
Quote:
Originally Posted by mjsas View Post
The schematic does not specify diode D. This diode can carry as much as the current Q4 does so it has to be sized correct. Actually at throttle it carries the same current as Q4.
Where did you learn of this? The back emf from a motor is a product of the RPM, and nothing to do with the duty cycle it is driven at. The current produced a product of both the current supplied to the motor and the inductance of the coil.
I'm curios
viperidae is offline Find More Posts by viperidae
Reply With Quote
Old Mar 03, 2013, 03:57 PM
CNC Cutter
birdofplay's Avatar
United States, IN, Indianapolis
Joined Dec 2005
8,706 Posts
And this is for Brushed motors.
birdofplay is offline Find More Posts by birdofplay
Reply With Quote
Old Mar 03, 2013, 08:57 PM
Registered User
--Oz--'s Avatar
United States
Joined Mar 2006
2,144 Posts
Quote:
Originally Posted by birdofplay View Post
Interesting Idea !

Thats the spirit I was hoping for in this thread.

IF there is ROOM to get AT the FET connections and IF we knew what TYPE it is
then they could even be OFFboarded with heatsinks and larger connectors.
Some might call it a Shield PCB .

BUT per the diode warning above in #6 ...
More than just the FET might have to be taken off board and I wonder about the
"Little" boards' ability to handle the higher currents.
I raced RC on-road cars for 21 years, I quit about 10 years ago, right before the LiPo batteries and BL motors came out. I was averaging 45 amps for 5 minutes, batteries and motor (mod 8 turn) would come off to hot to touch. In the ESC they had a schotky diode to give more speed (to clamp the CEMF) coming out of the motor, the higher the rpm the higher the CEMF. Most times we would put the schotty diode on the motor for slightly more effectiveness.

The Syma X1 has 2 dual 4A SO-8 FET packages. Simply remove them and off board use larger fets (with heat sinks if needed) and diodes, the high current would no go though the little board. The X1 runs off 1S (4.2Vmax), but with the external larger fets, you could run pretty high voltage and current. A popular 60A fet is the IRLZ44, they can be bought on ebay for $4 shipped for Qty=10.
http://www.ebay.com/itm/10pcs-DIP-IR...item53e9edf20b
While they preform at their best with higher than 4 or 5 volt gate voltage, they still preform nice at the lower gate voltages, this allows direct uP control like the X1 does (no gate driver in between the uP and FET) and simplifies the circuit quite a bit. Also, fets can be parralleled up for more current. I did a quick search on digikey for 10A to 60A fets with what they call TTL logic level fets, this link may not work
http://www.digikey.com/scripts/dksea...tity=0&PV69=80

off topic, the syma X1 quad is a great little stable quad, it taught me to fly quad's, so I built a larger 400mm 1000watt 4S quad with KK2 flight controller. I thought was you could put something together pretty low cost with the Syma X1 board, it's a Rx, 3 axis gryo, flight controller and only $10 shipped via slow boat. The X1 is compatible with the 9x Tx.

Link to X1 board.
http://www.banggood.com/Wholesale-Sy...2-p-54191.html
--Oz-- is online now Find More Posts by --Oz--
Reply With Quote
Old Mar 04, 2013, 09:12 PM
Registered User
Joined Nov 2007
908 Posts
Quote:
Originally Posted by viperidae View Post
Where did you learn of this? The back emf from a motor is a product of the RPM, and nothing to do with the duty cycle it is driven at. The current produced a product of both the current supplied to the motor and the inductance of the coil.
I'm curios
Any motor, brushed or brushless, is a motor, is an inductor and is a generator. At throttle the PWM is 50%, on for half the time and off for half the time. Assumming a PWM frequency of 8KHz the current in the motor cannot change that fast. The current will change very little during the off time due to the inductance.

During the on time the FET carries the current and during the off time the diode carries the current. Therefore the diode carries the same amount of current as the FET for the same amount of time.

Actually at any power setting less then WOT the current in the diode is nearly the same as the FET, just the duty cycle changes. At low power settings the duty cycle is long but the current is low, at high power settings the duty cycle is short but the current is high.
mjsas is offline Find More Posts by mjsas
Reply With Quote
Old Mar 07, 2013, 08:55 PM
Registered User
--Oz--'s Avatar
United States
Joined Mar 2006
2,144 Posts
Ot

Off topic, if you have read about servo disk motors, then skip this post. They are unquice style of motor. Here is one of the benifits (copy and paste from catalog)

A typical ServoDisc motor can accelerate from 0 to
3000 rpm in only 60 degrees of rotation. In some applications,
the entire move can be performed in less than 10 milliseconds.

Manual in PFD found here;
http://www.kollmorgen.com/en-us/prod...dc/servo-disc/

I attached one page of the manual.
--Oz-- is online now Find More Posts by --Oz--
Reply With Quote
Old Mar 07, 2013, 08:58 PM
CNC Cutter
birdofplay's Avatar
United States, IN, Indianapolis
Joined Dec 2005
8,706 Posts
We'll need titanium Props and CF motor mounts !!!
birdofplay is offline Find More Posts by birdofplay
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Designing a FET board for a TURNIGY 100A HV 5-12S p901P901 DIY Electronics 9 Feb 20, 2013 03:34 AM
For Sale Pitch Gauges for larger helis balsabasher2 Aircraft - Electric - Helis (FS/W) 0 Jan 14, 2013 02:02 PM
Cool Feed the Invisible Man... Norman Adlam Life, The Universe, and Politics 1 Jan 10, 2013 09:52 AM
Wanted Trade for larger twin Ice Aircraft - Fuel - Engines and Accessories (FS/W) 2 Jan 09, 2013 10:39 AM
Discussion Anyone else feed the wild birds? 56S Life, The Universe, and Politics 57 Jan 05, 2013 11:30 PM