RC motor current amplification

[Stupidspencer]

First of all, Im pretty good with electronics, but I have no formal training.
I have a medium sized, twin-rotor RC helicopter (specifically a SYMA S031) and I want to give it more "go" by modifying the tail motor operation. On these, the tail motor is what makes the heli tilt forward/backward and thus go forward and backward.

I installed a longer, higher performance tail motor, but the driving circuit barely has enough power to start it spinning and sometimes not at all. So what I want to do is use an NPN transistor to amplify the current available to the motor without sacrifycing the PWM linear controllability and the forward/reverse controllability.

The 9.6V LiPo battery has more than enough current to drive this motor 3 times faster than the original motor which = FASTER FLIGHT!

I believe the original +lead should go to the base, the battery + to the collector, and the new motor +lead should come from the emitter, but Im not sure if this is correct or if it will allow for reverse?

Here are the specifics:
PWM speed control operates at 9.6V forward and 4.8V in reverse
Max current output is 400mA
New motor current draw at 9.6V: 1920mA (3120mA start surge)

So yeah, I want to amplify the output from 400mA to about 2000mA and still maintain PWM and forward reverse operation.

Correct me if Im wrong and please help determine which NPN transistor to use, but please dont nit-pick. Nobody likes nit-pickers: those are the people that would tell me my new motor is too heavy and I will need to re-balance. I Already did that and, despite my screen-name, Im not stupid.

Thanks for helping!



If you want to know why I want to put any effort into a cheap chinese toy, I shouldnt have to tell you how much a real model RC helicopter costs. This thing only cost 45 bucks, is well built, and it handles great! With this mod it should totally kick-@ss.

 

[Resqueline]

You'll need a full bridge, meaning two NPN and two PNP transistors. You'll loose a volt or two through this arrangement though.
It would be better if the existing full bridge driver could be replaced but I guess it's all in one IC so it could prove impossible.
An alternative could be to use MOSFET's instead and put them in an inverting configuration. This would not drop any voltage.
Are you comfortable soldering surface mount devices?

 

[Stupidspencer]

Are you comfortable soldering surface mount devices?

Yes- Ive done it many times before.

The circuitry does not appear to put the inverter in an IC. There are two power-FETs which I assume drive the 2 main rotor motors. But other than that, there are four other transistors that are the standard circle-with-a-flat-side shape. Could these all be for driving the rear motor? It is the only channel that requires both forward and reverse.
2 of those transistors are marked "B772" and two are marked "D882" and all four have "JC 98" below those markings.
They are all physically located near the connector for the tail motor.

But please tell me more about using MOSFETS in an inverting configuration. How do I select the proper ones?

 

[Resqueline]

Ok, that sounds promising.
The 2SB772 is a 30V 3A PNP transistor, and the 2SD882 is an NPN transistor complementary to the 772.
Your best bet may be to replace those transistors with MOSFET's, depending on their circuit configuration.
If their emitters are connected to the power rails then you're in luck, as you can pop in MOSFET's as direct replacements.
If their collectors are connected to the power rails then you'll have to put in better bipolar transistors or make an inverting MOSFET buffer.
The general idea is to have P-channels at the positive rail, and N-channels at the negative rail. This ensures a minimal voltage drop.
It's not entirely trivial to design that however, as one needs to ensure they don't conduct simultaneously, thus shorting the power rails.
A quick search came up with this bridge circuit/discussion, as an example, and also this circuit.

 

[Stupidspencer]

OK, I went to the local electronics store before your post, Resqueline. But I cross referenced those to NTE parts and came up with NTE184 and NTE185. The specs are:
Polarity/material: (184: NPN-Si) (185: PNP-Si)
Application: Audio Power Amp & Switching
Package: TO126 (although this is a different package than whats on the heli)
Ic: 4A
Collector-Base Volts: 80
Collector-Emitter Volts: 80
Emitter-Base Volts: 5
Typical Forward Current Gain: 7min
Collector Power dissipation: 40 watts
Typical Frequency: 2MHz min

And based on that info I selected NTE377 and NTE378
Heres the specs on those:
Polarity/material: (377: NPN-Si) (378: PNP-Si)
Application: Power Amp Driver, Output, Switch
Package: TO220
Ic: 10A
Collector-Base Volts: 80
Collector-Emitter Volts: 80
Emitter-Base Volts: 5
Typical Forward Current Gain: 60min
Collector Power Dissipation: 50 watts
Typical Frequency: 50Mhz

These were the closest match I could find with higher amperage values. What is the typical frequency? Is that gonna matter here? And do you think these will work as direct replacements?
Thanks again for your help. I really appreciate it.

 

[Resqueline]

Yes they seem like they could do a better job, with the higher Ic and higher gain.
Just remember the TO-220 pinout is b-c-e as opposed to the TO-126 which is e-c-b. I don't know the pinout of the "oversized" TO-92's in your 'copter though.

 

[Stupidspencer]

The board has "B" "C" "E" markings for the transistor pins. Isnt that nice? I wish all manufacturers would make it that easy!

 

[Stupidspencer]

Not my best work, but it was a bit harder than expected getting those big TO-220s to fit where TO-92s were before. Some of the solder connections are on the top of the board which made it even more difficult getting the soldering iron in place.
And I had to put shrink tube over the pins so the base and collector pins could cross over each other since the TO-92s were c-b-e and the TO-220s are b-c-e.
I hooked up the battery and it appears to work, but I wont know for sure until I put the heli back together and test it with the motors connected.

EDIT-- I dont know why the picture wont show- Whatev
If you want to see it, right click the image icon and copy and paste the url into a blank tab's address bar.

 

[Stupidspencer]

Well, it only works in reverse, so I think one of those tough-to-reach solder points on the top of the board isnt making a connection.

 

[Stupidspencer]

Havent figured out the reverse thing yet, but I measured 1020mA @ 4.8V in reverse! Now if I can just get forward to work.

Dang! These new FETs (the two that are making reverse work) get really hot! I think they will need heat-sinks. How did those little TO-92s not overheat?

 

[Stupidspencer]

I cant get forward to work. All the solder joints connect. I dont know whats the dillyo, but I tell you what, Im frustrated and I think Im gonna just put the old transistors back on and use a relay to make it go full speed only in the forward direction or something. But that will definately make it hard to control- maybe even want to do a loop! The thing flew right off the workbench JUST WITH THE TAILROTOR when I hooked it directly to the battery. I want that power available, but not necessary.

UNLESS somebody has a better idea.

 

[Resqueline]

Double sided boards usually has through plated holes that takes care of the connection & solder flow to the other side. Maybe you pulled a plating out of its hole?
I would expect fitting TO220's in place of TO92's to be a challenge, and certainly even more so with a different pinout.
You first said you bought bipolar transistor but then you mention FET's? What did you put in there (part #'s)?
You'll also need to verify & communicate which one of the three possible bridge configurations the board is configured for. (Check against both power rails.)

 

[Stupidspencer]

Sorry, I was talking about the bipolar transistors. What makes them different than Field-Effect transistors? I'm pretty new to the workings of transistors. I want to learn more. I need to find a good book about em. Any recommendations?

As for this board, I'm gonna put wires where the 4 transistors are supposed to be so I can hook it up to a breadboard and experiment with different ones.
If I blow something up, no big deal. A replacement board is only 12 bucks.

 

[Stupidspencer]

Heres a quick drawing of how this part of the board is layed out.
The red lines are all the connections I can see from both sides of the board.
The lines that go to nothing in the drawing go off to more complex circuitry elsewhere on the board.
I dont know which are the power rails since this thing has a safety circuit that only gives power to the tail motor when the main rotors are running.

If the image doesnt show, click this link: http://www.freeimagehosting.net/892ce

 

[Resqueline]

Bipolar transistors are current controlled, whereas field-effect transistors are voltage controlled. They each have their advantages and disadvantages.
I don't have any particular recommendations, other than just to try to Google and see if something useful crops up.
From the board layout I can tell that it's not just a matter of popping in MOSFET's in place of the bipolar junction transistors (BJT's).
You'll have to stick with BJT's to avoid a more complex redesign. Can you read the resistor value, the smd transistor code, and write in what NPN/PNP is where?
The heating issue may be due to insufficient base drive. Perhaps it's possible to change the resistor value somewhat, or use Darlington transistors instead.
Save drawings in gif or png format if you can, it gives a better image quality (easier to edit/ work with later) and saves on file size too.
Here's the bridge straightened out into a more diagram-like drawing btw.

 

[Stupidspencer]

Yes, I will work on a more detailed drawing with all the resistor and transistor codes. The only components that arent marked (as always) are the tan surface-mount capacitors. I will post that drawing in a day or two.
I did consider trying darlington transistors, but the local electronics store didnt have a matching set in stock.
And thanks for that clarification on MOSFETS. Im learning a lot from this!