View Full Version : Another PCB mod and Schematic

10-21-2002, 12:38 PM
- This is my first ZZ mod and my first time thinking seriously about learning SMD.

- I wanted more power for my ZZ, and I have seen lots of good mods. Since I want to keep reverse, I decided to monkey with the H-bridge. I decided to take a look at the circuit board and try out this mod, http://tinyrc.com/forums/showthread.php?s=&threadid=1736. The resistors to be bypassed were not connected to what I expected. They seem to be going to the gate of the mosfets. After i checked the voltage levels, I suspect that the transistors are not mosfets but are NPN and PNP transistors instead. There is a voltage doubler on the board (which can be used for other mods!) but I doubt this is for operation of mosfets.

- I couldn't find a schematic of the H-bridge so I have included my best guess as to the SMDs. R20 and R21 go to the low side transistor's bases and R16 and R19 go to the high side transistor's bases. R17 and R18 are pullup resistors (For stability?).

- Now for the mod: I highly suggest that the four driving resistors be lowered. I have gone ahead and shorted out R16,R19,R20 adn R21. Originally I could pull 80mA through the circuit. With the mod, I can now pull 150mA! The best part is I am only pulling 166mA from the battery. So I get over 4 minute run times and my ZZ can now run on carpet with red-red. Yaaaa! I also tried to give more power to the steering coils but their resistance is so high that I could only pull an extra 13mA. So I might change the magnet to a higher quality one instead.

- Here is another thought to those that really wan't a challenge. Use N channel and P channel mosfets that opperate at 3 volts; install another battery for the voltage to run it and you will then have a propper H-bridge.

- Anyway, my car runs better than ever. If anybody else experiments with changing the resistors or adding more transistors (you still can't get around the 0.2 voltage drop, I dont think) then please let let us know the results.

10-21-2002, 04:43 PM
I'll try this mod on my other zip and see what happens.

Thanks for the tip.

10-22-2002, 12:11 AM
If your schematic is correct (and I suspect it is) that would explain why people are seeing mixed results by only shorting out the 68's in the H-bridge and why you are seeing much better by taking care of the 220's also.... nice work...


10-22-2002, 05:23 PM
- I have some low voltage SMD mosfets that I thought should be tried on my one and only Zip Zap. Naturally I am extra careful not to destroy it. So, after verifying voltages and currents I disconnected a NPN transistor and replaced it with my mosfet. And I could not believe it. It worked! So I tried two mosfets and got even better results.

- Measuring from battery negative to the blue motor wire, running the motor forwards and holding the wheels this is what I got on the oscilloscope:

Original configuration: 0.42v
Shorting R20 and R21: 0.35v
Replaced NPNs with N-Mosfets: 0.2v
Stacked 2 Mosfets: 0.09v
(I suspect that 3 Mosfets would give even better results, but I have gained 0.3v and that is enough for me)

The advantages to this mod are more voltage to the motor, less power wasted in the transistor, and less strain on the IC. My mosfets were pin-for-pin compatable and only cost 44 cents each. The values of R20 and R21 have no effect on the performance. It should also be noted that apparently the IC can sink and source current, so there is no need for a pulldown resistor to turn the mosfets off.

- The mosfets I used were N-Channel 25v SOT-23 from Digikey. But I somehow don't expect too many people to try this mod out.

Oh I almost forgot, I can pull 260mA from the H-bridge. But I have no idea why anyone would wan't to do that.

10-22-2002, 09:45 PM
I am so interested in this procedure, I must say. I understand how to short the four resistors, and I know which they are, but I totally do not understand anything else you are saying. Is there a way you can post that info in layman's terms? I'd love to try it, but I'm electrically clueless.

10-23-2002, 01:08 AM
Try this: It may not work or cause problems.

Using above diagram.

Disconnect R18 & R17 ( Remove )
Leave R19 & R16 ( or cut in half or double value )

And either short or cut ohm's for R20 & 21

The problem is you dont want too much base current or will fry the transistors.

Now is the + voltage comming strait from the battery or through another resistor?

Both R18 & R17 are biasing the transistor and not letting it fully switch.

Tnks Dayfall for the part number. :)

PS: Dayfall. Have you try a cobo of P-Channel on top and N-Channel on the bottom?

10-23-2002, 10:28 AM
It does seem like removing R18 and R17 would help. I havn't noticed much loss across the upper transistors so I havn't bothered with them yet. Now that I have more current going through the motor, I might should go back and look at how much current is going into the transistors gate. It is probably to much since 10mA would be way more than we need.

Well, I just checked it and sure enough, 50mA. Woops. I am sure it can take 100mA. but that reading seems kinda high. With the 68ohm resistor it takes only 5mA. I am guessing 10mA is what I need. 30ohms is probably close to optimal. The problem now is to find a resistor small enough. Regular SMD resistors are huge compared to the ones in the ZZ.

The reason I don't think P-Mosfets will work is because the maximum voltage avaliable to the gate is 1.2v (for those of us with only 1 battery.) Yes, + on my schematic is battey positive. I just looked at the charts for my mosfets and it says if Vgs=2.7v then Rds=0.24ohm. I am betting that the resistance will be way too high for 1.2v.

John: I wouldn't suggest trying this mod unless you have a good soldering iron and steady hands. The procedure is easy though. Just take off the two transistors and replace them with the new ones. Here is a picture of what I have done so far:

10-23-2002, 10:37 AM
Hmmm. Okay, I think I'll wait a bit on this one. I can solder, and I use tools to steady my hands (a surgeon, I am not :) ) I am just clueless about what little electical part does what. I'm learning though. I've been scouring the web for info on DC electricity. I do the electrician's work in my house, but it's much simpler to wire in an AC light fixture or an outlet than it is to hotwire a ZipZap.

I think I'll start with something a little easier... like headlights that turn on when the car is going forward or somthing. I just read how to do that, so perhaps I'll start small, and just watch this for a while and see what happens.

Thanks for the subtle warning and the pic, I'll heed the warning, and study pic!

10-23-2002, 11:46 AM
The only reason I suggested the P-device is the top of H is the "source" and the bottom is "sink". So you use P's on top an N's on the bottom. Im thinking of max volts/amps right now.

Wait a minute... Wonder if they used NPN for both! Usualy the setup is PNP on top and NPN on bottom!

Check around the PCB. They may be running a "switching inverter" PS for the receiver and decode IC. The chip set need at least 2.2 volts I think to run. If so there may be 3v running around that could be used. :)

PS: Good job! :D

10-23-2002, 12:23 PM
Yep, there is a voltage doubler on the board. It is labled A3_3V. The output seems to be the center pin between the other two. Right now I am trying to steal from it and run some lights, but I don't think I can pull more than around 50mA or so without the noise interfering with the RF stuff and IC.

The reason the N channel works is because there is 3 volts coming from the IC. The Source is connected to ground and so Vgs=3v. We normally want more but you know... The upper side of the H-bridge is diffrent. Assuming no voltage less than ground, we would need 6volts to the gate, that is because the motor would take 3volts away from the Source. That is if we used and N channel mosfet. A P channel one would need either 3volts on the Source (meaning running the motor at 3v) or putting -1.5v on the gate. Either could be done if someone wanted to.

I think the upper transistors are PNP because it looks like the intention of the circuit was when one side of the motor went to ground, that would pull down the gate of the transistor on the other side. And since that is the other transistor we want to be turned on I think it is a PNP type.

Also there is somthing funny about the current going to the PNP transistors. Even with 10ohms instead of the 68ohms I still don't get the power I do when I short the resistor out. I am going to try a PNP darlington and see what happens.

10-23-2002, 01:34 PM
You guys seem to know a crap load about electronics. I'm guessing you went to school for this but do you happen to know of any books or anything online that explains some of this stuff. I was going to go to school for this but I may be losing my job that would pay for the schooling. :(

10-23-2002, 02:17 PM
The base design seems like most designed on the TX2/RX2 chip set. They usualy will put a micro switching PS to stablize the voltage feeding the regenative receiver and the RX2 chipset on the board, since voltages range goes all over the place. I agree would be hard press to pull much out of that circuit. Could beef it up.

Wonder if switching from the 68ohm resistors cross to a more TTL scheme of flipping would help?

The thing is what is the exact different between a Bit-Chrg-G H bridge and a ZZ one? Got a MicroQ and my Tomy Bus is still on order so could check with the Micro-Q varrent.

PS: lowerdfool. Did in high school but work with this over a lot of years. But mostly reading, doing and working at companies. One of my books I always keep on the shelf a ARRL handbook. They have a new one every year. Has base info. Check around grab one used. The new one are expensive but the theory has not changes. Proabbly no further back than 1974 or 75. Else you start into tube therory. :eek: Check Radioshack they have some help books if they still stock them.

PSS: Got to run out to a client to demo a pice of SW I cook up for them. They was a site I ran across a few months back on using a PIC Processor to add multy speed to a bit but cant remember the site right now. From Japanes I think. It has the diagram of a Bit H bridge I think.

10-23-2002, 09:40 PM
What FET are you using?

10-23-2002, 10:48 PM
DayFall: Good Job.

Just wanted to let everyone know that they can get the parts online at http://www.digikey.com/. Also I was woundering what % of increase you seen after doing this mod?...


10-24-2002, 12:37 AM
Finaly got back in. Wow love putting out fires and bored clients! :)

The site I was thing of that did have the H configuration of a Bit Char-G H bridge and was a Japnaes site! Also it the guy that has the Japanes semi with the umpteen lights on it.

This is it under dual and tripple speed section on his site.

Its basically the same but missing R18 & R17, direct connection to RX2 and the two cross resistor. A lot cleaner.

Could it be the spects on the transistors are different? Be fun to take a "set" from a bit pc board and replace the ones on a ZZ.

Since I am taking the next couple of days off, will get to "have fun" also! :D

PS: Wondering how close to ground the NPN are really getting down to?

10-24-2002, 06:27 PM
Hey, Dayfall, is there any chance you could provide an actual picture of the PCB with arrows pointing to the points that we should be jumpering?? This will help out alot for the people who dont know how to read electrical schematics.


10-25-2002, 11:10 AM
- This thread has a good diagram of two resistors you should jumper (or at least jumper or replace with smaller resistors, which did)

I am posting a picture of my board with the other two resistors to jumper or replace. They are marked in red.

- The NPNs (before I took mine out) got within 0.35 volts of ground. Darlington NPNs might get you to 0.2 volts.

- I definatly noticed an increase in power when I jumpered the resistors. I got around 5 to 10% increse I think. When I put in the MosFETs I got around another 5 to 10% increase. My testing was motly how well it ran on carpet and what slope it could climb. Electronics wise, I went from about 0.85v to about 1.1v to the motor.

10-26-2002, 04:46 PM
Promoted to Bit Science. :)

10-30-2002, 01:54 PM
Try as I might, I can not increase the voltage to the high side of the motor. I bet the no reverse mod DOES work great. The voltage drop is still about 0.3v. I tried a darlington transistor and (DUH) that didn't work. Darlingtons need lots of voltage to thier bases. Mosfets didn't work much better. The only solution was to just short out the resistors like in the original post.

We simply need more voltage. A negative voltage generator perhaps?

10-30-2002, 03:10 PM
There has got to be a difference between how a Bit driver and a ZZ. Wonder why unless the transistor are not getting the same level of signal to switch? Unknown.

Oh Did this particle diagram of the front end receiver of a ZZ. Its not complet and has not been check fully for errors. This is mostly from the "top" side of the board. May get a chance this weekend to finish up but need something to crack the black expoxy when I get to the IC side of things. :)

11-26-2002, 10:06 AM
I was wondering- could you jumper the resistors listed in this post, on a dual cell car, or would that fry the whole thing?
It seems that if done wrong, they will fry on a single cell.

11-26-2002, 01:04 PM
Just reading over this again since it has been bumped.

Won't shorting the resistors promote decreased transistor and MOSFET life? Aren't they there in part to limit current flow?

Or are these transistors designed to handle more current than what is actually being used?
I might actually be brave enough to try this.

11-26-2002, 01:43 PM
You do not want to excede either base current ot gate current if a FET.

Around on this board is a diagram for the H-Bridge.

12-02-2002, 11:35 AM
If you are using a duel batt mod then I don't think you should try shorting the resistors unless you really know what you are doing. In the duel battery case I would suggest using 4 mosfets if you really want to get serious.

Jumpering can not hurt mosfets, but can hurt transistors. I don't remember the current they drew, so hopefully someone will measure it and post the results. Since I am no longer using the upper two transistors, I can't say if their life would have been shortened.

12-02-2002, 11:50 AM
Thanks for the reply.

I have read in another thread here where someone is using 100ohm resistors to replace the 220's, then someone else took that farther and used SMD resistors from an old hard drive.
I think I'm going to try 100ohm SMD resistors.

12-02-2002, 12:01 PM
Here is the thread I was talking about where a few people are using 100 ohm resistors.


12-18-2002, 10:49 PM
Hi Dayfall,
can you please tell us what's the exact part number of the n-fets from Digikey you used?
I tried to find N-Channel 25v SOT-23 mosfets, but they seem to have none. (only 30V)

12-23-2002, 11:48 AM
The part number is FDV303NCT-ND for $.55

I can't wait till I get a big enough order so I can get some SMD LEDs for some light mods.

12-23-2002, 12:12 PM
Instead of trying to remove the little nightmares you could parallel another 220ohm across each one. This will drop the effective restance down to 110 if you parallel a 220 across it

If you uses a 190 will make it around 101 ohms.

For the math incliend, here are two different formulas.

1/(1/R1)+(1/R2)) = effective value

or if you have two resistors

(R1 * R2)/(R1 + R2) = Results

So something around 190ohm will give you 100. :)

12-24-2002, 11:51 AM
I thought this might help in your quest for more speed etc. It's the full schematic for the receiver.

Craig Falknor

12-24-2002, 01:00 PM
Cool Grab! :)

For the ZZ group the receiver side of the circuit is different. But odds are the IC on is the same.

Again good grab.

12-24-2002, 01:05 PM
What I have piced together for the front in.

12-24-2002, 01:06 PM
And now for the secmatic.

12-27-2002, 04:00 PM
Would be REALLY nice to have learned Japanese, Chinese and Electronicese before these things came out... Hehe

Any English-language translations for the various mod-sites out there?

(desperately trying to absorb all of the engineer-speak in here and LOVING the pics on the Japanese sites... just feel like I'm missing steps due to language barriers... *grumble*)

12-27-2002, 10:19 PM
There is Babble fish at Altavista.com search site. For the Greek of electronic it wont handle it. Uses it for both French and Japanes but its pritty bad on some things.

PS: I think there is a ground missing on the left side of the circuit diagram.

01-07-2003, 06:00 PM
Ok, reading this thread it seems like DayFall and Nightwing know an awful lot about what they're talking about. I wont pretend to know most of what they're on about.

I may be asking for too much, but would it be possible for one of you guys to do a 'basic theory of operation' of the receiver PCBs?

It seems collectively, and through experience their is an awful lot of knowledge about Bit Char's out there, but i'd say the most complex and mysterious bit remaining is the PCB.

Any electronics gurus fancy doing a tutorial on the PCB board?

I only know basic electronics but would love to learn more about the board.

PS - Craig, where on earth did you get that detailed schematic from? Kudos to you!

01-10-2003, 02:23 PM
I don't have any experience in the receiver portion of the electronics. Sorry. From looking at the RF from the transmitter, I can only guess that a number of pulses is sent; the resistors and capacitor on the PCB, that connect only to the IC, are what is used for the timing to count the pulses. I am guessing that the pulses duration determines the data. Either that or the pulses are simular to serial data, where each pulse is for each button on the controller.

Also I just had the experience of controlling a robot toy someone had with my ZZ controller. Combined from other stories I have heard, there seems to be quite a standard as to how the data is sent.

Could some one confirm if it is the same or diffrent than what is used for R/C cars and planes. I am betting that it is the same, and that teach pulse lasts 1ms-2ms depending on what data is sent!

01-15-2003, 06:20 AM
The Bit remote uses a different protocol than "real" RC cars and planes.
Models with an analogue control use the 1-2 ms timing you mentioned, where the duration of the pulse determines the position of the servo (1ms= 0% 2ms=100%)
description here (http://www.geocities.com/BourbonStreet/3220/servotx.html)

The Bit remote uses another protocol that is very common for simple RC-toys, which allows only on or off and no intermediate values.
like this one (http://www.howstuffworks.com/rc-toy3.htm)

I looked inside the transmitter of a Fit-Racer (OEM-clone) but unfortunately I can not find any documentation for the transmitter IC (PT 8A977BP Y0238JW)
But it seems that many toys use the same protocol. Of course the channels may be switched so that left becomes forward :)

My remote has a 5th channel that is not used. My guess is that the "Bit Char-G Booster Machine set" uses the 5th channel for the turbo feature.

If someone has a Bit with the booster function he can try to connect pin 7 of the IC to ground, maybe this is the booster. If this works there would be no need to buy a new remote, just add a switch to your existing one.

01-17-2003, 12:50 PM
Thanks! I suspected that may be the case. Now to find out if my ZZ has an extra channel.

01-17-2003, 06:29 PM
If you are using a duel batt mod then I don't think you should try shorting the resistors unless you really know what you are doing.

I was wondering if this multi cell mod (http://www.geocities.com/wdiaz03/minidragster.html) would work. I think it might since the extra cells r on a seperate circuit. Their circuit is closed when power is sent to the transistor* using the origonal signal from the pbc. After the power is sent through the motor and then through a transistor* before going back to the pcb, so the pbc shpuldn't be able to tell a differance. I think this will prevent the pcb from frying.

*:confused:I donít really know that much about electronics so Iím not sure if this will work. I was wondering if u could help me with a few questions? First, what exactly does a transistor really do? And does a resistor drop voltage, current or both? What do these mofets do, i haven't herd of them? And last, after energy goes through the motor y would it have to be droped by the resistor?

Hope this helps.

I have so far:
zz tubo motor (i think its faster than the nos)
this pbc mod
no reverse mod

Whats next?:confused:

01-20-2003, 12:07 AM
Iv'e had 2-cells for months. Works fine, cause resistors rduce current, so more voltage won't hurt anything. I cannot see how 2.4v would anyway:)

03-14-2003, 08:40 AM
just wondering if Dayfall
found an extra channel on the ZZ board. Great stuff in this thread...getting ready to do some mods to my first ZZ...just can't decide what to try first....too many good ideas here.

03-25-2003, 01:19 PM

Tried your mod.....I shorted r16, r19, r20, and r21, and I lost a lot of speed. Anyone else have this problem? would really like to find a boost without an extra battery...

b49m zztbo Yellow Mustang Cobra
b27m zztbo Mazda RX8