Hi, I'm new here to the forum/website, but know a little about very simple DC circuits, switches, relays, etc. This is a fun little project I am working on and just experimenting with.

What I have:
$10 RC car that runs on 3 AA batteries. There are 2 motors - one in front to make it turn L/R, one in back to make it go F/R. There is no graduation in these directions, they are full on or full off at the remote. I want to make the car go faster by increasing voltage to the rear motor. The batteries feed a "motherboard" which outputs 4.5-5V to the rear motor. When the vehicle is put in R, the "motherboard" reverses polarity. The throttle on the remote is like a center off switch: up is F, middle is brake/off, down is R.

What I've done:
I have a wall wart that can vary output voltage from 1.5-12v. I applied varying levels of VDC at the battery terminals and found that nothing happens when I apply more than 4.5V (I went up to 7.5V before stopping). Less than 4.5V and the car works fine. So there is something in the "motherboard" that protects the system from too much voltage. This tells me I can't get more power to the drive motor through the motherboard.

I've briefly applied up to 7.5V to the motor directly and it cranks wonderfully (for how long is unknown). So my next thought is how to get more power to the motor after the "motherboard. I've thought of several options, but the best I can think of is using a separate battery/power source to the rear drive motor that applies as much V as I want. I believe this will require relays, so I picked up a 5V reed relay and a 5V SPDT relay at my local radio shack.

The problem:
As I've searched/researched online and after much thinking, I've come to the conclusion that using a relay or relays in my circuit with a separate power source will work, but I will lose the ability to make the car go in R (or F, depending - only one direction will be possible).

I found this thread
(http://forum.allaboutcircuits.com/threads/reversing-polarity-with-spdt-relays.30679/)
which was helpful for me in understanding how to use 2 SPDT relays to reverse polarity, but I still can't figure out how to achieve F/R direction without one direction being "always on" in a setup like this.

Thanks!

 

Does this make sense to you?

 

If you look at the board you'll see four transistors in one place may be eight the four to turn it all you got to do is cut the track that feed them power and use your new power source

 

I've briefly applied up to 7.5V to the motor directly and it cranks wonderfully (for how long is unknown).

I would guess not for long . At start-up and when stalled the motor windings will be dissipating nearly 3 times their normal (i.e. when run from 4.5V) power and could overheat.

 

$10 RC car that runs on 3 AA batteries. There are 2 motors - one in front to make it turn L/R, one in back to make it go F/R. There is no graduation in these directions, they are full on or full off at the remote. I want to make the car go faster by increasing voltage to the rear motor. The batteries feed a "motherboard" which outputs 4.5-5V to the rear motor.
Thanks!

Are the motor DC brushed or BLDC? Is the 'Mother board' a controller of any type?
Max.

 

Found this the other day, which i think is #12's solution: http://electronics.stackexchange.com/questions/86437/increasing-voltage-to-motors

Going to try it out this weekend, but the logic makes sense.

Max - Not sure if the motor is brushed or not.

be80be - I don't think it's feasible to try to figure out the transistors and interrupt the power feed there. That's beyond my abilities.

 

That link you posted doesn't make sense to me. The transistors are not arranged correctly.

The principle of my circuit is that the mother board delivers either one polarity or another from only 2 terminals. The diodes route the mother board signal to one relay at a time. Caution: The diodes will eat up about 0.7 volts of your mother board voltage. If you are running 4.5 Volts and lose 0.7 in the diodes, you have 3.8 volts left, so a 3.3 volt relay coil might be required. This seems likely to me because (3) AAA batteries generally won't produce 5 volts.