Transistor motor control question

The basics: The existing controller could have one terminal of the motor to ground, or to 6 volts or to somewhere else. That is important to know. The controller output could just be a voltage that can be adjusted between 06 volts, or could be a switching type that produces a train of pulses whose width determines motor speed, or something else. That is important to know also. It may easily be that the controller itself could work off 12 volts with little or no modification. It could be a very simple design or a very complex one, with internal feedback loops that rely on the motor being present to work properly. Or something in between.
There is such a range of possibilities - posting you just a link or two is probably not going to be doing you much of a favour. If you post more information about the controller and the motor, you may get more focused advice.
So anything you can find out about the controller and post?
These include:
1) Identifying the type and value of the components used - especially for things like capacitors, which should have a voltage marked on them as well as a capacitance value.
2) Tracing the circuit board may be viable if there are few components.
3) Close up photographs of the circuit board may help an expert.
4) Looking at what the output does at various settings, with an 'scope.
5) What the motor is and what you are using the motor for.
So, the more information you can post, putting things like photos and diagrams on a web page and posting links to them, the more likely you are to get focused advice. Which could range from, "it should be fine on 12v - you don't need to do a thing" to, "forget it - the modifications/ new circuitry would simply be too complicated".

+-------||---+---[FwRelay]---+ (Forward Relay) | | +---|------+ | ControllerB ----------------------------+
+12 ---+------------------+ | | o o All Relay points \ Fw1 | Ba1 shown are N/O; o o shown with Forward | | relay energized +---[A]Motor[B]---+ | | o o | Ba2 / Fw2 o o | | Gnd ---+-----------------+
Conceptually for transistors, see below.
+12 ---+------+-----------------+-----+ | | | | e/ ----- ----- \e 1---| Q1 / \ / \ Q2 |---2 \ --- --- / | | | | +------+---[A]Motor[B]---+-----+ | | | | / ----- ----- \ 3---| Q3 / \ / \ Q4 |---4 e\ --- --- /e | | | | Gnd ---+------+-----------------+-----+
ControllerA ---+---[Diode>]---[R]--- To 1 | +---[]---[R]--- To 4 | +---[

Would need to see the schematics. There wouldn't happen to be a DPDT relay on that board, would there?

Thanks Ed I have a circuit that uses all NPNs. The bottom 2 are in emitter follower and the top 2 common collector.
If the rails are 12v and ground on the bridge and the input is 6v will it still work? (for s1 and s2) I don't suspect a problem with the emitter followers but if the "ground" on the 6v ends up being .7v it might trigger the wrong common collector. I suppose I can bias that one away.
What I am trying to do is use a 6v RC module to drive a 12 v motor.

Nope. The transistors on the top will have no B-E current when their emiiters are at ~5.4 volts, so they won't put 12 volts on the motor. In addition, the 2.2K base resistors limit the base current to ~2.7 mA on the lower transistors, way too low. Even at 12 volts with about 5.4 mA, current to the motor is anemic. (The best hfe shown on the datasheet is 50; 50*5.4 = 272 mA) That may be fine for a tiny motor, but that circuit would be better designed with power mosfets. Turn them on with 12 volts at their gates and the current to the motor is limited by their rds, which is low. Taking .2 ohms as representative (and you can get lower), the curent would be limited to 30 amps.
You need to design based on what your motor draws. Relays will work with any motor. Bipolars can be used with low current motors. You want to drive the bipolar hard into saturation. Mosfets can go with either low current or high current motors.
Ed
> I don't suspect a problem with the emitter followers but if the > "ground" on the 6v ends up being .7v it might trigger the wrong common > collector. I suppose I can bias that one away. > > What I am trying to do is use a 6v RC module to drive a 12 v motor.

I was afraid of that. I guess relay it is. I can replace S1 &S2 with reed relays I guess.
I am starting to wonder if the original unit would run at 12v. I can't read enough off the chips to figure out what they are.

There's a voltage drop/space penalty for bipolar transistors and a current limitation penalty for reed relays. Those penalties don't apply to mosfets or regular relays. regular relays suffer a space penalty, but some come in a DIP footprint and may be sufficient for you. Much depends on the current your motor needs.
Reeds aren't usually good for high current (which you may need) or double pole (which you do need). In context, high current would be > 500 mA
For low current, you could use RLY-625 from Allelctronics with a current limiting resistor in the coil circuit. $1.50, fits in a DIP foot print, operates on 5V and contacts are rated at 2 amps. RLY-642 is larger than a DIP (still fairly small) but will handle 5 amps is a 6V relay. I'd keep the current to 1/2 the contact rating.
If drawing a lot of current from the 6V source is an issue, use mosfets. Here's a sample circuit plus decsription: (watch the line wrap)
I would modify that circuit to put a fuse between each N-channel mosfet and ground. That will protect them if there is an error where both mosfets on one side turn on. Make the fuse a slow-blow at a few amps over motor current and a few amps under the mosfet max current rating. For example, an 8 amp fuse would be fine for say 5 amp motor current and the mosfets below. If the motor current is under say 6 amps, an STP12PF06 12 amp To-220 should be fine for the P-channels, and a 16 amp IRF640 for the N-channels, but you can use just about any power mosfet of the proper polarity and current rating.
Your input, using mosfets, would be as shown at the web page, but you would need to add 2 diodes: A----|

Hi,
Just to add somethings may be its can help a little bit. A transistor is used not only to gain currents but it also can act as ON/OFF switchs based on currents base (if not make mistaken). you may get supply from collector leg direct to motor as assuming forward and anothers collector legs (pair) as assuming reverse. Before you do so, please check and measure the voltage form each legs (pairs) after you varied the supply to pair transistor.
Tks
magic