transistor help

Hi I have added a transistor to the output of a servo controller board to allow me to power a bigger motor from the same board. But i only get control over the motor in one direction not in both,

I have read in the past about audio amplifiers, and that they use two or more transistors in order to produce the negative part of the sound wave as one cant do this,

so am i having the same problem ? is this why i can mak ethe motor go backwards like the original? is there a clever way to fix it with two transistorS?

cheers for any help

…show us the schematics…

is there a clever way to fix it with two transistors

You want to build a kind of H - Bridge

As it is I have proporcional control over the motor in one direction , the otehr direction deosnt work ,
I just hook the old servo board onto the RC Rx and control it with theTx.

the plan (other than learnign stuff and passing the time of day) is to make a speed controller for a brushed motor,
i know the mosfet I have used is massivly over specced for the job, but I have a few of them lying around, and can get a few more, they are in an old battery charger we use at work , and we have a few fried ones,

Looking at an h bridge, it uses four transistors, is that neccesary? i would have imagined that two would be good enough,

servo schem.png

Take a look at this, using an old servoboard to drive a H-bridge:

http://www.rcuniverse.com/forum/m_9236698/tm.htm

READING

in the meantime , im willing to be that this photo is the original h bridge,

servo h bridge.png

You need 4 to be able to isolate the current flow path.
In one direction, the current flows from upper left to motor + to motor - to gnd at lower right.
In the other direction, the current flows from upper right to motor - to motor + to gnd at lower left.

If you’re only switching direction occasionally, then you could get by with a DPDT switch and one signal to enable/disable the power transistor.

Will post a drawing in a minute …

Thanks for the drawing, I have been thinking about the h bridge during the afternoon and it seems that there are basically two circuits, and that when you turn one supplying transistor on, you also turn on the corresponding grounding transistor in order to not have the two powering ones short out .

Having thought a bit about the plan, (to convert the servo) it seems clear that i need two outputs in order to control the two halves of the h bridge, I guess that what i need to do, is look at the existing h bridge, trace the tracks back to the microprocessor on the servo board and build my h bridge from there, leaving the original one disconnected,

other than that i dont see a way of acheiving the goal, and i assume that if i just were to remove the fets on the board, and upgrade them that the diodes etc and board traces would burn out under the new, higher load,

do all four fets need to be the same? i soppose i need two npn and two pnp, as long as all four can handle the nominal amperage needed i can have the pnp ones at a higher rating without problems no ?

Ok
i have thought a bit more, and obviously the current two output cables on the servo board take turns in being the pos or neg depending on the pwm signal received from the RX,

so assuming that they change like that, could i do something like in the attached pic?

Are there part numbers on the current transistors? You say FETs, you have BJTs drawn.

they are currently smt, two say y1 and two say y2, i tried looking up the IC tht controls it and couldnt find a datasheet,

its: kc2462 1008b

that last b might be an 8 , with google, kc2462 brings an image of the chip , but its all in chino

I found out that the chip used in one of mine Noname 9g servo is the Mitsubishi M51660L.
A little googling around shows that this is more or less a standard among small servos.

It only use two external transistors as shown in the schematic below:

Datasheet:

so maybe it uses two of those pins as a switchable ground? because there are four pins for the transistors , one to trigger each one and one on its output, after the motor,