Electronics > Repair

Toy car forward and reverse speed difference issue

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murabat:
My son's toy car goes to full speed in reverse direction but moves very slowly in the forward direction. At first I thought that it was a motor resistance issue but when I checked the voltages on the motors, the reverse direction was getting 12.5V and the forward direction was only getting 7.6V.

Main controller board picture is attached below.

The gate of the forward high side mosfet is not being driven fully. Reverse direction high mosfet is getting about 22.5V at the gate which gives a gate to source difference of about 10v but the forward direction mosfet is only getting 14.5V which makes Vgs to be around 2V which is not enough to turn it on fully. Battery voltage being 12.5V.

I thought that the driver IC had some issue. I swapped the ICs, no change. I swapped the charged pump capacitors, no change. Then I started checking inputs of both drivers and found that the PWM signal coming from the microcontroller is around 3V 21KHz in the reverse direction but only 1.7V 21KHz in the forward direction. LDO running the whole circuit is 3V.

At this point I am a little confused why the microcontroller is doing this? It is still fully functional taking inputs and outputs of other functions in the car. There is no speed control in this car. Just a rocker switch with forward and reverse. 

So my idea of posting here is to know if I have missed something in checking the mosfets and their driver. It seems to me that I have to either get a new circuit board or just wire the reverse output of the microcontroller to the forward driver input with some logic in between.

Xenon:
The simplest way to make it go forward full speed is to swap the motor + and - connectors and the wires to the forward and reverse buttons.

coromonadalix:
normally you need more control while reversing than being in forward mode,   the motor should spin the same  in forward or reverse mode,  but a circuit can add a limitor for reverse mode ??

maybe poking the controls signals, you coud see a signal level difference ??  or maybe some resistors values ??

an  ''H'' bridge should be simmetrical

abdulbadii:
First must get MC ID, specs, description which is null in question now

murabat:

--- Quote from: Xenon on December 04, 2021, 04:52:19 pm ---The simplest way to make it go forward full speed is to swap the motor + and - connectors and the wires to the forward and reverse buttons.

--- End quote ---
The motors are controlled by the microcontroller with forward/reverse inputs pulled high through resistor. When both forward/reverse button and the accelerator switch is pressed, the signal goes low and microcontroller sends the appropriate PWM signal to the mosfet driver. So changing wires on the rocker switch won't help solve the slow speed issue. It just makes forward button into reverse and reverse button into forward.


--- Quote from: coromonadalix on December 04, 2021, 05:29:36 pm ---normally you need more control while reversing than being in forward mode,   the motor should spin the same  in forward or reverse mode,  but a circuit can add a limitor for reverse mode ??

maybe poking the controls signals, you coud see a signal level difference ??  or maybe some resistors values ??

an  ''H'' bridge should be simmetrical

--- End quote ---
As I said in the first post that the problem seems to me that the high side mosfet in the forward direction is not being turned on fully causing a very high voltage drop. The mosfet driver signal is almost half of what it should be. Microcontroller is controlling the mosfet driver directly. There is no resistor in between.

--- Quote from: abdulbadii on December 04, 2021, 08:53:55 pm ---First must get MC ID, specs, description which is null in question now

--- End quote ---
The microcontroller markings are ground off. No way to know what it is. The four mosfets are N-channel 120N03. The mosfet driver is HY2105 which after probing seems to have a very similar pinout to NCP5104

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