RC car

[Dr_Doggy]

k, I got this 40$ RC car that originally ran off of 4AA batteries, I ripped out the control board and hooked in a PIC and H-bridge driver, but now when i go forward the car struggles to get up to speed,

I am thinking a capacitor in parallel with my 6v supply(to the H-bridge) to give it an initial boost, but I am bad with capacitor math, ,

would a cap work? what size cap could boost it sufficiently?

 

[DemonicSandwich]

Did you decouple the control hardware from the rest of the hardware? (IE, the motor?)

If not then the PIC may be reseting from the sag of the motor's stall current.

 

[Dr_Doggy]

Idk how to do that, so i isolated the supplies, the pic is run off 3AA, while the motor is run from 4 separate AA batteries(6V),

 

[DemonicSandwich]

And with the configuration the motor still struggles?
If so, how much current does the motor draw at stall speed? ......at running speed?
How is the H-bridge constructed?

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Anyway, to run the pic from the same 4 AAs, you would place a diode in series with the PIC's Vdd pin and the positive side of the supply. Then place an electrolytic cap across the PIC's supply pins.

Then have the positive rail of the H-bridge connect directly to the + side of the supply(before the diode).

The reason I brought it up is that the motor will draw high currents at stall speed. Currents high enough to drag the supply voltage down below what the PIC needs, causing the PIC to brown out, then reset and reinitialize when the voltage comes back up.

With the setup I mentioned above, the capacitor would supply the PIC with power during the voltage sag.

 

[Dr_Doggy]

ah yes, i did have that problem , that's y i isolated it,, but my bridge is an L298 with shotky(L6210) diode protection, i think it runs at a little over an amp, IDK though what the current the AA batteries spit out

it doesn't stall with the isolated configuration, but it works hard to speed up, that's y I figure a little boost could help it

 

[DemonicSandwich]

Ya, the PIC was probably resetting every time that H-bridge fired off until the car was getting fast enough that current draw settled down.

 

[Dr_Doggy]

yes before it was resetting and i would need to hit the start button again.....now its just a little slow when speeding up,,, so would a capacitor help me to get that extra bit of energy into the motor for the first half second?

 

[DemonicSandwich]

To supply a large current spike ≥ 1A? A very impractically large capacitor maybe.

I think I found your problem though. Reading through the datasheet I found the likely issue to be the saturation voltage of the drive transistors.

At 1Amp, the saturation voltage of the Source-side transistors is ~0.95-1.35v. (min-typical)
At 1Amp, the saturation voltage of the Sink-side transistors is ~0.85-1.2v.(min-typical)

Another words, with a 6 voltage supply, that motor will get a maximum of ~3.5-4v of it.
The motor is basically running as if it was using dead cells.

I'd say just construct your own H-bridge with a few transistors.

 

[Dr_Doggy]

....or another battery!? THNX, that was alot of info.... I just had that IC due to its max ratings...so i could upgrade the chasis, maybe.

btw is a 0.1uF cap good enough to couple it? or should I go closer to 10u?

 

[DemonicSandwich]

There is an equation to determine the capacitor needed. But if the only things it will power are the PIC and a few logic inputs(a few mA at most), then that 10µF cap will easily do the job.

As for the motor...you could either construct your own H-bridge, get a different one altogether or increase the supply voltage.
Just keep in mind your PIC can take ~5.5v.

You could actually use a DPDT relay and a single power transistor to power the motor. The relay to control direction and the power transistor to control the PWM. That would mostly rid the voltage drop.

 

[Dr_Doggy]

K, thnx again!!!!