bi-directional soft start for childs electric car

[spec]

I've just read your post #19. The potentiometer together with a PWM speed controller from Ebay would probably be the easiest for the OP to implement. It may also be worth adding a circuit so full power could not be applied for about 0.5 second
Les.

Ah yes! I think you have it- practical as usual Les.

spec

 

[MaxHeadRoom78]

There are 6v PWM controllers on ebay for <$5.00.
You could buy a pair, one for each wheel and fit a dual-ganged potentiometer that controls each.
With a bit of simple engineering the foot pedal could control the pots to simulate an accelerator pedal.
With these controllers you need a reversing relay.
Max.

 

[KeepItSimpleStupid]

These https://www.jameco.com/z/K8004-Vell...h-Modulator-Kit-Control-DC-Motors_120539.html have a soft-start built into them. I used one for a shutter.

How does to toy steer? My guess, conventional steering, not motors at different speeds?

 

[MaxHeadRoom78]

The Vellman kit shows 8v minimum.
Max.

 

[KeepItSimpleStupid]

I didn't check. Bad me. 6V likely makes things difficult.

 

[MaxHeadRoom78]

6v would sort of imply using logic level gate Mosfets for the output.
Max.

 

[KeepItSimpleStupid]

That PWM chip has a 8-35 V range of operation and it generates a 5.1 V reference.

 

[crutschow]

Here's a LM339/393 PWM circuit to generate a turn-on ramp.
It uses a logic-level MOSFET that will work at 6V.
The ramp time is determined by the R7C2 time constant.
For the values shown, it goes from full off to full on in about 0.7s.
R7 or C2 can be changed as needed with a ramp time ≈0.7*R7*C2.
The middle plot shows the startup pulses, and the bottom plot shows the ramp end pulses.

But it may be more complex than the OP would want to build.

 

[camerart]

That would certainly do the job.

Presumably you would use one of the two approaches in post #17 above to generate the PWM signal.

spec

Hi S,
Correct! Switch ON topleft and PWM bottom right = One direction. Switch ON top right and PWM bottom left = other direction. Switch OFF before changing direction.

A bit over the top but, it should work.
C.

 

[crutschow]

After looking at the Velleman PWM circuit, it's the sg3525 PWM generator used that has a minimum operating voltage of 8V.
The MOSFET used is a buk9535, which is a logic-level device specified down to a Vgs of 4.5V, so that's not the limiting factor.
Bummer.

 

[jarlel]

Hi, I am looking for something similar for a child electric car. It has two motors (24 V DC) and a simple on/off pedal, so it "jumps" when it starts.
I would like to add some circuit that makes the start softer without using any potentiometers for the "gas pedal".

Did any of you "solve" this?

Cheers

 

[rjenkinsgb]

I'd experiment with something like this, replacing the speed pot with a resistor and capacitor so it ramps up to full speed over a second or two each time it is started.

Depending how it works, you may need to use a relay on the start pedal to switch the start circuit on that and short the cap via a low value resistor to "reset" the soft start each time the pedal is released.

**broken link removed**

 

[jarlel]

Thanks, I will look into that! So, when pressing the start pedal it should engage a relay that opens the short for the capacitor and powers/enables the regulator/output, right (a relay with two switches, one NO and one NC)? The resistor in series with the capacitor? And two systems/regulators, one for each motor (the motors need opposite polarity)?

 

[rjenkinsgb]

And two systems/regulators, one for each motor (the motors need opposite polarity)?

It depends how much current they draw - if less that 20A total, you should just be able to wire them in parallel but wires swapped on one.

Everything else sounds OK, though I've never used one of those modules so it's part guesswork.

Just for clarity, the resistor would be from put clockwise terminal wire to the wiper wire, then the cap from the wiper wire to the anticlockwise / minimum terminal.

Try something like 10K and 47uF to start with, and eg. a 47 or 100 ohm resistor in series with the discharge contact to prevent high current spikes on that.

 

[jarlel]

Thanks rjenkinsgb. Not sure if I understand completely the wiring scheme. Can you please explain in more detail?
I just want 100% all the time, so will it be OK to remove the potentiometer, or do I need to have it in place?

Thanks.

 

[rjenkinsgb]

Remove the pot and replace it with the capacitor and resistor.

I used the the pot terminal identifiers relating to what connections the wires originally went to, to try and clarify what connected where.

 

[jarlel]

Ok, thanks. I made a connection diagram - does this look OK? Going from the original on the right side to the modified version on the left side?

 

[rjenkinsgb]

Schematic wise, exactly right.
(Though the pot V+ may well be a rather lower voltage, eg. 5V or 10V).

The relay contact would be normally closed. Common symbols for contacts, for reference:

https://www.omron.com.au/service_support/FAQ/FAQ03206/index.asp

For info, you have the pictorial pot connections reversed - the wiper sweeps all the way around the "back" (in that view) as it's turned clockwise, so the near terminal is the max speed voltage connection to it and the far one is the ground / 0V.

 

[jarlel]

Hi again, short update:

It took some time, tested in lab and seems to work great with the PWM regulator. Almost everything is mounted in the toy car now, tested with direct connection to the motors. Just need to wire up both +/- correctly from the regulator output to the forward/reverse switch before I can test it fully under load Thanks again to all the great suggestions

 

[jarlel]

Hi again! As it is now the regulator goes from 0-100 percent. What would be a good way to introduce a switch that makes it go from 0-50 percent? Still with "soft start".

Thanks in advance and take care during these challenging times.