Slow automatic increasing variable resistor

Thread Starter

P6RAF

Joined Jan 4, 2017
9
I've looked through some previous posts but nothing seems to cover what I'm after.
I have modified my son ride on car from 6v to 12v. The motors can take the increase (I know of several others that have done this and still running fine several years later) However the cheap plastic gearbox often fail due to the extra power. I'm trying to find some kind of automatic variable resistor that will slowly increase the power to 12V to stop the instant surge the gearbox goes through. In essence I want to regulate the power to begin at a low voltage and increase it to 12v over 2 or 3 seconds.
Any suggestions ?
 

dannyf

Joined Sep 13, 2015
2,197
Any suggestions ?
The answer depends on how you want to ask the question.

You are unlikely to find a variable resistor that meets your particular needs.

However, there are. Countless ways to increase power gradually to your motor.
 

Thread Starter

P6RAF

Joined Jan 4, 2017
9
The answer depends on how you want to ask the question.

You are unlikely to find a variable resistor that meets your particular needs.

However, there are. Countless ways to increase power gradually to your motor.
Are any of these ways automatic ie when he pushes the pedal (simple on/off) it will gradually increase the power without needing to do anything else ?
 

OBW0549

Joined Mar 2, 2015
3,566
Try googling on the phrase, "simple dc motor soft start circuit"; a bunch of information comes up, including a number of circuit diagrams that might be useful.
 

Thread Starter

P6RAF

Joined Jan 4, 2017
9
Try googling on the phrase, "simple dc motor soft start circuit"; a bunch of information comes up, including a number of circuit diagrams that might be useful.
Great, I've been guessing lots of phrases to search but those seems to be the golden word. Many, many thanks.
 

Thread Starter

P6RAF

Joined Jan 4, 2017
9
Sounds like a PWM circuit to slowly increase the voltage would work.
But we need to know the maximum motor current.
I am a real novice, so please bear with me.. From what I just read a PWM circuit is fixed based on the duty cycle. This would need to be adjusted manually to increase the power to the full 12v.
 

AnalogKid

Joined Aug 1, 2013
10,987
Modifying a PWM circuit to replace an external control signal with an external ramp signal is not difficult. Once you've settled on a PWM driver that can handle the power, making it ramp up (and down? is relatively easy.

ak
 

#12

Joined Nov 30, 2010
18,224
This would need to be adjusted manually to increase the power to the full 12v.
Yes. It's called the throttle or the gas pedal. What you want is a slower response to the throttle in the "go" function. That's easy with a PWM.

I think I remember doing one of these about a year ago. Maybe I'll find the old drawing...
A resistor to a slow charging capacitor with a diode bypass for the "off" direction.
 

hp1729

Joined Nov 23, 2015
2,304
I've looked through some previous posts but nothing seems to cover what I'm after.
I have modified my son ride on car from 6v to 12v. The motors can take the increase (I know of several others that have done this and still running fine several years later) However the cheap plastic gearbox often fail due to the extra power. I'm trying to find some kind of automatic variable resistor that will slowly increase the power to 12V to stop the instant surge the gearbox goes through. In essence I want to regulate the power to begin at a low voltage and increase it to 12v over 2 or 3 seconds.
Any suggestions ?
Try a handicap scooter store that repairs scooters. I buy my scoter parts from Monster Scooter Store on-line. I suppose they might carry motor controllers.
 

crutschow

Joined Mar 14, 2008
34,284
Below is the LTspice simulation of a PWM circuit that will go from full off to full on, as determined by the MOD control voltage.
It requires only one active device package, an LM324 quad package op amp.
U1 acts as a comparator with hysteresis to generate a square wave, which is then integrated by U2 to generate a triangle wave.
U3 also acts as a comparator, with its output going high when the MOD voltage is greater than the triangle wave voltage to generate the PWM output pulses.
The PWM pulses can drive a power MOSFET to control the motor.

The MOD voltage can be supplied by a voltage or the pot shown.
For the test, the voltage is provided V_Test, which overrides the pot voltage.
For a slow start circuit, the MOD voltage could be generated by an RC ramp.

upload_2017-1-4_22-40-50.png
 
Last edited:

JUNELER

Joined Jul 13, 2015
183
Hi,
I had a simple and very cheap controls this is just a suggestion but not yet test only my ideas.

I saw a pedal type control on one of the sawing machine. The pedal is composed of a small rounded

type of carbon . This is in series of one line in the ac input and goes to motor.

By merely pressing the pedal light or hard the resistance of the carbon decrease while the motor
go on high speed. I guest in ac it works fine. Try it in dc supply.
 

crutschow

Joined Mar 14, 2008
34,284
I saw a pedal type control on one of the sawing machine. The pedal is composed of a small rounded
type of carbon .
Those are designed to carry the relatively small current of a sewing machine motor.
They won't handle the likely much larger current of the toy car motor.
 

Thread Starter

P6RAF

Joined Jan 4, 2017
9

Thread Starter

P6RAF

Joined Jan 4, 2017
9
Below is the LTspice simulation of a PWM circuit that will go from full off to full on, as determined by the MOD control voltage.
It requires only one active device package, an LM324 quad package op amp.
U1 acts as a comparator with hysteresis to generate a square wave, which is then integrated by U2 to generate a triangle wave.
U3 also acts as a comparator, with its output going high when the MOD voltage is greater than the triangle wave voltage to generate the PWM output pulses.
The PWM pulses can drive a power MOSFET to control the motor.

The MOD voltage can be supplied by a voltage or the pot shown.
For the test, the voltage is provided V_Test, which overrides the pot voltage.
For a slow start circuit, the MOD voltage could be generated by an RC ramp.

View attachment 118081
Thanks for such a detailed reply.
This looks excactly what I need, not sure I have the skills to build it though. Do you think the
SoftStart Reversible Motor Speed Control PWM Controller PWM DC 20A 12V I found one Ebay is the same kind of thing. I'll have to get a meter of the motor and find out how many amps the motors draw. Might have to have 1 for each motor.

Many thanks again for your time and advice and everyone else that's offered me suggestions. I really appreciate it
 
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