How to Build a Soft Start RC Ramp Circuit for 12V 1.2A DC Motor to Prevent Wheel Slip

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#1 21660416 20 Dec 2011 17:46

Brian Buick Brian Buick

Anonymous

Post #1
21660416 20 Dec 2011 17:46

I have a small 1.2A 12v dc motor that is geared and has a small 3inch diameter wheel that buts up on the inside of a rockler bearing my problem is when I connect 12v to the motor, it starts and spins too quickly, causing the wheel to slip.

What I need is a soft start ramp circuit, so that it takes its time to start and spins slowly to start. I'd recon say 100ms to 200ms to obtain 12v and so drive the motor slowly up to full speed and avoid the slip.

My guess is a resistor and capacitor. so resistor in line with the positive lead to the battery ? and then a capacitor , with capacitor across the motor terminals ?

So this would mean the 12 would connect but be limited through the resistor and capacitor so taking 100ms to charge the capacitor ? Is this right ?

Whats the resistor and capacitor size and with the resistor in series and 1am what's wattage of resistor going to be and will it not be wasteful in heat dissipation once capacitor is fully charged.

What will happen when I remove the 12 v presumably the capacitor will then discharge through the motor pretty quickly - do i then need a diode to assist discharge

Struggling to come up with an RC ramp slow start circuit

many thanks

Brian
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#2 21660417 20 Dec 2011 18:02

Steve Lawson Steve Lawson

Anonymous

Post #2
21660417 20 Dec 2011 18:02

I would use PWM.

BTW: What's a "1am"? Does anybody proofread their posts!?!
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#3 21660418 21 Dec 2011 01:45

Cody Miller Cody Miller

Anonymous

Post #3
21660418 21 Dec 2011 01:45

The problem with the resistor in series with the positive lead is that it will cause a voltage drop across the resistor. Even if the value was as low as 1 ohm it will cause a 1.2V drop. I like Steve's suggestion drive the motor with a PWM circuit.
#4 21660419 21 Dec 2011 13:21

David Deleonardo David Deleonardo

Anonymous

Post #4
21660419 21 Dec 2011 13:21

Here is a simple way to do what you want to do: Connect a FET between the motor's "-" terminal and COM (or ground). Then use the 12 input signal to turn on the gate of the FET via an RC net such that the R is in series with the 12 volt input and the gate. (The cap is connecte from Gate to COM.) To allow the gate circuit to discharge on power down, you can either connect a resistor from the gate to COM or connect a diode from the gate to the 12 volt input such that the Anode of the diode is connected to the gate. This will force the gate to track the input to within a voltage drop on power down.

Thoughts?
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#5 21660420 21 Dec 2011 14:16

Steve Lawson Steve Lawson

Anonymous

Post #5
21660420 21 Dec 2011 14:16

Regarding David's suggestion, realize that as the current ramps up, the FET is going to be in it's "active region", thus, at one point it will be dissipating just as much power as the motor. If that motor draws 1.2A at steady-state, then it's probably going to be drawing a lot more current as it ramps up, so for that 100 to 200ms it's going to have to handle a great deal of power, which means heat--heat that needs to be dealt with either with a heat sink or with a FET designed to take a great deal of power (or both).

There are a lot of variables, so you might need to experiment with various different FETs. Perhaps use a storage scope to get a read of the power over time dissipated by the FET as the motor ramps up, and use that to zero in on a particular FET. You also might need to consider environmental factors, such as ambient temperature, and how it will affect the ramp and the ability to dissipate the heat generated.
#6 21660421 21 Dec 2011 15:34

David Deleonardo David Deleonardo

Anonymous

Post #6
21660421 21 Dec 2011 15:34

To get you started on a solution, I ran a quick LTspice simulation with the attached circuit. I used the trick of adding “Miller Capacitance” between the drain and gate of the FET to slow the rise time. I assumed a DC resistance in the motor (L1) of about 7 ohms which may be a bit high, but seemed about right given that data you provided.

As you can see from the attached plots, the current rise time it a bit longer than you asked for. You can correct these by changing the values of the various R’s and C’s. I have also attached the LTspice source file in case you want to play with that (great) tool.

Lastly, I added a “power meter” to a subsequent schematic that allowed me to plot the power in the FET as a function of time. For the values I have shown in the attached plots, you could expect to see an average power of about 4 watts in the FET for a period of about 1 second. However, if you speed things up, this number will come down proportionally. Still, it is good to be cautious and plan for the worst, so add a small heat sink to your TO-220 FET and you should be good to go.
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#7 21660422 21 Dec 2011 18:23

Brian Buick Brian Buick

Anonymous

Post #7
21660422 21 Dec 2011 18:23

Steve, Cody David

Thanks so much for your help

I love the FET solution, so I'll look at that with a heatsink

It's for an r2d2 robot I've made, to drive the head. Its powered by a 12v motor which is activated for short bursts that moves the head .

The dome sits on a rockler bearing - like a lazy Susan , and so a half second motor activation moves the dome through 90 degrees

It moves ok sometimes but occasionally the motor slips and it squeals as the wheel on the motor moves too fast

I have the motor powered via a relay and an arduino I programmed to drive the relay
#8 21660423 21 Dec 2011 18:59

Steve Lawson Steve Lawson

Anonymous

Post #8
21660423 21 Dec 2011 18:59

Sounds like fun! Any chance you can post a photo?
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Topic summary

A 12V, 1.2A geared DC motor with a small 3-inch wheel experiences wheel slip due to rapid startup when powered directly at 12V. The goal is to implement a soft start ramp circuit that gradually increases voltage over 100-200ms to prevent slipping. A simple RC (resistor-capacitor) approach in series with the motor is ineffective due to voltage drop and power dissipation in the resistor. Instead, a PWM (Pulse Width Modulation) control or a MOSFET-based soft start circuit is recommended. The MOSFET method involves placing a FET between the motor's negative terminal and ground, controlling the gate voltage with an RC network to ramp the gate voltage slowly, thus controlling motor startup current. This approach requires careful selection of the FET and heat dissipation measures, as the FET will dissipate significant power during ramp-up. LTspice simulations demonstrate the feasibility of this method, showing current ramp times and power dissipation in the FET. The motor is used in a custom R2D2 robot head, driven by an Arduino-controlled relay, where short bursts of motor activation rotate the dome on a Rockler bearing. The soft start circuit aims to reduce motor slip and noise during these movements.
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How to Build a Soft Start RC Ramp Circuit for 12V 1.2A DC Motor to Prevent Wheel Slip

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