Simple DC motor ramp up circuit

Thread Starter

doby

Joined Aug 17, 2011
49
I'm working on a project where I need a DC motor (12V, 350mA) to build speed from stationary to full speed within say 10 seconds and then remain at full speed until the power is cut. I've done a little research into it and have seen PWM's recommended however having a quick look at prebuilt PWM circuits they all have potentiometers that vary the speed manually.

I however just want an automatic pre defined ramp up that happens everytime 12V is applied, being able to alter the speed thereafter is not required.

I would like to also do something similar with about 60 leds (2V 20mA each, 10 sets of 6) and have them slowly fade in in about 10 seconds and then remain at full brightness.

Any help appreciated :)
 

MikeML

Joined Oct 2, 2009
5,444
I recently did this using an Arduino. I move a 16 foot drive-through swing gate with a 12Vdc powered linear-actuator with gearmotor. Because of the huge inertia of the swing gate, I need a relatively long ramp-up time during acceleration, and then a somewhat shorter ramp down to stop it. Reversing requires ramp down, dead time, followed by reversing, and then ramp up...

My Arduino gate controller does a lot more, but I would be happy to share the ramp up, ramp down code, along with the H-bridge hardware to implement the PWM speed control and reversing...
 

Thread Starter

doby

Joined Aug 17, 2011
49
I recently did this using an Arduino. I move a 16 foot drive-through swing gate with a 12Vdc powered linear-actuator with gearmotor. Because of the huge inertia of the swing gate, I need a relatively long ramp-up time during acceleration, and then a somewhat shorter ramp down to stop it. Reversing requires ramp down, dead time, followed by reversing, and then ramp up...

My Arduino gate controller does a lot more, but I would be happy to share the ramp up, ramp down code, along with the H-bridge hardware to implement the PWM speed control and reversing...
I'm aware of Arduino and it's advantages but I'd rather not get into it if there's a simpler way.

The ramp up in my project is purely aesthetic rather than needing to solve a problem, maybe 'ramp up' is the wrong term? I basically want the motor to go from 0 RPM to 200RPM (it's geared speed) in about 10 seconds.

If arduino is indeed the simplest way to achieve this then I'll go with it, I just wondered if it could be done with a simple circuit rather than having to get into programming a microcontroller.
 

AnalogKid

Joined Aug 1, 2013
9,296
12 V and 350 mA are numbers almost any power transistor can handle, so the question is how to drive it. Two thoughts:

1. Boring PWM chip drives the motor (or drives power transistors that drive the motor). Input to PWM is a simple ramp circuit, either a linear ramp (1 transistor or 1 opamp) or an exponential ramp (1-R and 1-C).

2. Drive the motor through an ***exciting*** low-dropout linear regulator and drive that with a ramp. With one opamp you could ramp the motor current rather than the voltage for a more linear response. With a PNP pass transistor or P-channel MOSFET it would saturate at the end of the 10 second ramp and not need a heatsink. No switching, no clock, only one pass device.

ak
 
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Thread Starter

doby

Joined Aug 17, 2011
49
12 V and 350 mA are numbers almost any power transistor can handle, so the question is hot to drive it. Two thoughts:

1. Boring PWM chip drives the motor (or drives power transistors that drive the motor). Input to PWM is a simple ramp circuit, either a linear ramp (1 transistor or 1 opamp) or an exponential ramp (1-R and 1-C).

2. Drive the motor through an ***exciting*** low-dropout linear regulator and drive that with a ramp. With one opamp you could ramp the motor current rather than the voltage for a more linear response. With a PNP pass transistor or P-channel MOSFET it would saturate at the end of the 10 second ramp and not need a heatsink. No switching, no clock, only one pass device.

ak
Sounds promising.

I'm not going to pretend I understand everything you wrote but I'll certainly look into it. Thanks
 

MikeML

Joined Oct 2, 2009
5,444
Here is a try at it. Requires a dual rail-to-rail 12V opamp. It starts out with low-duty cycle PWM, and transitions toward full DC applied to the motor after a few seconds. The green trace shows the puesdo-ramp that controls it V(rc). The red trace is the pwm signal which controls the gate of the NFET. The violet expression is the "filtered" average voltage across the motor vs time (the integrator R8 C4 is not actually part of the real circuit; it is there only for simulation to show the filtered effective motor voltage). Disconnecting the circuit from the 12V should reset it for the next start-up.

With R1C1 as shown, the pwm rate is about 500Hz. With R2C2 as shown, the ramp-up is over after about 10s. D1 is a snubber for the motor inductance. M1 could be any of about a 100 NFETs whose gate will stand a 12V swing.

75e.gif
 

Alec_t

Joined Sep 17, 2013
12,160
BTW, what is the preferred method for capturing LTspice into a JPEG or GIF?
To capture a schematic I use the LTS 'Print Preview' function, then the computer's 'Print Screen' function to copy the screen to the clipboard, then paste from the clipboard into a graphics program and crop the image as required.
 

AnalogKid

Joined Aug 1, 2013
9,296
That's pretty much what I am doing, but I dump the image into a Word doc, select the image, and save as. I was hoping for a secret command within LTS that went straight to JPEG or GIF. Oh well...

ak
 

MikeML

Joined Oct 2, 2009
5,444
That's pretty much what I am doing, but I dump the image into a Word doc, select the image, and save as. I was hoping for a secret command within LTS that went straight to JPEG or GIF. Oh well...

ak
Make the suggestion to the keepers of the LTSpice bits...

I use just the LTSpice window. I use "PrtScn" to copy the image to clipboard. I then use IRFanView to paste the clipboard to, and use the "Crop Selection" to discard everything outside the range of interest. I save as .gif, because I found that .gif is lossless, while .jpg sometimes makes the image look muddy...
 

Thread Starter

doby

Joined Aug 17, 2011
49
Thanks for all your inputs but I've actually found a circuit on here (designed by Bill Marsden) that should work nicely. Originally designed to soft start (fade on) LEDs when powered on and then fade out after power is cut. I'm going to use a relay to switch on the circuit which will be powered from a separate relay PIR circuit (prebuilt).

As I want the motor and LED's to have the same ramp effect it should be able to do both given the current draw the circuit was designed for, plus it has the added bonus of being able to define exactly how long the ramp time is.
 

mcgyvr

Joined Oct 15, 2009
5,394
That's pretty much what I am doing, but I dump the image into a Word doc, select the image, and save as. I was hoping for a secret command within LTS that went straight to JPEG or GIF. Oh well...

ak
Don't you guys use the "Snipping Tool" in windows?.. Its great and allows you to capture just what you want
 
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