Slowly stop small dc motor

Thread Starter

Matt D.

Joined Nov 10, 2013
65
Hello I have a dc geared motor, about 6v and 500ma, and I want there to be a small delay in stopping it.

When the motor is on it goes through a SPST normally closed switch. When I open the switch to turn off the motor I would like about a 1-2second delay before the motor cuts off completely.

Could a large capacitor across the switch provide the necessary current to drive the motor for the brief amount of time? If so how large of a cap would it take?

I tried doing this with a 470uf capacitor I had laying around and it didn't seem to do anything.

Thanks for your help and (obviously) my electronic knowledge is limited.

Matt
 

ian field

Joined Oct 27, 2012
6,539
Hello I have a dc geared motor, about 6v and 500ma, and I want there to be a small delay in stopping it.

When the motor is on it goes through a SPST normally closed switch. When I open the switch to turn off the motor I would like about a 1-2second delay before the motor cuts off completely.

Could a large capacitor across the switch provide the necessary current to drive the motor for the brief amount of time? If so how large of a cap would it take?

I tried doing this with a 470uf capacitor I had laying around and it didn't seem to do anything.

Thanks for your help and (obviously) my electronic knowledge is limited.

Matt
Across the switch probably won't do much - except sputter the contacts each time you close the switch and short the charge in the capacitor.

The capacitor might be more effective across the motor, but you'll still need a lot to keep the motor going more than a fraction of a second.

If your 6V is well regulated - you can get a bunch of 2200uF 6.3V electrolytics from a scrap PC motherboard.

With a big bank of capacitors; the switch has to handle the instantaneous charging current - total current is limited by source impedance and leads etc, so not quite as bad as dumping the capacitor directly into the switch contacts. But you should keep mindful of the switch on surge.
 

Brownout

Joined Jan 10, 2012
2,390
It would be much better to drive the motor with an active device, say a transistor. The you can use a much smaller capacitor to drive the transistor. Since the transistor amplifies current, a much smaller capactor can be used, as it will drain more slowly.
 

KL7AJ

Joined Nov 4, 2008
2,225
Across the switch probably won't do much - except sputter the contacts each time you close the switch and short the charge in the capacitor.

The capacitor might be more effective across the motor, but you'll still need a lot to keep the motor going more than a fraction of a second.

If your 6V is well regulated - you can get a bunch of 2200uF 6.3V electrolytics from a scrap PC motherboard.

With a big bank of capacitors; the switch has to handle the instantaneous charging current - total current is limited by source impedance and leads etc, so not quite as bad as dumping the capacitor directly into the switch contacts. But you should keep mindful of the switch on surge.
If you put a large inductor in series in addition to the shunt capacitance, this will prevent some of the relay sputtering (unless the inductor happens to resonate with the capacitor!) A protection diode across the inductor will fix this.
 

MaxHeadRoom

Joined Jul 18, 2013
19,753
If you want to retain full voltage for the 1-2 sec, then a 555 could be set up for a delay off timer.
A mosfet or relay could control the motor.
Max.
 

MikeML

Joined Oct 2, 2009
5,444
I'm slowly starting and slowly de-accelerating a DC gear motor using Pulse-Width-Modulation generated by a $4 Arduino mini....
 

Thread Starter

Matt D.

Joined Nov 10, 2013
65
Thanks guys for the help. I'm trying to keep it as simple and cheap as possible. And don't need to keep the full voltage. A lower voltage would be good as it would slow down the motor and give a delay.

Thanks for the circuit Colin. I didn't think of this until now but since I'm using batteries do you think this circuit would use much quiescent current?
 

ian field

Joined Oct 27, 2012
6,539
Here is the circuit you need:
View attachment 97573
The TS has 6V to play with - easily enough for a logic level MOSFET (a regular one might work, but not quite make its headline RDSon).

The MOSFETS on old PC motherboards are usually at least 50A and somewhere close to logic level. The voltage rating is usually 30V, and sometimes as low as 20V - so it'd have to be protected from motor back emf.
 

BR-549

Joined Sep 22, 2013
4,938
When you hit the stop switch........do you want the motor to wait 2 sec. before stopping?

OR when you hit the stop switch....do you want the motor to take 2 sec. to go from running speed to a full stop?
 

Thread Starter

Matt D.

Joined Nov 10, 2013
65
BR-549 I think either would be fine. They both kind of seem the same to me.
It doesn't have to be too precise - about 1 to 2 seconds.
 

Colin55

Joined Aug 27, 2015
519
"Then the 1-2 sec 555 timer would do it. It would keep full voltage on the motor for a period.
Max."

Didn't you read the original requirement ???????
 

GopherT

Joined Nov 23, 2012
8,012
BR-549 I think either would be fine. They both kind of seem the same to me.
It doesn't have to be too precise - about 1 to 2 seconds.
The chip is a 555 timer. It is wired as a monostable (one-shot) timer. The resistor and capacitor to the left of chip set delay, currently set to 1.1 seconds.

image.jpg
 

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Colin55

Joined Aug 27, 2015
519
"It would be far more constructive to offer your viable alternative suggestion rather than abrasive comments!:rolleyes:
Max."

I have already provided a solution.
If you look above you will see he is using batteries and a 555 circuit does not drop to zero current.
A 555 is an absurd suggestion.
This problem has already been solved. Get on with something else.
 
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