Connecting Circuits

Thread Starter

Incredible-Bots

Joined Sep 12, 2020
6
Hello !
I'm trying to make a circuit connection, I want to control a DC motor using two push button with reverse polarity.
When I press one button it runs in one direction and when another button is pressed then it must run in opposite direction.
Should I use a relay or something else.?
 

jpanhalt

Joined Jan 18, 2008
10,221
Welcome to AAC.

A relay will work. So will something else. If you don't have any ideas about something else, then use a relay.
 

DickCappels

Joined Aug 21, 2008
6,584
Not sure what you want. Do you want a push button to reverse the motor, two buttons -one for each direction. Do you mind making a circuit with transistor switches?
 

MisterBill2

Joined Jan 23, 2018
6,998
But I want to use a push button to run motor in two directions and need something which changes polarity like a DPDT relay
LIke the answer that you have been given TWICE, you can use two single pole two position push buttons. What I am describing is pushbuttons that make one contact when not pressed and a second, different contact when pressed. That is a classic circuit that has been used for MANY years.
For the circuit, the motor must be connected between the common terminals of the pushebuttons, and the negative supply source connected to the two normally closed contacts, with the positive source connected to both normally open contacts. This is the arrangement that was already posted in #2 and #4..
Using a single relay will not allow the motor to be off, only to be instantly reversed, which is a rather poor choice.

The complicated part of this arrangement may be finding pushbuttons that have both N.O. and N.C. contacts. Of course, in any case, the buttons must be adequate for the motor current.
 

jpanhalt

Joined Jan 18, 2008
10,221
Hello !
I want to control a DC motor using two push button with reverse polarity.
You have not described how the device will be turned on or off. How do you plan to do that?

Since it is a DC motor, it does not need to stop before reversing. That might be fine in some situations; however, if it is high speed, high power (>1/3 HP) or carrying a load, I suggest stopping before reversing. Many lathes run on DC motors including mine. I can't recall seeing one where there not not a stop position before changing directions.

Questions for you:
1) How will the device be turned on or off?
2) Do you want to reverse direction while the device is running? Why?
3) If not #2, do you want protection from reversing while running? Do you have an active stop as opposed to coasting to a stop?
 

MisterBill2

Joined Jan 23, 2018
6,998
You have not described how the device will be turned on or off. How do you plan to do that?

Since it is a DC motor, it does not need to stop before reversing. That might be fine in some situations; however, if it is high speed, high power (>1/3 HP) or carrying a load, I suggest stopping before reversing. Many lathes run on DC motors including mine. I can't recall seeing one where there not not a stop position before changing directions.

Questions for you:
1) How will the device be turned on or off?
2) Do you want to reverse direction while the device is running? Why?
3) If not #2, do you want protection from reversing while running? Do you have an active stop as opposed to coasting to a stop?
It is not ever, at any time, under any condition, a good choice to plug-reverse any motor except possibly a stepper motor. Not only is there a probability of mechanical damage, there is certainly the presence of electrical over-stress.
AND, by the TS specifying the buttons as they did, it is implied that both buttons released was the non-running condition.
 

jpanhalt

Joined Jan 18, 2008
10,221
@MisterBill2
I never said or suggested that instant reversing was a "good" idea. I said it could be done, and for lower power, damage may be small. My lathe is only 1/4 HP Baldor DC, for example, and it uses electrical braking.

As for what's implied and what's not, I think the TS is a better/more reliable source for that information than you. Perhaps, he didn't consider it?
 

MisterBill2

Joined Jan 23, 2018
6,998
All of my electric lawnmowers except the most recent one,came with electric braking, whereby switching off short-circuited the motor, stopping it almost instantly. That monetary extreme overload damaged the motors on the first three of them so that they were not worth repairing. On the next two I disconnected that function and they lasted much longer, until they simply wore out. Dynamic full braking on any motor is a very high stress action. The lathe motor may use a resistor to reduce the torque to a reasonable value, or it may be that it just has not filed yet.

And given that the TS asked about two buttons to run the motor in opposite directions that rather obviously implies that with no buttons pressed the motor would not be running. So that is a rational presumption.
 

Thread Starter

Incredible-Bots

Joined Sep 12, 2020
6
You have not described how the device will be turned on or off. How do you plan to do that?

Since it is a DC motor, it does not need to stop before reversing. That might be fine in some situations; however, if it is high speed, high power (>1/3 HP) or carrying a load, I suggest stopping before reversing. Many lathes run on DC motors including mine. I can't recall seeing one where there not not a stop position before changing directions.

Questions for you:
1) How will the device be turned on or off?
2) Do you want to reverse direction while the device is running? Why?
3) If not #2, do you want protection from reversing while running? Do you have an active stop as opposed to coasting to a stop?
Actually, What I want is that I have a DC Johnson Gear Motor of 60 RPM and I gave a 12v supply to it and what I want is using Two Push buttons to reverse polarity.
Let say, on pressing one of the button, it is running in Clockwise direction and then I stop it by releasing the push button which I pressed for running it and after a minute or two, by pressing another push button it should run in another direction that is Anti-Clockwise.
So for running it in another direction from previous, I have to change its polarity.
So for changing polarity, what can I do.?
 

MisterBill2

Joined Jan 23, 2018
6,998
The answer was provided in posts #2 and #4, and then in post #7 I provides a detailed description of exactly how the circuit would be wired. So what more are you asking for? I am not able to draw the circuit but that post describes the connections exactly. It does change the polarity of the voltage applied to the motor, depending on which button is pressed, and an added benefit is that if both buttons are pressed no short circuit of the power source is caused.
 

MaxHeadRoom

Joined Jul 18, 2013
21,403
Hello !
I'm trying to make a circuit connection, I want to control a DC motor using two push button with reverse polarity.
When I press one button it runs in one direction and when another button is pressed then it must run in opposite direction.
Should I use a relay or something else.?
If you are switching this motor manually, IOW visually wait until the motor is at stop position before reverse. you can most probably get away without having any built in zero speed connection.
This motor shows that it is 800ma off load, with a maximum current of 7.5a at stall.
At 18,000 rpm max, It is surprisingly high RPM for a DC brushed motor.
You could use a DPDT toggle switch with a centre off position, this just reverses the motor leads. Wait at centre before reverse, if the output shaft is operating in to a very high inertial load, this would be important.
https://www.instructables.com/id/HOW-TO%3a-Wire-a-DPDT-rocker-switch-for-reversing-po/
Max.
 

Tonyr1084

Joined Sep 24, 2015
5,039
If it's a DC motor that reverses when the polarity changes, you can use two SPDT switches in a bridge configuration.
This is what Crutschow is suggesting:
1599924883666.png
The only problem I see is that when either switch is connected to positive (or negative) and the other is opposite, if the motor is a permanent magnet type - it creates the dynamic braking others are warning about.

Another issue is that this is toggle switches. You're asking for push buttons. If you go with a relay DPDT you can get reversing, but the motor is constantly running, and the switch in direction is instantaneous. What I envision is two push buttons with an H-Bridge using MOSFETs. When you push one button you provide power to run the motor clockwise. When you let go the motor comes to a stop. Push the other button and the motor runs counter clockwise (anti-clockwise). There, too, I see a potential issue: Accidentally pushing both buttons simultaneously. That would produce a direct short and could damage the battery or the wiring. Or possibly the PB switches.

Would you consider using one DPDT switch and a momentary PB switch? The DPDT sets the direction and the PB runs the motor as long as you press it. But now you're looking at a larger PB switch, one that can handle the amount of power your motor will draw at startup. There's a whole lot more engineering that needs to go into this project before you start building.
 

MaxHeadRoom

Joined Jul 18, 2013
21,403
The only problem I see is that when either switch is connected to positive (or negative) and the other is opposite
That is why the one I show has a centre off, the max current on this motor is 7.5a at stall. 800ma off load.
With the toggle sw, there is no accidental double push.
Max.
 

Tonyr1084

Joined Sep 24, 2015
5,039
MaxHeadRoom Max; I know. The TS wants push buttons. I'm envisioning simple single pole contacts that are open until pushed and held. When let go the motor comes to a stop but not suddenly. A center off position would be another option, but again we're getting away from the required push buttons. Either an H bridge or two separate SPDT relays.
 

MaxHeadRoom

Joined Jul 18, 2013
21,403
It is possible that the OP did not consider the toggle method, it is just another option.
No relays involved!
Plus I also read it that he wants a PB to latch, which involves relays etc.
Plus he does not mention an off condition between them, which as his post implies a FWD-REV toggle. Which is not recommended.
Max.
 

MisterBill2

Joined Jan 23, 2018
6,998
Post #14 covers it, although with that sized motor it will need some heavier duty push buttons, which are available but cost several dollars each. I can solve the hard dynamic braking by adding two diodes and a dynamic braking load resistor, keeping the same basic circuit but adding three parts. Why make this much harder and more complex than it needs to be???
The diodes would be installed between each normally closed button switch and the positive supply , and then the braking resistor would be connected between the two normally closed PB terminals, in place of the jumper wire. The diodes will need to be ten amp devices rated at least 25 volts for a 12 volt motor. The resistor wattage rating will depend on the resistance value. My suggestion is 100 ohms and five watts.
 
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