Help with DC Motor Back-EMF w/o Diode?

Thread Starter

WhenYouNeedHelp

Joined Feb 20, 2021
6
Hey all. I was doing some research for a project and came across the issue of back-EMF and, not being super experienced with electronics, it has me scared of frying my Arduino. Almost every solution I come across makes use of a diode but unfortunately, they were not provided with our course kit.

Here is the link to the motor I am using. Not mentioned on that site is the fact that there is a single 104 Capacitor bridging the terminals of the motor. Now one site I read says that for low voltage motors (I wouldn't be pushing this past 5V worst-case scenario), a capacitor should be sufficient for dealing with the back-EMF but then other sites say otherwise so I am frankly confused.

If the capacitor won't cut it, can I make use of any of the following components in order to operate the motor safely: LEDs, 1k Resistors, NPN "Low Power" Transistors (2N 3904), NPN Power Transistor (N5 192G)?

I will mention that I do not need to vary the speed/torque at all, I just simply need to turn the motor on and then off without any load attached to it.

Thank you in advance!
 

Alec_t

Joined Sep 17, 2013
12,064
Welcome to AAC!
You could use either of the two junctions of the NPN power transistor, reverse-biased, as a BEMF suppressor across the motor.
Drive the motor via a power transistor; not directly with the Arduino.
 

MaxHeadRoom

Joined Jul 18, 2013
23,092
I will mention that I do not need to vary the speed/torque at all, I just simply need to turn the motor on and then off without any load attached to it.
Thank you in advance!
A DC motor that is turned off and cycles to a stop generates a voltage in the same polarity as the supply. ;)
As it does while powered.
 

dendad

Joined Feb 20, 2016
3,835
And you will find adding a reverse diode makes the motor run smoother, particularly at low PWM, as the back EMF will help keep the motor running.
Do use a FET or transistor. Arduinos cannot drive much from their port pins.
 
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Thread Starter

WhenYouNeedHelp

Joined Feb 20, 2021
6
And you will find adding a reverse diode makes the motor run smoother, particularly at low PWM, as the back EMF will help keep the motor running.
Do use a FET or transistor. Arduinos cannot drive much from their port pins.
Unfortunately, I don't have a diode available to me as part of my specified project components.
 

Thread Starter

WhenYouNeedHelp

Joined Feb 20, 2021
6
So the better looking image is from the Arduino site but couldn’t we just do it like in my drawn image using just the transistor? I’m just trying to understand the right method of attaching components.

My experience with electronics has mostly been limited to the lecture setting unfortunately so I’m learning haha.

51F6C3EF-C604-48B9-9C3C-32FEC411C0B1.png0C703698-C170-41A0-B7F4-16AD300BA38D.jpeg
 

MisterBill2

Joined Jan 23, 2018
8,740
In the drawing of post #8 there is no return connection from the 5 volts negative to either the arduino or the motor, thus no current can flow..An actual functional circuit requires a complete current path, supply and return connection. That is a very elementary part of circuit theory.
 

djsfantasi

Joined Apr 11, 2010
7,689
And an NPN transistor works best by switching the ground connection on the motor.

And you’ll need a diode to protect the Arduino
C3742848-744C-45BF-8383-5D72B6271550.jpeg
 

MisterBill2

Joined Jan 23, 2018
8,740
That diagram can work. And one thing about motor circuits: Many of them do not have a diode across the motor but rather a disc capacitor, I am not sure of the value, probably in the range between 0.1 mFD and 0.01 mFD.
 

Thread Starter

WhenYouNeedHelp

Joined Feb 20, 2021
6
That diagram can work. And one thing about motor circuits: Many of them do not have a diode across the motor but rather a disc capacitor, I am not sure of the value, probably in the range between 0.1 mFD and 0.01 mFD.
Yes my motor uses a capacitor. My whole goal here was to protect the Arduino against B-EMF without using a physical diode (instead using a facsimile made from the parts I originally listed). Would the capacitor be sufficient to protect the Arduino or is more needed?
 

MisterBill2

Joined Jan 23, 2018
8,740
The capacitor is used to reduce the noise, which includes the spikes of back EMF. There is another concern that may be more of a problem, if any external source mechanically spins the motor, which will act as a generator and produce a voltage that may be either polarity, depending on which way the motor is spun. A diode will provide protection against THAT hazard.
 

Thread Starter

WhenYouNeedHelp

Joined Feb 20, 2021
6
The capacitor is used to reduce the noise, which includes the spikes of back EMF. There is another concern that may be more of a problem, if any external source mechanically spins the motor, which will act as a generator and produce a voltage that may be either polarity, depending on which way the motor is spun. A diode will provide protection against THAT hazard.
Yes that’s exactly my concern. Is there any chance you could provide a diagram of how you would replicate a diode with an NPN transistor? I found an example online as seen below, but I need to include another transistor connected to 5V that uses a digital pin as the gate switch. I’m just having trouble wrapping my head around how I should implement that.

41223F7B-507C-4379-AEDB-28D94455C24E.jpeg
 

Tonyr1084

Joined Sep 24, 2015
5,969
A PN junction is a diode. Within a transistor you have at least one PN junction. NPN has a PN in it. PNP has a PN in it. In the case using an NPN transistor as a diode you use the base as the Anode and the emitter as the Cathode. If using a PNP transistor then the emitter is the Anode and the base is the Cathode. You can ignore the collector in either case. Just think of current flow and the arrow acting like the diode directing the current.
 

MisterBill2

Joined Jan 23, 2018
8,740
I am quite aware of a transistor being able to function as a diode, BUT for this application, a diode with adequate ratings should be simpler and probably less expensive, and certainly easier to get the correct connections than using a transistor. Diode voltage and current specs are simpler to understand, and that may be a benefit to those who are not extensively experienced in the field..
 
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