How to determine the orientation of diode to prevent damage by inductive spikes? - Page 1
Forum section: electronics / beginners

author: fishandchips
author_rank: Regular Contributor
timestamp: October 10, 2020, 09:45:04 pm
content: Hello, I read that to avoid damage of circuit caused by inductive spikes from DC motor, one could put a diode in parallel to the motor terminals. How do I know which side to connect the end of the diode that has a bar on it?

author: pwlps
author_rank: Frequent Contributor
timestamp: October 10, 2020, 10:22:05 pm
content: Definitely you can only use the diode in the reverse-bias direction, otherwise it will short the motor and most probably blow. The bar indicates the cathode and thus should be connected to the positive side.Google for "freewheeling diode" for further explanations.

author: TimFox
author_rank: Super Contributor
timestamp: October 10, 2020, 11:20:58 pm
content: The mnemonic for the diode orientation is that the (positive) current flows in the direction of the "arrow" towards the "bar". As stated above, you don't want current to flow through the diode with normal voltage across the motor, only in the reverse direction. Do you apply both polarities of voltage to the motor to reverse the direction of rotation? If so, a simple diode won't work and you might use a pair of Zener diodes instead to keep the voltage "finite", but not conduct when "normal" voltage is present in either polarity.

author: David Hess
author_rank: Super Contributor
timestamp: October 10, 2020, 11:54:15 pm
content: Put the diode in the direction that does not short out the motor when it is powered.

author: fishandchips
author_rank: Regular Contributor
timestamp: October 11, 2020, 03:19:51 am
content: I bought Schottky diode. So if I put the Schottky diode in the wrong direction by mistake, the motor will most likely be damaged? If I do not add a diode, the external circuit connected to the motor may be damaged or it may not?

author: Paul Rose
author_rank: Regular Contributor
timestamp: October 11, 2020, 02:05:51 pm
content: Quote from: fishandchips on October 11, 2020, 03:19:51 amNo. The diode will be damaged

author: pwlps
author_rank: Frequent Contributor
timestamp: October 11, 2020, 02:18:30 pm
content: It is easy : if you see a smoke coming out of the diode it means it was put in the wrong direction. You will need at least two diodes for this method to work though.

author: Zero999
author_rank: Super Contributor
timestamp: October 11, 2020, 02:21:35 pm
content: Quote from: Paul Rose on October 11, 2020, 02:05:51 pmThen, when it blows open circuit, the switching transistor will die.Quote from: pwlps on October 11, 2020, 02:18:30 pmWhy do you think two diodes are required?

author: pwlps
author_rank: Frequent Contributor
timestamp: October 11, 2020, 02:28:19 pm
content: Quote from: Zero999 on October 11, 2020, 02:21:35 pmTo replace the burned one, of courseEdit: I thought it was clear it's a joke, but I now precise it before it becomes a bad joke, of course the trial and error method can do more damage than just burning the diode.

author: Paul Rose
author_rank: Regular Contributor
timestamp: October 11, 2020, 03:18:03 pm
content: Quote from: Zero999 on October 11, 2020, 02:21:35 pmQuite right. I just meant that themotoris the least likely thing to suffer damage.Attached schematic of the correct direction. As mentioned in one of the earlier replies, this does not work if you are switching directions (i.e. using both polarities).

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 11, 2020, 10:17:37 pm
content: Forget all those rules of thumb.Also forget about voltages, as far as inductors are concerned. They do not mean much.The thing to remember is that current through an inductor is a "continuous" function. It has no discontinuities.In practice it means that if you open a switch in an attempt to stop a current through an inductor, the current will just keep on flowing (for a short while) after the switch is opened.It is your job to make some kind of path for that current to flow in circles without damaging parts.

author: wizard69
author_rank: Super Contributor
timestamp: October 16, 2020, 06:02:40 am
content: Quote from: fishandchips on October 10, 2020, 09:45:04 pmThis isn't as simple as everybody is making it sound. The problem is DC motors are also generators, if there is any possibility of the motor being mechanically driven while off, you can potentially generate enough current to damage the diode across the motor. This can dramatically outweigh any potential inductive behavior that have been considered. even more importantly the sign of that generated DC voltage will change with direction so if somebody is manually cranking on that motor shaft the motor will free wheel in one direction and brake in another. So you really need to careful consider what the motor is doing when it comes to the application of its controller.For a minute here I was under the impression that you might be concerned about the coil of a relay that then controls the motor. If that is the case then yes a diode makes sense across a relay coil. A relay though is not a motor and a motor is not an inductor (at least not in the sense that is the big problem here). That isn't to say that there is no inductance to consider just that turning off a motor turns a lot of mechanical energy into electrical energy for a considerable period of time (relative to inductive kick back from shutting off a coil). A diode might be the correct solution but you really need to consider all factors that apply to your usage.

author: Zero999
author_rank: Super Contributor
timestamp: October 16, 2020, 09:31:24 am
content: Quote from: wizard69 on October 16, 2020, 06:02:40 amThat's true: motors, act as generators.A few additional points.When the motor is turned off, the magnetic feild in the coils has collapsed and it's freewheeling due to inertia, the voltage generated will be lower than the supply voltage and the same sign, so it won't cause any damage.The freewheel diode will only be subject to reverse current, if the motor's shaft is rotated in the oppositite direction to it normally spins.If it's possible the motor will be driven in reverse, or at faster than the operational speed, then the diode, or driving circuitaty can be damaged.The easiest solution to the above issues is mechanical, rather than electrical. A clutch could be used to disengauge the motor, when it drops below a certain speed. This could be magnetic clutch, powered by the circuit, or centrifugal, so it engauges drive, then the motor shaft exceeds a certain speed.