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Topic: [Solved] Where should I put the flyback diode? (Read 688 times) previous topic - next topic


Nov 06, 2020, 05:00 pm Last Edit: Nov 06, 2020, 05:01 pm by cdb101

in the middle left schematic, you say that the relay (for millisecond) will not have reached ground yet. During that small period, the current will flow through the diode and go back in the motor.
The bottom left diode will be passing current when turning off the forward motor state while the bottom rigth diode will do the same for backward motor state.

At what point the two top diode do something?

Thank you


At what point the two top diode do something?
That would be relevant if both relays were actuated together - which has the same effect as both released.


Karma thanks to all of you for the explanations. 

Since I don't plan to use both relay on high position, I will only use the to diode tied to ground. 
The final diagram for those of you who want to do it is the following: 


Typical flyback diodes would protect the transistor or mosfet that was switching the power. Relays are less sensitive to the flyback voltages but they could still use some protection.

When the relay switches from closed to open, there is current flowing through the inductance of the motor and the inductance will increase the voltage to try to keep the current constant. That will cause sparks across the opening relay contacts. Over time that will tend to burn the contacts. A diode will divert that current away from the contacts. The diodes to both ground and battery are useful during that short interval when the contacts are transitioning between high and low. 


More to the point without the diode there's a massive pulse of EMI from the sparking contacts, this often
plays havoc with nearby circuitry and AM radios. You never want this, whether or not the relay contacts
last a reasonable time (and generally they dont).  

A common issue that crops up in these forums is un-snubbed relay contacts freezing a nearby Arduino...
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]


Nov 10, 2020, 08:36 pm Last Edit: Nov 10, 2020, 08:37 pm by cdb101
I have one last question about this topic.

In a similar application but with only 1 relay that control a PWM which in turn control a pump. Should I add a flyback diode to protect the PWM from the pump inductor?

The PWM is of course the cheapest one we can buy on internet. 


You cannot PWM a relay, it will die soon.


I am assuming he's got a PWM source he wants to pass through a relay to run the motor forward or reverse at variable speeds.

The relays could benefit from flyback diodes but the PWM source will require those same flyback diodes if they are not already part of the PWM source circuit.


Sorry if it was not clear. 
In fact, the PWM is powered with 12 volts. When powered, it produce a PWM output that drive the motor. 
My relay only toggle on and off the 12 volts of the power input of the PWM. I set and forget the PWM rate with the potentiometer for example at half duty cycle. Then I turn the relay on which in turn transmit power to the PWM. This drive the motor at half-duty cycle or other preset rate. 

If a flyback diode is installed. I would fix it just between the + and - prong of the motor since there is no other place.

Thank you 


One must assume the PWM module is itself protected (with a diode - I can see at least one on the board) against the motor, so there will be no impulses passed through to to a relay that is switching its power input.

You would only have concerns if you were putting the reversing relays on its output to the motor.

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