Hi everybody, I'm designing a fairly simple circuit to switch on and off a 12V load using my arduino.
The circuit has to be multi-purpose, meaning that it must be able to switch both inductive and purely resistive loads.
The load will be connected at the screw terminals J1.
I was thinking of placing a flyback diode right before the screw terminals, directly on the pcb. Is it ok, or should it be always at the load's terminals? It is possible that the load will be approx. 1mt far from the pbc. In case I want to hookup a DC motor, can I add in parallel to the diode also a capacitor?
No that is too far away. Ack EMF voltages are big and sending it a long way before being clamped will increase the radio frequency interference that you generate. You might get away with it but you might not. Not a good idea.
Grumpy_Mike:
No that is too far away. Ack EMF voltages are big and sending it a long way before being clamped will increase the radio frequency interference that you generate. You might get away with it but you might not. Not a good idea.
Is that a problem of EMF only? Would still the diode work protecting the mosfet?
I can solder a small ceramic capacitor on the leads of a DC motor load. However I would like not to add many things at the load's location, and keeping the flyback diode on the PCB would help a lot.
Grumpy_Mike:
No that is too far away. Ack EMF voltages are big and sending it a long way before being clamped will increase the radio frequency interference that you generate. You might get away with it but you might not. Not a good idea.
If the long wires to the device have significant inductance then you would want to have the flyback diode close to the switching element, so that it allows the current in the inductors to continue to flow.
Now the wires won't have a significant inductance but thinking about the extreme example helps determine the "best" location.
Also - why do you think Back EMF voltage should be "big" in this particular case? It will be diode forward drop + voltage drop caused by resistance of the wires which is hopefully low.
The instantaneous voltage on the cable is determined by the characterist impedance of the cable as that is all the inductor can see until the signal is reflected back from the diode. Since most cables have impedances in the 50 to 300 ohm range you can estimate the size of the initial voltage pulse given the current through the inductor.
