Freewheel diode placement question

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bconnoll113

Joined May 4, 2020
4
Freewheel diode placement question - I would prefer to locate my freewheel diode on my pcb instead of soldering it across the relay tabs, is this ok to do? pcb is just inches away (12v 140mA relay coil, diode is rated 1kV and av. fwd current of 2.5A)
 

crutschow

Joined Mar 14, 2008
24,997
It is usually only recommended if the relay/solenoid is in very close proximity.
I beg to differ.
Actually putting the diode at the coil switch rather than on the coil can be better since, in that location, it will suppress any transients from the wire inductance, (although that inductance is small, it can still cause switching spikes).
 
The wire to the solenoid from the PCB is an antenna. Placing the diode at the solenoid makes the transmitting "antenna" almost non-existant. The magnitude is close to the solenoid operating voltage. The first purpose is to product the solid state driver.
 

crutschow

Joined Mar 14, 2008
24,997
Placing the diode at the solenoid makes the transmitting "antenna" almost non-existant.
Not necessarily.
If the diode is at the switch, then the current in the wire drops slowly when the switch opens.
If the diode is at the solenoid, then the wire current stops abruptly (with possible ringing).
It would seem that the second scenario might be worst than the first, since it's the di/dt in a wire that radiates EMI.
 
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