Back EMF Diode (Freewheeling diode) selection for 24Vdc, 9ohm, relay

Thread Starter

mishra87

Joined Jan 17, 2016
851
Hi All,

How do i choose Back EMF Diode for below rating
I am driving relay with 24Vdc,9ohm coil.
How do i calculate the diode rating.
What all parameters needed to taken into consideration.

Thanks !!
 

AlbertHall

Joined Jun 4, 2014
10,395
Does this relay really need 2.7A coil current?

The maximum voltage across the diode is the voltage supplied to the coil
The maximum current through the diode is the same as the coil current.
 

Thread Starter

mishra87

Joined Jan 17, 2016
851
Does this relay really need 2.7A coil current?

The maximum voltage across the diode is the voltage supplied to the coil
The maximum current through the diode is the same as the coil current.
Yes, 2.7A
Is there any calculation available for that ?
what about diode power rating ?
 

Thread Starter

mishra87

Joined Jan 17, 2016
851
Calculation for what?
Diodes don't really have a power rating just maximum reverse voltage and maximum forward current.
Calculation for freewheeling diode selection?
What about energy stored in relay coil
E=1/2LI^2
P=E/t
t=5L/R

Same power has to dissipate with diode as well.
As far as think.
 

AlbertHall

Joined Jun 4, 2014
10,395
Calculation for freewheeling diode selection?
What about energy stored in relay coil
E=1/2LI^2
P=E/t
t=5L/R

Same power has to dissipate with diode as well.
As far as think.
While the coil is activated the voltage across the diode is the same as the coil voltage.
When the current to the coil is interupted the inductance of the coil causes the current to keep flowing through the diode. At this point the diode current is the same as the coil current. Then the current will decrease as the energy is disipated mostly in the coil resistance. The current in the diode will drop some voltage and hence some power in the diode. As long as the coil current is less than the maximum forward current of the diode then all is well.
 

ericgibbs

Joined Jan 29, 2010
10,806
hi mishra,
You could model the Relay coil in LTS, then measure the diode current.
The peak current thru the diode should be same as the current thru the coil at switch Off.
E
 
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