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 !!

 

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.

 

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 ?

 

Yes, 2.7A
Is there any calculation available for that ?
what about diode power rating ?

Calculation for what?
Diodes don't really have a power rating just maximum reverse voltage and maximum forward current.

 

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.

 

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.

 

What type of 24V relay has a 9 ohm coil?
Most have a resistance at least 10 times that.

 

What type of 24V relay has a 9 ohm coil?
Most have a resistance at least 10 times that.

Maybe it's switching the national grid?

 

By the resistance, sounds like a AC relay, unless a VERY large contactor!
Max.

 

By the resistance, sounds like a AC relay, unless a VERY large contactor!
Max.

In which case a diode is definitely contraindicated.

 

What type of 24V relay has a 9 ohm coil?
Most have a resistance at least 10 times that.

Load is water pressure boost up pump have the same resistance.

Regards,

 

So what is it, a pump (type?) or a relay??
Max.

 

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

 

And while at it, are you switching AC or DC into it?

 

Load is water pressure boost up pump have the same resistance.

What does that have to do with the relay coil resistance?

 

What does that have to do with the relay coil resistance?

Thanks all for your reply.

Pump type is AC but relay coil is 24DC @ 4.3K

Regards,

 

1n4007 or any of the 1n400x versions.
Max.