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

#### mishra87

Joined Jan 17, 2016
1,010
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
12,293
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.

#### mishra87

Joined Jan 17, 2016
1,010
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 ?

#### AlbertHall

Joined Jun 4, 2014
12,293
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.

#### mishra87

Joined Jan 17, 2016
1,010
Calculation for what?
Diodes don't really have a power rating just maximum reverse voltage and maximum forward current.
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
12,293
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.

#### crutschow

Joined Mar 14, 2008
33,325
What type of 24V relay has a 9 ohm coil?
Most have a resistance at least 10 times that.

#### AlbertHall

Joined Jun 4, 2014
12,293
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?

Joined Jul 18, 2013
28,020
By the resistance, sounds like a AC relay, unless a VERY large contactor!
Max.

#### AlbertHall

Joined Jun 4, 2014
12,293
By the resistance, sounds like a AC relay, unless a VERY large contactor!
Max.
In which case a diode is definitely contraindicated.

#### mishra87

Joined Jan 17, 2016
1,010
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,

Last edited:

Joined Jul 18, 2013
28,020
So what is it, a pump (type?) or a relay??
Max.

#### ericgibbs

Joined Jan 29, 2010
18,213
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

#### DickCappels

Joined Aug 21, 2008
10,103
And while at it, are you switching AC or DC into it?

#### crutschow

Joined Mar 14, 2008
33,325
Load is water pressure boost up pump have the same resistance.
What does that have to do with the relay coil resistance?

#### mishra87

Joined Jan 17, 2016
1,010
What does that have to do with the relay coil resistance?

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

Regards,