Suitable device to suppress back emf

[Murugesh_89]

Hi,
I am using a 24v dc actuator/contactor which will load the ac motor.
I am driving this actuator through a MOSFET. Since the actuator contains the inductor coil, the mosfet should be kept safe from back emf.

My doubt is to suppress the back emf which of the following is best..
1. Connecting a free wheeling diode across the load
2. Connecting a MOV(VDR) across the load
3. Connecting both free wheeling diode and MOV.

Thanks,
Murugesh

 

[crutschow]

A diode will work. The only downside is that is slows down the release of the contacts somewhat which could increase the burning of the contacts if they are carrying high current.

A MOV would reduce the relay operate time at the expense of a higher transient voltage. The MOSFET must have a voltage rating at least 25% greater then the sum of the supply voltage and the MOV voltage rating.

You shouldn't need a diode with the MOV as long as the MOV is bidirectional (breakdown voltage the same in both directions).

 

[jit_singh_tara]

use RC ckt to reduce noise ....like 0.1uf,400V(ctr) and 4.7k 2watt etc.....

 

[Murugesh_89]

Thanks crutschow.

I am using the 24V as the supply voltage.
What should be the voltage rating of MOV i need to select?

 

[crutschow]

I am using the 24V as the supply voltage.
What should be the voltage rating of MOV i need to select?

It's somewhat arbitrary. The higher the voltage, the faster the relay operates. About 30-40V is probably a good number

 

[Murugesh_89]

ok..

i have given the one end of load(solenoid coil) to 24v and another end to drain of mosfet where as its source terminal is grounded and gate terminal is used to switch the mosfet.

where i need to connect the mov to avoid noise due to back emf?
1. across load ( between 24v and drain ) or
2. between drain and ground or
3. some where else.

 

[crutschow]

Either 1 or 2 will work.

Option 2 has the advantage of suppressing any transient voltage from the stray wiring inductance.

Another difference is that the maximum voltage across the transistor with option 1 equals the supply voltage plus the transient suppressor voltage. The maximum voltage across the transistor with option 2 equals the transient suppressor voltage only. But for equal turn-off times for the relay you would need to increase the transient suppressor voltage for option 2 by an amount equal to the supply voltage as compared to option 1. In that case the peak voltage across the transistor would be the same either way.

 

[Murugesh_89]

Since my supply voltage is 24V, i had searched mov with the working voltage(Vdc) = 30V. For this spec, i did not find a part whose clamping voltage is below 75V. I mean, for the operating voltage(Vdc) = 24V, the clamping voltage i found was 75V minimum.

Since mov perform its operation when the transient voltage is well above its clamping voltage, how it could handle when the spikes or surges comes in the range of 60V which is well above the supply voltage and below the clamping voltage.

Please correct me if i am wrong:...

 

[crutschow]

Put the 75V MOV across the transistor drain to source and pick a transistor with a voltage rating of at least 100V.

 

[FvM]

Since my supply voltage is 24V, i had searched mov with the working voltage(Vdc) = 30V. For this spec, i did not find a part whose clamping voltage is below 75V. I mean, for the operating voltage(Vdc) = 24V, the clamping voltage i found was 75V minimum.

For more precise clamping use zener/TVS diodes.

 

[Murugesh_89]

So will it be best to use TVS diode with the following spec?

Type = Unidirectional
Reverse standoff voltage = 30V
minimum breakdown voltage = 33.3V
maximum breakdown voltage = 38.3V
clamping voltage = 64.3V
Peak pulse current = 35A

I planned to place this between drain of mosfet and ground(Vss).

I am almost using 30 outputs. Off this 20 are relay coils and 10 are actuator coils. all these are triggered using mosfet in single 24v supply voltage. there may be chance of switching on/off 10 outputs simultaneously. i am too concerned about suppressing back emf using flywheel diode(parallel to load) which dissipate energy by conducting towards supply direction and which may lead to malfunction. hence i want to dissipate energy due to backemf through ground connection.

Please tell me whether i am thinking correct about the bold statement...