Protection diode for polarity reversing circuit

[kwame]

Hi
i have bought a general purpose Omron MK3P-I 28V DC plug in relay for a project.The relay is going reverse polarity to to reverse a 12V DC,5.5A permanent magnet linear actuator hourly.
When inductive loads like linear actuators get switched off,there is high current generated by which is about 8 times its rated current;which can 'roast' the relay.There is ver little information online about how to protection diodes effectively in applications involving polarity reversal.
I have read that wiring one diode in both wires between the relay and the actuator can take care of the situation.Or not?
Any suggestions?

 

[ronv]

Diode Clamps

The current through the diodes will not be any higher than the current through the motor was. Without the diodes the voltage will be very high. A circuit like below should help keep your relays happy.

 

[kwame]

So what happens is that ,the actuator is going to be in the middle right?

 

[ronv]

Yep, where it says motor.

 

[kwame]

Ronv
can you give a brief explanation of how the set up is going to look like?The 12V represents the relay right?What of the arrow?Kindly give a brief explanation of the circuit thanks.
For exactitude,the load is a 12V DC permanent magnet actuator 5.5A not a mere motor.

 

[kwame]

Ronv
can you give a brief explanation of how the set up is going to look like?The 12V represents the relay right?What of the arrow?Kindly give a brief explanation of the circuit thanks.
For exactitude,the load is a 12V DC permanent magnet actuator 5.5A not a mere motor.

 

[carbonzit]

The way I've always seen it done is far simpler: just a single diode (in reverse bias position) across the relay coil. Why don't you suggest that, instead of this 4-diode scheme? It only needs to absorb whatever reverse "EMF" the coil generates.

 

[ronv]

The +12 is the 12 volts at your power supply. The arrow represents ground. Also at your power supply. The motor represents your linear motor. Your relays from your other threads attach to the two motor leads.

 

[kwame]

As it stands right now,it appears there will be no protection for the relay.WoNT this expose the relays to EMF surges and even damage?

 

[Jaguarjoe]

The 4 diodes around the motor will protect the relay contacts from arcing.
Like Carbonzit said, you need another diode across the coil of the relay to protect whatever contact or transistor controls the relay coil from damage.

Buy plenty of spare relays.

 

[kwame]

So this should bring the total number of diodes to... . And most importantly,what must be the rating of the diodes .As already stated ,the actuator takes close to 6 Amp (but a little less than 6Amp).What must the rating of the diodes be?Can you specify a particular diode since i a m not very good when it comes to diodes.

 

[Triode]

He's just saying to put the diodes pointing into the high and out of the low voltage from the lines connected to the inductive load. Under normal circumstances the lines to the actuator are at a higher voltage than the low end so no current will flow through the diodes, likewise the voltage at the actuator is normally lower than the voltage of the high end of the supply, so no current will flow from the actuator lines to the high end. The diodes allow current to flow into the high voltage or out of the low voltage supply, this means if a spike jumps above your intended supply it simply shorts to high and if it spikes below the low supply, it simply shorts to low. It's like a water tank with a float valve near the top that drains water and one near the bottom that adds water, the water level ends up staying between the two.

Sorry if I over explained I'm just trying to make it clear.

 

[kwame]

And What must be the rating of the diodes/ what type of diodes is best suited for this purpose?

 

[kwame]

I have elsewhere that it is possible to control/curb the high EMF with a resistor+capacitor.When the motor is switched off,the capacitor stores the the EMF on its negative and discharges it through the resistor without doing harm to the relay contacts.The modus operandi of those junction diodes is above my head.
The capacitor + resistor has been used successfully to control arcing on this relay board successfully :https://www.electro-tech-online.com/custompdfs/2011/07/relay_faq.pdf

 

[ronv]

To do either method accurately the inductance of the actuator is needed. Do you have that spec.? If not overkill is in order.

Diodes:

MBR350
SB350

 

[kwame]

Life expectancy

Hi
I want to know how long a relay would last when it is used for reversing a 12VDC 6A linear actuator without reverse polarity protection against EMF from the linear actuator.I am using a plug in Omron MK3P general purpose relay.I have read online that most standard relays can work for about 100,000 cycles be getting ruined.

 

[Reloadron]

I have elsewhere that it is possible to control/curb the high EMF with a resistor+capacitor.When the motor is switched off,the capacitor stores the the EMF on its negative and discharges it through the resistor without doing harm to the relay contacts.The modus operandi of those junction diodes is above my head.
The capacitor + resistor has been used successfully to control arcing on this relay board successfully :https://www.electro-tech-online.com/custompdfs/2011/07/relay_faq-1.pdf

The device you are talking about is a RC Snubber Netowrk. Read about them here includintg how they are selected. Additionally a Google of RC Snubber will bring up a dozen more links with formulas. You are going to have to start working out some of this project on your own.

Ron