Relays, diode protection?

[inerratum]

Im ordering a few relays to start making progress on my PLC reef controller and see that there are some that have built in diode protection, is this necessary for a reef project? It looks like it doubles the cost of the relays which really isnt a problem if I will see some benifit to them.

 

[smcdonn]

I take it since you are building your own reef controller you know what a diode does. Other than for certain applications where you don't need reverse electrical flow, I don't see where you would need one for a "REEF" application. Hope this helps a bit. Cheers

 

[stugray]

Typically diode protection on a relay is used to protect the driving circuitry from 'reverse EMF'.

When you apply current to a relay to make it 'contact' or 'transfer', there is a large Backwards Voltage that comes out of the relay when you release the drive current. If there is nowhere for this energy to go, it can damage the driving circuit.

Usually a diode is placed near the relay coil to dissipate the reverse EMF energy.

Stu

 

[smcdonn]

Good one stugray, I was trying to recall to my electronics courses but could only remember the basics of the diode. Cheers

 

[ronc98]

Spend the extra money. You need the diode to protect the driver. Keep in mind however some drivers have this built in already. I use a 1416 darlington driver and it has it built in already.

I would think the PLC drive circuit would have it built in as well

 

[BeanAnimal]

I use the ULN series drivers...

However I use a diode at each relay. The diode needs to be as close to the coil as possible.

I will dig up the app note I have on relay protection... but in a nutshell a diode will work for our purposes. The diode does however substantially slow the action of the contact closure and therefore causes more arcing at the contacts. Other means of protection can be used to compromise between protection and closure rates. These would include resistors, RC networks with oru without diodes.

...looking for that silly app note....

 

[easttn]

Is your PLC switching AC or DC?

 

[BeanAnimal]

Relay Coil Suppression

Other interesting app notes


These are not the exact ones I was looking for... but they will shed some light for those interested in the dynamics involved.

 

[inerratum]

Thanks for all the info.

easttn: The plc is switching 24V dc.

 

[greenstarman]

<a href=showthread.php?s=&postid=7896907#post7896907 target=_blank>Originally posted</a> by stugray
Typically diode protection on a relay is used to protect the driving circuitry from 'reverse EMF'.

When you apply current to a relay to make it 'contact' or 'transfer', there is a large Backwards Voltage that comes out of the relay when you release the drive current. If there is nowhere for this energy to go, it can damage the driving circuit.

Usually a diode is placed near the relay coil to dissipate the reverse EMF energy.

Stu

Why not an inductor? Diodes are current directional meaning current only goes one way, right?

 

[BeanAnimal]

Yes diodes only conduct current in one direction (at least until they reach their breakdown voltage). The diode acts as a current shunt when the field around the relay coild (an inductor) collapses and the iron armature us pulled back through it by the spring (creating an induced current). The diode does not conduct otherwise as it is placed in reverse polarity parallel to the coil.

As mentioned the side effect is also to damp the action of the armature by slowing the collapse of the field.

Adding an inductor in series or parallel with the relay could would surely change it's operating characteristics including it's hold current and closure time, where the diode realy only effects the opening time. In addition that inductor would also have a field that would collapse when current is removed.

Somebody please correct any errors I have made.