Which side of the transformer do I control ?

Need advice to control power to a device with a relay.

Am making a system that allows me to reset a remote CCTV DVR device using a GSM receiver.

The GSM receiver is working perfectly, but I am having a problem with the control relay.

The relay is used to maintain the mains line ( 220V ) to the CCTV system with the 220V Live wire through the ‘Com’ and ‘NC’ contacts. So under normal conditions the power is supplied to the device.

When the GSM receives a signal, it pulses the relay for 10 seconds. This cuts power to the DVR and 10 seconds later the relay is ‘off’ and the DVR boots up again.

My initial test involved controlling the mains line to the complete CCTV system, but the relay kept failing - dissected the failed relay and the ‘Com’ and ‘NC’ contacts are ‘pitted’ so I assume they are welding themselves together as a result of an arc.

Then discovered that the line is actually for the DVR itself AND all the cameras. Powers each of 3 transformers ( 220V 1.5A converting to 12VDC 5A ) and only 1 required for the actual DVR itself.

So next step is to replace the relay, and use it to power only the DVR power.

But which side of the transformer is best to control to cause the least damage to the relay ? The 220VAC or the 12VDC ?

The relay is marked :
NT72C-S10
DC12V
10A/125VAC,28VDC
50/60Hz
10A/250V ~
10A/28V = ( DC symbol )
coil symbol 12V = ( DC symbol )

DaveO:
Powers each of 3 transformers ( 220V 1.5A converting to 12VDC 5A )

What exactly are the “transformers” - are they switched-mode power supplies? If so, then they will take a large surge current when you power them up.

When using a relay, switching AC is generally preferable to switching DC because the zero-crossings will naturally extinguish the arc, unless the load is inductive.

If you continue to have problems with the relay, you could use either an SSR to switch the AC, or a mosfet to switch the DC.

Not certain about the transformers, but would it not be better to control the 12VDC side, as I am guessing that the power out of the transformer would be reasonable 'startup surge' free ( compared to the 220VAC supply side ) ?

Also, should the Watts / energy that will be passing through the relay contacts not be a consideration ?

Am thinking : 220VAC x 1.5A = 330W ( excluding startup surges ) but on the other side, only : 12VDC x 5A = 60W

  1. I think you will find that the average current drawn at the "transformer" input is much less than 1.5A.

  2. The output side is probably subject to a surge as well, because there will be decoupling capacitors in the DVR that need to charge. However, it may be a smaller surge than on the primary side.

  3. In principle, the relay you have chosen should be suitable for switching either the input or the output of the transformer. So if switching the input causes the relay contacts to wear out, try switching the output instead.

dc42: 1. I think you will find that the average current drawn at the "transformer" input is much less than 1.5A.

  1. The output side is probably subject to a surge as well, because there will be decoupling capacitors in the DVR that need to charge. However, it may be a smaller surge than on the primary side.

  2. In principle, the relay you have chosen should be suitable for switching either the input or the output of the transformer. So if switching the input causes the relay contacts to wear out, try switching the output instead.

Many Thanks dc42

The first relay that failed was controlling the 220VAC line to the 3 transformers, so I think the problem was simply too much current in total. Having said that, it is only 3 transformers 220V x 1.5A each = 4.5A and the relay is marked 10A/250V ~.

On physical examination, the surface of the relay contacts, although about 2mm diameter, are slightly curved, so the actual contact point is a much smaller area. Would the 220V x 4.5A = 990W have contributed to this problem ?

I hear your point about the SSR or Mosfet, but I need the power to the DVR to be ON by default, even if there is no power to the board ( currently using the 'Com' and 'NC' contacts ). Is this even possible with the SSR or Mosfet ?

DaveO: The first relay that failed was controlling the 220VAC line to the 3 transformers, so I think the problem was simply too much current in total. Having said that, it is only 3 transformers 220V x 1.5A each = 4.5A and the relay is marked 10A/250V ~.

On physical examination, the surface of the relay contacts, although about 2mm diameter, are slightly curved, so the actual contact point is a much smaller area. Would the 220V x 4.5A = 990W have contributed to this problem ?

No, the contacts are rated for switching a 2200W resistive load @ 220V.

Did the contacts weld themselves together, or did the relay fail in some other way? If they welded themselves together, then it was probably caused by surge current.

DaveO: I hear your point about the SSR or Mosfet, but I need the power to the DVR to be ON by default, even if there is no power to the board ( currently using the 'Com' and 'NC' contacts ). Is this even possible with the SSR or Mosfet ?

It's possible with a mosfet. Here's how:

Take one IRF4905PBF p-channel mosfet. Connect source to +12V output from transformer. Connect a resistor (about 1K) between mosfet gate and 0v from transformer. Mosfet drain provides +12V power to the DVR. The resistor biases the mosfet on, so the DVR is powered.

Now take an opto isolator. Connect the input side and a series resistor (about 180 ohms) between an Arduino output and Arduino ground. Connect the output side between mosfet gate and mosfet source (collector of opto isolator to mosfet source). When the Arduino drives the opto isolator, it connects the mosfet gate to the source, turning off the mosfet and removing the DVR power.

This spec sheet for the relay - particularly the last graph on the last page - seems to answer why your relay was failing:

http://www.altronics.com.au/download/ndb/specifications/S/S4202B.pdf

Basically, you were trying to switch upwards of 220 VAC @ 4.5A (somewhere around there) - and that graph shows a quick dropoff in contact current carrying capability once you go beyond about 125 VAC...

cr0sh: This spec sheet for the relay - particularly the last graph on the last page - seems to answer why your relay was failing:

http://www.altronics.com.au/download/ndb/specifications/S/S4202B.pdf

Basically, you were trying to switch upwards of 220 VAC @ 4.5A (somewhere around there) - and that graph shows a quick dropoff in contact current carrying capability once you go beyond about 125 VAC...

Many Thanks for the link.

In the specs it also says : Max. Switching Power 336W 2400VA

I thought that W and VA were basically the same thing ( appears not especially when used for DC or AC ) is this spec not saying that the contacts should not exceed 336W ?

The supply voltage of 220V 1.5A = 330W, which is very close to this limit. The secondary side of 12V 5A = 60W would appear ( to my very limited understanding ) to be a better choice.

I am most likely missing something here, and please don't think I am trying to just argue this - I simply want to have the best solution and to also understand WHY it is the best solution.

Did the contacts weld themselves together, or did the relay fail in some other way? If they welded themselves together, then it was probably caused by surge current.

I don't know if they were physically welded, but the relay coil was getting power, the contacts were pitted ( after less than 10 switchings ), and they were not switching. They were most likely stuck together ( even if just enough to prevent the coil from switching them ) and separated when I cut the relay cover open for inspection.

DaveO: I don't know if they were physically welded, but the relay coil was getting power, the contacts were pitted ( after less than 10 switchings ), and they were not switching. They were most likely stuck together ( even if just enough to prevent the coil from switching them ) and separated when I cut the relay cover open for inspection.

I've known relays fail to operate as a result of mechanical defects that are nothing to do with the contacts. Also, if you are driving the relay coil with too little voltage, the contacts will open more slowly and arcing will be prolonged. I suggest you measure the voltage across the relay coil while it is turned on. What transistor are you using to drive the relay?

Best solution is to eliminate the mechanical relay, e.g. by using a mosfet instead.

Watts and VA are not the same thing for AC. You can have much larger VA than watts for a reactive circuit (capacitors and inductors), the switching device doesn't know that the circuit is storing and releasing energy, it only knows about the current and voltage at the switch. So VA is a much more pertinent rating.

dc42: I've known relays fail to operate as a result of mechanical defects that are nothing to do with the contacts. Also, if you are driving the relay coil with too little voltage, the contacts will open more slowly and arcing will be prolonged. I suggest you measure the voltage across the relay coil while it is turned on. What transistor are you using to drive the relay?

I tested the voltage on the relay coil : 12.15VDC. It is being driven by a relay on a different device.

I have replaced the failed relay with another, and have connected to the 12VDC secondary side line from the transformer to the DVR. Before connecting the relay, I measured the current at around 1.2A and saw a momentary peak of 2A when the hard drive in the DVR started up.

Have been testing for a few hours now, having reset the unit around 15 times, and still working as expected.

I will look at the mosfet option next.

Thanks to all for the input.