Electrical Fire in my roof

[daveob]

Need advice from the power wise members.

My question relates to current and wiring capacity.

I have a PC based I/O controller and am going to use it to control a few external relays, that in turn will control different appliances around my property. The writing of the software should be reasonably simple with a touch screen interface to show status / override, etc.

I will probably be using either a 5V or 12V DC supply ( around 3A ).

The distance to the external relays, ranges from 10 to 40 meters. The external relay is rated at 10mA - I assume this is the amount of power that it is going to pull from the supply when fed power, and this is the amount of power that the wiring needs to be capable of carrying.

If this is the case, and the fact that the supply transformer (at 3A) is irrelevant, can I use my existing wiring for my alarm system to carry the power to the relay ? The existing wiring is already in the right places, is 12 core standard alarm system cable, and I have at least 4 unused strands.

All advice appreciated.

[AndyD]

Okay so you want to use a pc to operate remote 'appliances via a 5 or 12vdc relay mounted at the appliance. What are the 'appliances' and do they already have a power circuit to them?

[Justloadit]

The typical alarm wire is more than adequate to operate the relays. However I would use a minimum of 12V to drive relays on long lines. Ensure the relays that you use to power your electrical devices can carry the load. Most 11Pin plug in relays can handle approximately 10 Amps per contact. However putting more than one contact in parallel does not double the current capability of the relay. So what is important there for is to check what devices you are planning to switch on and off. If it is a geyser element, then the relay will be more than capable of handling the task, however if you wish to switch your pool pump on and off, then you require to place a MOV of 275V across the relay contacts, to absorb the energy released by the motor when the relay switches the pump off.

[daveob]

First, thanks to all for the replies.

I would use a minimum of 12V to drive relays on long lines.

OK. How long is a piece of string ? What distance would you class as a cut off between using 5V and 12V DC 'signal' ? I remember the days when I was told I could not use an RS232 bar code scanner over more than 15 meters - the experts were adamant about this. Dropping the baud rate to 2400 allowed me to get reliable data transfer over 100m without a booster on the line (production tracking scenario). My point is, there's theory and practical - what's your advice ?

So what is important there for is to check what devices you are planning to switch on and off. If it is a geyser element, then the relay will be more than capable of handling the task, however if you wish to switch your pool pump on and off, then you require to place a MOV of 275V across the relay contacts, to absorb the energy released by the motor when the relay switches the pump off.

I will be mindful of this advice to ensure that the relay has the capacity to handle the required current. From what you're saying, I also need to pay special attention to anything with a motor ? Most of what I intend to use this for will be lighting circuits. I actually just want to experiments with a USB based digital I/O module used for automation tasks.

[Justloadit]

The problem with 5V is two fold. Firstly you can not get a 5V 11 PIN plug in relay. Secondly, the 5V relays are usually PCB mount, and will probably draw about 100mA when pulled in. A criteria is that the voltage must not drop below 3.75V or the contact pressure will change and cause damage to the contacts. Now 1.25V is not much, on a long line of say 50 Meters on the thin twin flex as used by the alarm companies, could cause a drop of 1 volt with the internal resistance of the wire, so one would be very close to the drop out voltage.