12v Dual Supply Relay Problem

[jvenning]

Hi all, first of all sincere apologies for the lack of experience and inappropriate use of symbols! I'm putting together a remote mobile broadband station for use at events that can take a mains supply (transformed to 12v DC) or leisure battery.

I've obtained a 12V switching relay which works, if I supply the relay with two power sources it automatically switches to the strongest and should one be disconnected then it falls to the other. The issue that I'm having is that I have switched the supplies on entry to the box, if either of the switches is opened it doesn't take a supply from the other source but instead cuts the supply completely and in turn the 240AC - 12V DC transformer starts to buzz slightly - I'm guessing this is a bad sign! I would be most appreciative if someone could advise me what I'm doing wrong... I'm expecting it to be rather obvious but I can't seem to get my head around it. I've attached a diagram of my circuitry below.

Many thanks
James

View attachment 35053

 

[Barry White]

Can you post a link to the relay you are using please

 

[jvenning]

Can you post a link to the relay you are using please

Thanks for your reply. It's a TF1170-4 smart combi relay, link:

http://www.rydertowing.co.uk/downloads/instructions/TF1170-4.pdf

James

 

[HandySparks]

What are the two boxes, top left, with 3 wires to each?

 

[jvenning]

What are the two boxes, top left, with 3 wires to each?

Switches, sorry!

 

[HandySparks]

Switches, sorry!

Why do they have a -ve 'earth' connection? (Do they have an indicator light?)

 

[jvenning]

Yes they do

 

[HandySparks]

Yes they do

OK, thanks.

 

[Barry White]

Unsure you will get this relay to work as it relys on the voltage rising when the alternator is started on the vehicle to changeover.
How are you coupling up the charger? for the batterys

@PEG this is more your style any ideas

 

[dmxtothemax]

I would say that the problem lies with your switching relay system,
maybe it is not designed to cope with switching off and on the input supplies !
you will need to contact the maker of the relay and ask there advise ?

 

[Richard Burns]

The description of the relay to which you link says that it is intended to switch off power to the load when the voltage at the supply drops below a certain voltage.
The relay is intended to be supplied from one source and has two internal relays.

What you are doing is connecting together two power supplies (presumably one battery and one 230Vac to 12Vdc transformer) together so that they back feed each other.
This is not a good idea as you cannot tell what the effects will be, it is possible the transformer would charge the battery, but it is also possible the battery would overload the output of the transformer as they are not designed to be interconnected.

If the interconnection did not cause a problem then if the supplied voltage was above probably something like 14 Vdc then the relay would allow power to the load, however this voltage would not be seen from a standalone battery or a DC driver so the relay would not come on.

You would need a different type of change over relay that was monitoring the battery voltage and would switch over to the mains powered supply if the voltage dropped below say 11Vdc. This is not what the relay you have does.

View attachment 35062

 

[jvenning]

Many thanks for your help everyone, that's cleared the matter up quite a bit. Would you have any recommendations on where I would find a suitable relay for this purpose?

Happy New year
James

 

[Richard Burns]

Probably the easiest and cheapest method is to assume you have the system powered mainly by the battery and buy a unit to monitor the battery voltage and if it falls too low as it is being discharged then a battery cut off switch can operate a changeover relay that switches over to the supply from mains power.
The 12V driver supplied from a 230V supply is not worth monitoring as if you have no 230V supply available then you can't use it and if you have a supply available then it will be always on.
If you wanted to run on the driver supply you just switch off the battery supply.
Most automotive type (cheap) relays will not be double pole but this is not critical as a single pole double throw would work OK if required.
There are a variety of battery protector/ cut off / low voltage disconnects available, I have pictured one off amazon but anything that will handle to current should be OK (adjustable can also be useful if you find the battery does not cope with being discharged too low)

View attachment 35083

 

[HandySparks]

Probably the easiest and cheapest method is to assume you have the system powered mainly by the battery and buy a unit to monitor the battery voltage and if it falls too low as it is being discharged then a battery cut off switch can operate a changeover relay that switches over to the supply from mains power.
The 12V driver supplied from a 230V supply is not worth monitoring as if you have no 230V supply available then you can't use it and if you have a supply available then it will be always on.
If you wanted to run on the driver supply you just switch off the battery supply.
Most automotive type (cheap) relays will not be double pole but this is not critical as a single pole double throw would work OK if required.
There are a variety of battery protector/ cut off / low voltage disconnects available, I have pictured one off amazon but anything that will handle to current should be OK (adjustable can also be useful if you find the battery does not cope with being discharged too low)

View attachment 35083

I'd generally agree with this arrangement, although surely it would be best to run the relay coil off the mains-derived supply so that would be the one normally used when available, with the battery as a backup?

 

[Pat H]

To avoid any disruption during any potential automatic switch over of supply isn't it easier to just run the 12V DC to mains continually and just use the mains to charge the 12V battery.
If the 12V battery is under load then the incoming mains charge will increase.
This is effectively a UPS arrangement as used in computers and IT systems. You can just buy one off the shelf.