Piggy back main 12v 15A supply

[rsgaz]

what way a fuse goes in a car

With ordinary fuses in a 'normal' fuseholder it doesn't matter at all which way round they go. It's only when you introduce fancy fuse taps and whatnot that you need to check which way they go.

 

[PaulBerkshire]

so, learnt about the 12v and load side ( would never have thought of that) and it looks as first thought the new load being added needs to be a lower fuse rating than the one you piggy back. If i do have an empty slot that has power +12v permanent can i assume it will take 15A? If not i am going direct to battery, probably easier and safer Thanks all for your input Also just realized battery is in passenger footwell, so looks like it is out the firewall under the van and in to battery.

 

[rsgaz]

If i do have an empty slot that has power +12v permanent can i assume it will take 15A?

Look in the manual to see what fuse would be there if your vehicle had that particular feature. If it's 15A or higher, then you're good to go.

Any items that are permanently powered in a fusebox ought to be fed via some sort of busbar supplied via a nice thick wire from the battery, but I couldn't be sure for all brands/models/etc.

 

[PaulBerkshire]

Will run direct to battery, safest option and won't interfere with anything else, keep it seperate. run some 25A cable, can use it for something else too then if needed

 

[PaulBerkshire]

Started installing today, where the alarm brains is going is close to the large loom at the steering column, tapped into the 12v that is +12v when ignition is on, would it be ok to use the permanent 12v on there too with the 15A inline fuse or could that damage other things? Be handy as very close to where i need the power.

 

[rsgaz]

Altering the wiring there is asking for trouble!!

Cost saving and weight saving is crazy in vehicle manufacture, every single wire is calculated to be as thin as they dare get away with! You can't just go popping an up to 15A load wherever!

Just run it to the battery, 15 amp fuse as close to the positive terminal as practical. Extend the wire with the same or larger CSA wire than the one provided.

 

[PaulBerkshire]

OK ran to battery on a 21 amp wire protected with the 15a fuse, alarm seems to work fine, added a microwave detector too which works fine in back of van. Currently i do not have lights connected, so if i wanted to get indicators to flash, where would i look to connect, at the relay? Read somewhere I could use 2 diodes one on each wire from the back of the hazard switch and just connect the single signal wire from alarm to them both.

 

[rsgaz]

each wire from the back of the hazard switch

Without a wiring diagram of the vehicle and the alarm, or actually being there, there are way too many possibilities to list of how it should or could be done I'm afraid.

For example, on a lot of Fords, the hazard switch doesn't provide a +12v output when you turn it on, nor are there two outputs. It switches the negative and sends that to the "passenger junction box", which then does the timing and switching of the relays to power the indicator lamps.

If your vehicle uses the same system, you need to know whether the wire from the alarm is a "positive output" or a "negative output". On some you can choose via a DIP switch.

You could intercept the wires that actually go directly to the lamps and use the diode trick (making sure the diodes can carry at least 4 amps forward current), but then you need to know whether your alarm can output 8 amps down that wire or does it say in the manual something like "200mA max"?