Relay And Trailer Wiring

Discussion in 'General Electronics Chat' started by lostincable, Oct 1, 2010.

  1. lostincable

    Thread Starter New Member

    Oct 1, 2010
    Hi everyone, I am wiring up a couple of relays to a hotwire on my trailer plug as per the attached schematic. I had a couple of questions before I start the job and was wondering if anyone could help.


    1) Does a fused relay do away with the need for an inline fuse and if so are they any good? Or am I better of with a relay and separate fuse?

    2) For the ignition circuit wire through pin 86 can you join the two relays together or should they be individual wires to the ignition circuit?

    3) Is there any negatives / positives to using a circuit breaker as well as a fuse with a relay?

    Thanks all!
  2. SgtWookie


    Jul 17, 2007
    1) I have never heard of a "fused relay". Your Jaycar link does not work.

    2) Your schematic does not make much sense. What is it that you are attempting to do?

    3) If you use a suitably rated circuit breaker, you don't need a fuse in series with the circuit breaker. Keep in mind that numerous automotive relays have internal diodes across the coils. If you connect them up backwards, you will fry the internal diode.
  3. majsyd2010


    Aug 29, 2010

    Just to response to your 1st question, there is a fused relay and it does the job the same in line fuse. The good thing about fused relay is that 1 less connection and can be replaced if blow up. Therefore, I would say that there is no difference and yes you can use a fused relay.
  4. eblc1388

    AAC Fanatic!

    Nov 28, 2008
    From the above Jaycar link, a fused relay is a relay with a fuse socket included, as shown below:

  5. lostincable

    Thread Starter New Member

    Oct 1, 2010
    Yep that's the ones, I got two 30AMP ones links is below :)

    Thanks for the response hopefully this will clear it up...

    1) Jaycar link is this one (apologies pasted the wrong one)....

    2) In the schematic I need to achieve the following -

    a) Wire from trailer plug pin 2 via relay to battery to power fridge. Needs to be sent through a relay so when you turn the ignition of the power cuts out. Note 86 will be feed to an ignition circuit in the engine bay probably from the window wiper motor.
    b) Wire from trailer plug pin 9 via relay to battery. Needs to be sent through a relay so when you turn the ignition of the power cuts out. Note 86 will be feed to an ignition circuit in the engine bay probably from the window wiper motor.
    c) Earth lead from trailer plug 10 and 3

    3) Thanks I was reading another guide on another site and they advised to use a circuit breaker, fuse and then the relay. I thought it was a bit over the top myself and had assumed that a fuse and relay would be enough and wanted to make sure I did not need the additional circuit breaker?

    I think I will remove the circuit breaker so I will just have the fused relay and then direct to the battery and the other side direct to the trailer plug.

    Finally when you say "if you connect them up backwards" what do you mean by that?

    Thanks so much for your help!
    Last edited: Oct 2, 2010
  6. SgtWookie


    Jul 17, 2007
    Much better, thanks.
    Interesting; I'd not seen those before. Good idea.

    I don't know how your particular refrigerator works, but common household refrigerators use freon that's compressed with a pump. These types of units do not "like" for their power to be interrupted once it has received initial power.

    Simply using the "ignition power" circuit is not a good indication that the engine is running; only that the key has been turned to the "ignition on" position.

    Once power is applied to a typical freon compressor, pressure starts to build in the condenser. As long as the power is kept on, and the motor stays near full speed, it can keep up with the increased load. However, if the power is applied for a brief time (so that some pressure can build up), and then the power is removed and re-applied, the motor will likely be locked in a stall condition, drawing lots of current until it overheats, usually tripping an internal thermal breaker. This is very hard on the compressor, and will place an extra load on your alternator.

    I suggest that what you need is a circuit that can read your engine's tachometer output, and only turn on the refrigerator relay's coil after the engine has been running for a period of time; turn the relay off immediately if tach pulses cease, and prevent the relay from turning on again until at least 5 minutes has elapsed to allow the pressure in the refrigeration system to bleed off.

    This will require a bit of thought. However, you could help things along by finding out where a tachometer signal is available on your vehicle, and what it's specifications are.

    I suggest that your refrigerator may require a good deal of current on start-up; and it would be unwise to power it from another circuit; particularly a safety type circuit like the windshield wipers or lighting. If a cigar/cigarette lighter or aux power outlet is installed in the vehicle, that would be a good candidate.
    I don't know about vehicles where you are, but in the States, the wipers are powered by the accessory circuit; wipers will operate with the switch in either "accessory" or "run", but switched off when starting.
    If your relay will be located right where it's getting power, then an additional fuse/breaker should not be necessary. However, if you have a length of wire between the relay and the power source, you need a breaker or fuse at the power source. Otherwise, if the wire should come loose or insulation get chafed, you risk having a melt-down.

    The relay that you linked to do not have any indication whether or not there is a built-in diode across the coil terminals (86 and 85). Some relays come with such a reverse-EMF protection diode installed. If they do, it will probably be shown on documentation supplied with the relay, and/or on the body of the relay itself in schematic form.

    If they do not have a reverse-EMF diode, I highly suggest that you provide your own. A simple and inexpensive 1N400x series diode will work just fine. I suggest using any of that series from 1N4002 to 1N4007. The cathode of the diode gets wired to the more positive side of the coil, anode to the more negative side.

    Just a note or two on automotive wiring...
    Never use "electrical tape" aka "electrician's tape" on automotive wiring for insulation purposes! The adhesive will get "gummy" during hot weather and eventually fall off, leaving bare wires.
    Don't use "wire nuts" like for household wiring, either. They do not seal the connection against fluids/atmosphere, and the connection will soon corrode and overheat.

    Some people use "crimp-on" type connectors. I don't like those, as the typical commercial crimpers do not provide a good crimp, nor do they seal against fluids/atmosphere.

    I solder my connections using Sn63/Pb37 or Sn60/Pb40 tin/lead solder and rosin flux. I use two layers of heat shrink tubing for insulation purposes. The best type of heat shrink tubing to use has glue on the inside; this stuff won't ever come off by itself.

    Use nylon "zip-ties" to secure your wiring to the vehicle's frame, or other places that will keep your wires out of harm's way, and to minimize rubbing/chafing I don't know what they call them downunder; but "zip ties" are flat strips of nylon with ribs in them, one end is pointed, the other end has a rectangular slot with a locking device; you insert the pointed end into the slot and pull to tighten the resulting loop.

    Use "service loops", or loops of wire, where the wires connect to relays, power, or trailer connections; as this distributes the flexing over a longer piece of wire, and provides a bit of spare wire for when repairs are necessary.

    Do not use solid wire. Use stranded wire; the finer the strands the better. Finely stranded wire is much more flexible than coarse stranded wire, and will stand up much longer against breakage. Do not use "lamp cord" or "speaker wire"; use only wire that is rated for automotive use or better. I like to use mil-spec Teflon insulated wire; a local surplus store carries large spools of it.

    Be certain to use wiring and connectors that are rated for more current than the fuse you are using. Failure to do this may result in a fire or melt-down. The absolute minimum size wire for 30 Amperes is AWG14; or 1.63mm. I suggest that you use at least AWG12, or 2mm stranded wire.
    Last edited: Oct 2, 2010
  7. debe

    AAC Fanatic!

    Sep 21, 2010
    Hi Sgt Wookie, dont know where Lostincable is situated but here in Australia most caravans have absorbtion (3way) fridges & can draw 15A on 12V power. People tend to use a relay to disconect the power to the van when the tow vehicles engine is switched off to stop them from flatening there vehicle battery. Usualy when they camp for the night the fridge is then run on LPG. hope that explains it. Daryl