Which side of a relay do you put the fuse?

[macmattom]

<font face="Comic Sans MS"><font size="2"><font color="blue">I am now running out of jobs to do on my FHC before all I have left to do is to try to get the engine running. However, standing between me and that are 2 wiring questions. Once these issues are out of the way, I only need to put the inlet manifold back on (properly) and we're ready to convert chemical energy into carbon monoxide and noise.

1) <u>Front Fog Lights</u>. I am putting some front fogs on and have wired them up, but which side of a relay does the fuse go and what amp rating should I use? Should I fuse the low-current side (power to switch the relay) or the high-current side (power to front fog lights)?

2) <u>In-Line Kenlowe Fan Switch</u>. I have fitted an in-line temperature switch for my Kenlowe fan (Demon Tweeks - absolutely superb!). The low current side is pre-wired so all you need to do is connect the red high-current wires to the +ve supply and the blue high-current wire to the fan input - simple. My questions are: what fuse rating should I use, and can I put a manual overide switch on the low-current side to operate the fan if I ever want to? If so, is it just a case of splicing my switch into the existing low-current wires? Not sure I will, but if I did ..........[;)].

Cheers,

Mac</font id="blue"></font id="size2"></font id="Comic Sans MS">

The pheonix rises from the ashes - just give it time!




(two and a half years and counting - but who's counting!!)

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[jclay (RIP 2018)]

I am only guessing for your description. You have a relay set up to trip on power from the fog light switch. Then you are bringing in power to the relay to go to the lights when the relay is closed. If this is correct, then the fuse should go between the power source for the lights and the input side of the relay. That is how mine is wired up and it has never been an issue.

Fan over ride switch, wire the switch in parallel with the in-line switch. Not in series.

Clay

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[FI Spyder]

Clay is right, in addition it is good practice to put the fuse as early in the circuit as possible. If the wire shorts out before the fuse you will have burnt wiring or worse a fire, if it happens after the fuse, the fuse will blow and nothing further happens.


TR7 Spider - 1978 Spitfire - 1976 Spitfire - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra - Yellow TCT

[macmattom]

<font face="Comic Sans MS"><font size="2"><font color="blue">Thanks guys. As I'm more mechanically than electrically minded can I please clarify: place the fuse (will 15A suffice?)on the 12V feed on the high-current side (which I've taken directly from the battery +ve terminal) before it goes into the relay, and then continue from the other side of the relay to the lights.

I've taken the feed for the low-current side from the main light switch via the fog light switch, so I can only get power to the relay when I have lights on. My front fogs come on with sidelights and headlights - my main thought here was if my headlights fail for any reason I will always have some kind of front light. Does anybody know how this fits with MoT legislation?

Thanks again,

Mac</font id="blue"></font id="size2"></font id="Comic Sans MS">

The pheonix rises from the ashes - just give it time!




(two and a half years and counting - but who's counting!!)

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[jclay (RIP 2018)]

Yes, put the fuse between the battery and the relay!

Clay

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[Bobbieslandy]

The fuse is to protect the cable, not the lights so the fuse rating should ALWAYS be lower than the rating of the cable. power in Watts divided by Voltage will give you the total current draw, 2 x 55w spot lamps = 110W / 12V = 9.2A. Buying a fuse carrier which bolts to the battery + post is a good idea as it totally eliminates any unfused cables trailing about.

As a rule try and fuse any + cables you install as close to the source as possible, it's ever so easy for a cable to chaff on something and keep heating up until the copper wire melts filling up the cabin with the most horrid smoke and potentially causing a fire. Not nice at 70mph.

this is off the top of my head, all my leccy books are at work but it's a good enough rough guide. I Don't know what the equivilent is in SWG though.


1mm 15A

1.5mm 19.5A

2.5mm 27A

4mm 36A

6mm 46A

10mm 63

[macmattom]

<font face="Comic Sans MS"><font size="2"><font color="blue">10A it is then. Cheers chaps.

Mac</font id="blue"></font id="size2"></font id="Comic Sans MS">

The pheonix rises from the ashes - just give it time!




(two and a half years and counting - but who's counting!!)

http://s733.photobucket.com/albums/ww338/macmattom/

[Bobbieslandy]

Current draw will be "10A" but you may find that arcing within the relay will cause the power to spike above that. See how you go with a 10A fuse and if it blows when switching on and off a few times in sucession then go up a size (12.5A i believe). Just be careful not to go above the rating of the cable!

If you're switching the control side of the relay using the positive then you should really fuse it as you'll be bringing it in the cabin. you'll only need a 500mA fuse for that. Or you could bring the negative cable into the cabin and switch that instead linking the +'s together on the relay, if that goes down to earth it'll just mean your lights stay on!

Rob.