Some numpty electrical questions

P

PawnSacrifice

Veteran
 South Oxfordshire
I have a couple projects on the go that require some electrical work. Not 100% confident, I wanted to check a few things.

Please excuse my use of the term "power" - I am not sure if I'm using it correctly, but in context it should be easy enough to understand. Some of the questions may sound REALLY stupid, but I more checking my workings - rather look stupid now ( :oops: ) than get it wrong on the car.

Questions...
1. Switched (key to I) power supply, any good ones at the front of the car? Need to check the "power ratings" but was planning on nicking some from the centre switch panel.
2. With regards to amps, if I have 2 x 5amp devices, I still need a 5amp supply, rather than 10amp? ie, it's not cumulative with regard to supply / fusing.
3. If I have a 3 amp device on a 15amp fused supply, I assume that this will only below the device if it draws too much power and won't just below for being on a bigger circuit? This is sort of irrelevant as I would add an inline fuse before any device in such a setup.
4. If I can't find a switched supply I may just use the cigar socket feed and add a relay, triggered on/off by a lower powered switched supply, does this sound feasible?
5. I assume that I should be putting all fuses / switches / relays on the positive supply.
6. Does it matter what I am feeding back to the 0 connection.

Thankfully I have a 12V mains adapter so I can build everything and prove the circuit in the comfort of my living room before fitting to the car.

I will make sure that I match all of the wire ratings to the correct amps of the device and any switches / relays are appropriately rated. Anything else?

Thanks
 
2. If they are both drawing 5A and are on the same circuit you will need a 10A fuse. If they draw exactly 5A you may need to go up a fuse size i.e. 7.5A

I would put them on different circuits a fuse each seperately, you can't have enough fuses, the more the merrier
 
5. yes fuse on the positive
6. the negative cables have to be sized to match the current flow

3. don't really understand??????????

4. the cigarette socket may be over-loaded, not sure what fuse rating it is, having said that i assume it will be largeish
 
In all seriousness:

1 - not sure
2 - Taz's right, 10A
3 - Thats fine, better to go belt and braces wwith an inline
4 - why dont you just spur off the cigar lighter and switch that? As for relays - a little old. youd probably go for something solidstate - requires next to o current - check maplin.
5 - doesnt really matter but best practice to.
6 - ?
 
Taz x said:
2. If they are both drawing 5A and are on the same circuit you will need a 10A fuse. If they draw exactly 5A you may need to go up a fuse size i.e. 7.5A

I would put them on different circuits a fuse each seperately, you can't have enough fuses, the more the merrier
Click to expand...
Thanks. There will always be something else on the circuit - the original device I'm steeling from. I assume this applies if I am nicking the power for device B before device A?
I'll try and steel from sources that are used intermittently.

I will be checking everything with my multimeter first.

srhutch said:
Are you going to tell us what your doing, or do we have to wait until it's fitted.
Click to expand...
Fitting the parking sensors that PVR used on the Polo to the front bumper of the Z4 (obviously can't use the brake light) and an AE project - they're still on the way from Singapore, will have a better idea of the power requirements when they arrive. Could just hook them up to something else in the headlight, but I want the option to switch them on and off from the cabin.
 
Greeno said:
In all seriousness:
4 - why dont you just spur off the cigar lighter and switch that? As for relays - a little old. youd probably go for something solidstate - requires next to o current - check maplin.
Click to expand...

Cheers. I did think about that, but for the AE I want them to just come on when I switch the car on (DRLs. Tasteful DRLs), but with the option to turn off when needed. If I used a relay I could use this power supply then only switch it on when I need it, as one option. I also want to make sure I don't accidentally leave things switched on when the car is parked up.

And thanks srhutch for the vote of confidence :thumbsup:
 
Taz x said:
4. the cigarette socket may be over-loaded, not sure what fuse rating it is, having said that i assume it will be largeish
Click to expand...

That was my other concern - I've checked and there's some big wires going in. I would be a little worried about overload as I can't check what's going in. I could see how many amps I have available and then left and put the appropriate sized fuse in after the spur and before the connection to the cigar lighter. Most of my stuff is USB powered so I should be okay.

Given what Greeno said as well, this may be the best option for the AEs - meant to be low power.

I will check the parking sensor and see what that's drawing - I doubt it's too much and could probably ponce off something else.
 
obvioulsy your voltage is 12V, ampage is different, the more devices you add in series the more current they will draw, the supplying cables need to be sized to match the flowing current, if they are too small the cable could get too hot and melt/burn/short
 
Taz x said:
obvioulsy your voltage is 12V, ampage is different, the more devices you add in series the more current they will draw, the supplying cables need to be sized to match the flowing current, if they are too small the cable could get too hot and melt/burn/short
Click to expand...
Thanks. I'm think more that I will use the cigar socket - the cables there are quite meaty - I will go over the WDS twice to check and double check my sums. I can check what size the distribution box is fused at and the WDS will tell me the wire sizes.
I will also run everything in the cabin for a little while to physically check for heat.

srhutch said:
Are you going to have to split your headlights then for the AE's
Click to expand...
Yes - having seen gookah's post it looked a lot easier than other's I'd seen, quite probably misleading! Ordered the same kit as his - after speaking to someone a BMW dealer about how the E89 AEs work, I was told that the DRL versions are a lot brighter so thought the ones gookah had would fit with this. I'll need to prove everything in the living room before I start cutting things open... as CJ did, although I think his were lit up in the kitchen.

The one thing I will be interested to see, if the AEs when used with the standard halogens improves the dipped beam performance any.
 
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