Installing low power accessories- tap into head unit ground?

[jakesmith]

I'm installing a parrot kit, a bluetooth module and a dashcam. They're all low power items.
Is there any reason I can't just tap the ground for the head unit? It's not a massive hassle to ground them individually to the chassis but there aren't always points behind the HU and I don't want to start stripping out too much of my R8's nice interior
I am just wondering what the potential downsides are - is it about pulling the supply down too much for the head unit, creating interference etc?

[David Beer]

jakesmith said:

I'm installing a parrot kit, a bluetooth module and a dashcam. They're all low power items.
Is there any reason I can't just tap the ground for the head unit? It's not a massive hassle to ground them individually to the chassis but there aren't always points behind the HU and I don't want to start stripping out too much of my R8's nice interior
I am just wondering what the potential downsides are - is it about pulling the supply down too much for the head unit, creating interference etc?

Compared to the 15amp or so draw for the head unit, those bits would have no effect. After all the negative for the head unit will join the chassis, negative.

[Benbay001]

If they are fused separately i wouldnt ground them on the same cable
Could you take your 12v for the accessories from the same fused supply to the head unit?

Otherwise if you fuse the accessories at 5A then you have a potential maximum current of 20A conducting on the negative before you will pop a fuse (Worse case scenario, but not a risk I would take)

Edited by Benbay001 on Sunday 21st October 11:05

[David Beer]

Benbay001 said:

If they are fused separately i wouldnt ground them on the same cable
Could you take your 12v for the accessories from the same fused supply to the head unit?

Otherwise if you fuse the accessories at 5A then you have a potential maximum current of 20A conductingy on the negative before you will pop a fuse (Worse case scenario, but not a risk I would take)

Edited by Benbay001 on Sunday 21st October 11:05

The negative is always greater current wise, or it would be fused. Depending on the mounting method for the head unit, ie bolted to the chassis , even the aerial earth will be much greater than say the 15 or so amps positive .
In any case, I would say both device would add no more than two amps,
Benbay, what are you doing work wise?

[Benbay001]

David Beer said:

The negative is always greater current wise, or it would be fused. Depending on the mounting method for the head unit, ie bolted to the chassis , even the aerial earth will be much greater than say the 15 or so amps positive .
In any case, I would say both device would add no more than two amps,
Benbay, what are you doing work wise?

We may have had a misunderstanding.

"The negative is always greater current wise" I dont know what you mean.

The current remains the same through all parts of a series circuit.
If the positive is conducting an amp, so is the negative.

To suggest the OP should use the negative cable of his head unit as the negative conductor of a circuit on a different protective device is absolutely crazy.

The only exception i can see is if somehow the head unit is chassis earthed and youre suggesting he puts an additional, new ring crimp under a screw type chassis earth

Im doing an HNC in electronics. Going by your profile, you fit head units. But does this mean you understand electrical theory?

I feel ive set myself up for a massive fall here from missing something obvious, but so far.. i really cant see i have.

[jakesmith]

That makes sense. I was thinking as they are very low draw accessories it would be ok but I’m going to do it differently now

Would it be ok to run each accessory as a separate piggy back fused circuit from the fuse box, but have them share a common chassis ground cable (nice thick short one)

Or run one piggy back from the fuse box, to a distribution block, then individually fuse the accessories, and then ground them using another distribution block and shared thick chassis earth cable to save running 4 piggy backs?

[Benbay001]

jakesmith said:

That makes sense. I was thinking as they are very low draw accessories it would be ok but I’m going to do it differently now

Would it be ok to run each accessory as a separate piggy back fused circuit from the fuse box, but have them share a common chassis ground cable (nice thick short one)

Or run one piggy back from the fuse box, to a distribution block, then individually fuse the accessories, and then ground them using another distribution block and shared thick chassis earth cable to save running 4 piggy backs?

Without knowing your setup, id take my 12v from a splice into the stereo harness, then you can also splice the neutral as you initially wanted to.
I suspect some additional capacity was built into the cable sizing and fuse rating over and above the OEM stereo's demand, to allow for aftermarket headunits etc.
You need to make sure that the cables you select for this are rated for the fuse they are protected by (So 15A if you are splicing into the stereo) The device itself shouldn't necessarily be protected by the fuse. Most devices have a component inside which is designed to fail in an over current situation.

If this option doesn't appeal then you should try and stick another fuse into the fuse box or something.

[2Btoo]

Benbay001 said:

To suggest the OP should use the negative cable of his head unit as the negative conductor of a circuit on a different protective device is absolutely crazy.

Erm, why?

[Benbay001]

2Btoo said:

Erm, why?

Jesus Christ! Because it wont be protected by a suitable fuse!


Where do you guys come from with your condescending comments, offering nothing valuable to the discussion.
If the live going to the head unit is 2.5mm, protected by a 15A fuse, with the negative coming from the head unit also in 2.5mm then that is fine (cable sizes for illustration purposes only).

If the OP now goes and adds a 5A fuse to his fuse box, using 1mm cable as the positive to his accessories, that's fine.
However, if the negative from said accessories now uses the original 2.5mm cable from the head unit, then that is dangerous and totally bad practice!

That 2.5mm negative cable would in a worse case scenario conduct up to 20A of current, which it was never originally designed to do! Car manufacturers don't add extra cost and weight of copper just so you can hack your electrics around. If you want to make mods, do it properly and safely.

Please explain what part of this you think is wrong! This is getting ridiculous.

[jakesmith]

Benbay001 said:

Jesus Christ! Because it wont be protected by a suitable fuse!


Where do you guys come from with your condescending comments, offering nothing valuable to the discussion.
If the live going to the head unit is 2.5mm, protected by a 15A fuse, with the negative coming from the head unit also in 2.5mm then that is fine (cable sizes for illustration purposes only).

If the OP now goes and adds a 5A fuse to his fuse box, using 1mm cable as the positive to his accessories, that's fine.
However, if the negative from said accessories now uses the original 2.5mm cable from the head unit, then that is dangerous and totally bad practice!

That 2.5mm negative cable would in a worse case scenario conduct up to 20A of current, which it was never originally designed to do! Car manufacturers don't add extra cost and weight of copper just so you can hack your electrics around. If you want to make mods, do it properly and safely.

Please explain what part of this you think is wrong! This is getting ridiculous.

I’m not doubting you’re right, but surely if the device malfunctions it’s own fuse would blow?

Just to be clear can you tell me the correct and safe way of wiring up these 4 accessories? Would best practise be to give each its own circuit from the fuse box and their own connection to chassis?

Can I not use a distribution block connected to one decent chassis ground?

[2Btoo]

Benbay001 said:

2Btoo said:

Erm, why?

Jesus Christ! Because it wont be protected by a suitable fuse!


Where do you guys come from with your condescending comments, offering nothing valuable to the discussion.
If the live going to the head unit is 2.5mm, protected by a 15A fuse, with the negative coming from the head unit also in 2.5mm then that is fine (cable sizes for illustration purposes only).

If the OP now goes and adds a 5A fuse to his fuse box, using 1mm cable as the positive to his accessories, that's fine.
However, if the negative from said accessories now uses the original 2.5mm cable from the head unit, then that is dangerous and totally bad practice!

That 2.5mm negative cable would in a worse case scenario conduct up to 20A of current, which it was never originally designed to do! Car manufacturers don't add extra cost and weight of copper just so you can hack your electrics around. If you want to make mods, do it properly and safely.

Please explain what part of this you think is wrong! This is getting ridiculous.

Fair point about the rating of the earth wire for the head unit, although the OP said he would be 'tapping the ground for the head unit', which I took to mean he would be sharing the ground point on the chassis that is also used by the head unit.

[Benbay001]

jakesmith said:

I’m not doubting you’re right, but surely if the device malfunctions it’s own fuse would blow?

Just to be clear can you tell me the correct and safe way of wiring up these 4 accessories? Would best practise be to give each its own circuit from the fuse box and their own connection to chassis?

Can I not use a distribution block connected to one decent chassis ground?

You could splice into the cig lighter, using suitable cable FOR THAT CIRCUIT'S FUSE.
Then bring this off into a connection block thingy (again, suitably rated for the fuse that protects it), and then off to the accessories using cable that's suitably rated for the fuse that protects it.
For the negative side, the same principles apply. Make sure the cable is always suitable for the fuse that protects it, if for some reason you have to use smaller cable, you should add another fuse.

Its clear your going to get different opinions from different people, but that's how I would do it.

The important thing to remember is that youre trying to manage heat.
Cable has a resistance, albeit very small. when it conducts a current it will dissipate a small amount of power. This power is disapated in the form of heat. Your goal is to make sure that things don't get too hot.

You do this by using bigger cable that A) has a lower resistance B) can conduct more heat into the air.
You protect that cable by using a fuse. If the current exceeds the current that you had chosen the cable for, then the fuse blows which makes the circuit open, current goes to 0A.

[David Beer]

Audi R8, take it somewhere and have them to do it!!