Piggy-back fuseholders - a right way and a wrong way?

eugenel

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I'm about to hardwire in another dashcam using a piggy-back mini-fuse holder. Prior to doing this, I had a quick ferret at it with a multimeter and reckon ideally the piggy-back should be fitted one war around and not the other. It will work both ways, but one way will result in the current flowing to the dashcam flowing through both fuses in series.

How? Well, check out the illustration:

images


Let's say the left contact in the picture is A and the right contact is B. A is connected to the two sockets above it. B is connected only to the lower socket above it. The red wire is connected to the higher socket above B.

So, I say contact A should be inserted into the battery-side of the fuse socket, rather than the load-side of the socket. Why? Because if B is inserted into the battery-side of the fuse original socket, then the dashcam current has to flow through both piggy-backed fuses in series. This could overload the lower fuse, which has to now handle the current for its original load plus the dashcam.

Does this matter? Probably not, if your dashcam fuse is smaller than the original fuse it was piggy-backed off (so you're getting the intended protection) AND if the total load on the circuit is low, so doesn't blow the lower fuse. But I intend to try and fit it the 'right' way around - space permitting.

Or, maybe I'm completely wrong! Don't think so though. And I've seen comments on Amazon from people moaning about blown fuses in piggyback installations!

Eugene

PS. If I'm right, they really should provide installation instructions.
 
You are completely correct.

Although it works both ways, for the reason you mentioned, only one way is correct, also for the reason you mentioned. The intention is to use the A side to tap off power to go to the red wire (through a fuse rated for that circuit) while leaving the original circuit as it is.

If it is used correctly, both fuses do not need to be the same rating. They are two different circuits. If used incorrectly and the lower fuse for the original circuit is a lower rating than the new circuit connected to the red wire, it may blow before the fuse for the new circuit does.
 
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I reckon you plan to have it the wrong way around!

If you connect it as you plan and have both fuses the same rating as the original then you can draw twice the original power, however the cable providing the power to the A side of the fuse will probably only be rated to match the original fuse, or in my case the relay supplying power to the A side of the fuse is only rated to match the original fuse.

If you connect it the other way around and keep the original fuse in the bottom slot then the cable/relay providing the power is still correctly protected from overload.

Realistically, if you are only powering a dashcam and fit a 2 or 3 amp fuse in the top and the original 10 or 20 amp fuse in the bottom then there isn't much to worry about whichever way around you have it.
 
The right way is to insert the fuse tap with what you called A is the one that receives the voltage.
Break the connection in the middle of a spare fuse (the lowest amp rating - the thinner) you don't need (create a separation between the "legs" A and B).
Insert that fuse to the place you want to tap.
Check with a multimeter or a fuse tester (like this one below) which leg is A and which is B.
015391_new_show1.jpg



You can see the correct way in the picture here:
2upahs7.jpg
 
Yup, and that also means one can use empty fuse sockets - I have three of these piggy back things, two plugged into unused slots (easy to identify, even without your fuse chart, as they only have connectors on one (+in) side. The third one was used to create a manual breaker for a particular circuit.
 
The right way is to insert the fuse tap with what you called A is the one that receives the voltage.
Break the connection in the middle of a spare fuse (the lowest amp rating - the thinner) you don't need (create a separation between the "legs" A and B).
Insert that fuse to the place you want to tap.
Check with a multimeter or a fuse tester (like this one below) which leg is A and which is B.
015391_new_show1.jpg



You can see the correct way in the picture here:
2upahs7.jpg

Thanks. All fitted now and working like a charm ...

Eugene
 
Good thread, I hadn't thought about which way round when I fitted one the other day but there's a really simple way to check which way is correct.

Just pull the 'main' fuse from the piggy back module and plug it back in with just the 'accessory' fuse in place. It will only work one way round and that is the correct way.

(Then put the 'main' fuse back in again.)
 
Just pull the 'main' fuse from the piggy back module and plug it back in with just the 'accessory' fuse in place. It will only work one way round and that is the correct way.
I still say that is the wrong way!

Lets take a more extreme example, we decide to add two 100W spotlights to the front of the car 2 x 100 = 200W / 12V = about 17 amps. Now we decide to get the power from the fuse for the clock using a fuse tap, the clock uses 10 milliamps and is protected by a 1A fuse, so we put the original 1A fuse in the bottom of the fuse tap and the new 20A fuse for our new spot lights in the top of the fuse tap.

Now we have two options for fitting the fuse tap:

  1. If we fit the fuse tap your way around then when the new lights are turned on we draw 17A for the lights plus 10mA for the clock but the wiring for the clock is only rated at 1A so it will very quickly get very hot, melt it's insulation causing a short circuit and maybe burn the car to it's metal shell.
  2. If we fit the fuse tap the other way around then all that happens is that the 1A fuse burns out to protect the 1A wiring and the car lives for another day.

Which is the safer option?
Why would you not choose the safe option?

It is possible that every fuse in your fuse box is provided with power by 100A cable via 100A relays but copper cable is both expensive and heavy and 100A relays are expensive and rather large so that is very unlikely! More likely that they will be supplied by cable/relays rated for what they will be used for.
 
I still say that is the wrong way!

Lets take a more extreme example, we decide to add two 100W spotlights to the front of the car 2 x 100 = 200W / 12V = about 17 amps. Now we decide to get the power from the fuse for the clock using a fuse tap, the clock uses 10 milliamps and is protected by a 1A fuse, so we put the original 1A fuse in the bottom of the fuse tap and the new 20A fuse for our new spot lights in the top of the fuse tap.

Now we have two options for fitting the fuse tap:

  1. If we fit the fuse tap your way around then when the new lights are turned on we draw 17A for the lights plus 10mA for the clock but the wiring for the clock is only rated at 1A so it will very quickly get very hot, melt it's insulation causing a short circuit and maybe burn the car to it's metal shell.
  2. If we fit the fuse tap the other way around then all that happens is that the 1A fuse burns out to protect the 1A wiring and the car lives for another day.

Which is the safer option?
Why would you not choose the safe option?

It is possible that every fuse in your fuse box is provided with power by 100A cable via 100A relays but copper cable is both expensive and heavy and 100A relays are expensive and rather large so that is very unlikely! More likely that they will be supplied by cable/relays rated for what they will be used for.

Thank you for all your exclamation marks. And, yes, if we decide to do something stupid then your way around protects you.

But we're not.

I've piggy-backed off the supply for the cigarette-lighter, which is where the dashcam would be plugged in anyway.

We are talking about dashcams ... the clue is in the name of the forum. ;)

Eugene
 
And, yes, if we decide to do something stupid then your way around protects you.
Well that is the purpose of a fuse - protection.

The only advantage of putting it the other way around is that it allows you to overload the circuit which is dangerous.

Why do you want to bypass the protection?
 
Hi Nigel, I think you're wrong there. The 12v bus is on the one side of all the fuses. That's kind of the point. The piggy back unit should be fitted so both fuses are tapping power from the bus side. If fitted wrong way around, the accessory fuse is getting power via the main fuse and that can never be right. After all, if we are to fit the piggy back your way, there is no point in using a second fuse at all for the acc circuit, it might as well share the current from the main fuse directly. So I'm unpersuaded that my method is wrong. I'm always open to rethink with a logical argument of course, but I still think this is correct:

Just pull the 'main' fuse from the piggy back module and plug it back in with just the 'accessory' fuse in place. It will only work one way round and that is the correct way.
 
Hi Nigel, I think you're wrong there. The 12v bus is on the one side of all the fuses. That's kind of the point. The piggy back unit should be fitted so both fuses are tapping power from the bus side. If fitted wrong way around, the accessory fuse is getting power via the main fuse and that can never be right. After all, if we are to fit the piggy back your way, there is no point in using a second fuse at all for the acc circuit, it might as well share the current from the main fuse directly. So I'm unpersuaded that my method is wrong. I'm always open to rethink with a logical argument of course, but I still think this is correct:

Just pull the 'main' fuse from the piggy back module and plug it back in with just the 'accessory' fuse in place. It will only work one way round and that is the correct way.
Very few of the fuses in my car have a bus on the bus side, most have a relay or switch which has a current limit and is protected from overload by the original fuse, some even come direct from the ECU and you definitely don't want to overload that!

There is nothing wrong with going via two fuses, you should chose the new one to match the current rating of the wiring and device you are installing which may be less than the original fuse, if it isn't less then the extra fuse does nothing but complete the circuit.
 
Hi Nir,

Yep, saw that, and you're right of course. The second fuse should be getting power from the 12v input.

My contribution to the thread, which I thought was being helpful until all this contraversy, was that you can achieve that correct connection without using broken fuses or multimeters. Just pull the main fuse and test which way around turns your camera on, then put it back. Job done.
 
I still say that is the wrong way!

Lets take a more extreme example, we decide to add two 100W spotlights to the front of the car 2 x 100 = 200W / 12V = about 17 amps. Now we decide to get the power from the fuse for the clock using a fuse tap, the clock uses 10 milliamps and is protected by a 1A fuse, so we put the original 1A fuse in the bottom of the fuse tap and the new 20A fuse for our new spot lights in the top of the fuse tap.

Now we have two options for fitting the fuse tap:

  1. If we fit the fuse tap your way around then when the new lights are turned on we draw 17A for the lights plus 10mA for the clock but the wiring for the clock is only rated at 1A so it will very quickly get very hot, melt it's insulation causing a short circuit and maybe burn the car to it's metal shell.
  2. If we fit the fuse tap the other way around then all that happens is that the 1A fuse burns out to protect the 1A wiring and the car lives for another day.

Which is the safer option?
Why would you not choose the safe option?

It is possible that every fuse in your fuse box is provided with power by 100A cable via 100A relays but copper cable is both expensive and heavy and 100A relays are expensive and rather large so that is very unlikely! More likely that they will be supplied by cable/relays rated for what they will be used for.

If in your example of driving lights the safest way is to only run a relay off that circuit and get the power for the lights direct from the battery or a known high current circuit like the high beam wires.

Why would you run the circuit through two dissimilar fuses as the lighter one will always blow first.

negating the need for the heavier one.

A sensible person would of course use a circuit that is heavy enough to support whatever it is you are adding to it.
In this case it only needs a 2 amp fuse so any 10 or 15 amp circuit should be OK
 
Yup, and that also means one can use empty fuse sockets - I have three of these piggy back things, two plugged into unused slots (easy to identify, even without your fuse chart, as they only have connectors on one (+in) side. The third one was used to create a manual breaker for a particular circuit.

Thats the way i have done it also but should it be for a higher current device it would pay to find out what the socket was put there for and its rating.
EG if car doesnt have a sunroof there may be a 20amp spare
I use these Tidier and cheaper but as its almost impossible to solder onto a fuse I just buy them $6 each
 
Bumping up an old thread! but this is what comes up in Google,

how do I tell which is the 12v side? If I take a multimeter and check the resistance, one way shows -ve, one way shows +ve. I'm guessing the lead I'm touching with the red pointer when it shows +ve is the 12v side?

Thanks
 
See the quoted section in the post right above yours - it answers your question ;)
 
I'm still not sure, I feel like I'm missing something very obvious. I looked at the fuse box under my dashboard, and it's too tiny to tell which is +ve just by looking. A test by voltmeter would give me 12v on both sides, so how can I tell which is +ve?
 
it's all positive, but one side of the fuse is hot, the other isn't, if you power something from the hot side it still works even if the fuse were blown (no protection)
 
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