Hardwired chargers - fuse question

jumbone

Input 12v
Output 5v +/- 2.5% max 3 amp
Transformer is 96% efficient
Power rating 15W


What fuse would I need to fit for this transformer? Realsitically, the max output would only ever be 5v 2.1A.

I would be putting an inline fuse on the positive just after where I tap the cigarette lighter feed. Would I also need to uprate the fuse in the fusebox for the extra load (purely for safety, the lighter is never used)? The current fuse is a 30A one according to a diagram here. what current does a cigarette lighter draw @12v? Ive seen elsewhere online that other cars use 15A and 20A fuses for lighters/12V sockets.

Also if there was something charging using the 5v connection and the fuse was too small, would the lighter just heat slower or blow the fuse?

CiniO

jumbone wrote: »

Input 12v
Output 5v +/- 2.5% max 3 amp
Transformer is 96% efficient
Power rating 15W


What fuse would I need to fit for this transformer? Realsitically, the max output would only ever be 5v 2.1A.

I would be putting an inline fuse on the positive just after where I tap the cigarette lighter feed. Would I also need to uprate the fuse in the fusebox for the extra load (purely for safety, the lighter is never used)? The current fuse is a 30A one according to a diagram here. what current does a cigarette lighter draw @12v? Ive seen elsewhere online that other cars use 15A and 20A fuses for lighters/12V sockets.

Also if there was something charging using the 5v connection and the fuse was too small, would the lighter just heat slower or blow the fuse?

My understanding is that while max you will need is 3A at 5V, then it calculates to 1.25A at 12V.
Surely transformer efficiency comes into play here, but assuming 96% you mentioned, then it looks like you might not need anything bigger than 1.5A fuse (if such thing exist). Obviously no point making it just on the borderline, so possibly 5A fuse should do the job OK.

jumbone

:mad:

CiniO wrote: »

My understanding is that while max you will need is 3A at 5V, then it calculates to 1.25A at 12V.
Surely transformer efficiency comes into play here, but assuming 96% you mentioned, then it looks like you might not need anything bigger than 1.5A fuse (if such thing exist). Obviously no point making it just on the borderline, so possibly 5A fuse should do the job OK.

Sweet, so do you reckon that a 5 A inline fuse and just leave the 30 A for the lighter circuit? 1.25 A is basically nothing so surely it could even be used with the lighter? This will never happen, but just in case. More likely is a satnav running from the lighter socket which would presumably have draw similar current to the usb charger.

ironclaw

Lighter in use will pull a decent rake of current when in use. Most are 20 Amps. Technically speaking, you are tapping an already fused circuit so there isn't a necessity or an additional one, it it was me, I wouldn't. The hard wired charger will never pull enough to blow the standard cigarette lighter fuse.

For example, I have the same unit as you, and I can charge an iPhone 5 and Samsung S3 at the same time. No problem. You may run into issues if you had significant additional load on the cigarette lighter itself.

Just as a minor point, check the lighter circuit is ignition switched. You don't want it constant live if you can at all.

CiniO

jumbone wrote: »

:mad:


Sweet, so do you reckon that a 5 A inline fuse and just leave the 30 A for the lighter circuit? 1.25 A is basically nothing so surely it could even be used with the lighter? This will never happen, but just in case. More likely is a satnav running from the lighter socket which would presumably have draw similar current to the usb charger.

Hmm this 30A fuse makes me wonder.
12V at 30A gives 360W.
Hard to think about any device needing so much power, that would plug in to lighter socket.
Did you check in your car manual if that 30A for lighter circuit is what it should be?

I think in my car lighter socket is at 15A fuse.
I have extension allowing me to plug in up to 3 items.
So I have my dash cam, sat nav and charger sometimes all at the same time, and all those runs without any bother on 15A.

And answering original question - I think 5A fuse in line to your hard wired charger will be adequate.

ironclaw

CiniO wrote: »

Hmm this 30A fuse makes me wonder.
12V at 30A gives 360W.
Hard to think about any device needing so much power, that would plug in to lighter socket.
Did you check in your car manual if that 30A for lighter circuit is what it should be?

Sometimes, especially in heater circuits, you get an initial 'umph' when they kick in and the current is quite high for a short period of time. As they heat up, you get all sorts of high current behaviour, hence the heating effect.

But you are right, there is very little that you can plug into a cigarette that comes close to that. I think the biggest I ever saw was 50W fridge, and thats nothing at all.

jumbone

ironclaw wrote: »

Lighter in use will pull a decent rake of current when in use. Most are 20 Amps. Technically speaking, you are tapping an already fused circuit so there isn't a necessity or an additional one, it it was me, I wouldn't. The hard wired charger will never pull enough to blow the standard cigarette lighter fuse.

For example, I have the same unit as you, and I can charge an iPhone 5 and Samsung S3 at the same time. No problem. You may run into issues if you had significant additional load on the cigarette lighter itself.

Just as a minor point, check the lighter circuit is ignition switched. You don't want it constant live if you can at all.

Lighter circuit is switched in the 207. I know its fused already but the transformer is an ebay one from china that'll be charging a smartphone worth a few hundred quid so I think £1.85 on a fuse holder and 5A fuse is a wise purchase for piece of mind. Especially seeing as its the girlfriends car/phone !

CiniO wrote: »

Hmm this 30A fuse makes me wonder.
12V at 30A gives 360W.
Hard to think about any device needing so much power, that would plug in to lighter socket.
Did you check in your car manual if that 30A for lighter circuit is what it should be?

I think in my car lighter socket is at 15A fuse.
I have extension allowing me to plug in up to 3 items.
So I have my dash cam, sat nav and charger sometimes all at the same time, and all those runs without any bother on 15A.

And answering original question - I think 5A fuse in line to your hard wired charger will be adequate.

Haven't had a look at the fuse box yet, I am judging by the link to that peugeot forum thread in my OP

Also, I know they arent recommended but is it an absolute no-no to use scotchloks? I have them bought already and my soldering is a bit messy

Dont think I could reach this level
http://bolty.net/2010/11/15/how-to-solder-for-motorcycle-tapping-wires/

ironclaw

Fuse won't stop damage from occurring as the inline regulator should prevent a high current surge. It's not a transformer btw, it's a DC-DC regulator which are fairly stable. A fuse won't blow fast enough either way. By all means put one in, I'm just saying it's not going to do much.

Damage to a phone is more likely from a bad noisy / ripple output from the device which a fuse won't stop either.