Electronics > Projects, Designs, and Technical Stuff
USB charger circuit for car
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Charybdis:
Hi there I was wondering about F1 specs.
Is it ok to put a 3A fuse?
does it have to be fast, or slow?
Thank you so much :)
soldar:
For a while I had a car which would kill any 12V to USB adapter which was plugged in when cranking the engine. I had to remember to unplug before cranking and I killed quite a few when I forgot. Especially if I killed the engine in traffic and wanted to start in a hurry.

I guess I could have put some protection but I always thought I would remember to unplug it.
madires:

--- Quote from: Charybdis on April 16, 2019, 12:56:50 am ---Hi there I was wondering about F1 specs.
Is it ok to put a 3A fuse?
does it have to be fast, or slow?

--- End quote ---

The things to consider are the inrush current (mostly by filter caps), the maximum load current and over-voltage situations (suppressed by TVS). I'd suggest to go for a medium blow fuse rated slightly higher than the maximum load current to be able to deal with short overloads by inrush current or over-voltage. BTW, you could add a power resistor in series with the fuse to help with the power dissipation (less for the LM339) and to limit current. In this case a fast blow fuse could be used.
jfiresto:

--- Quote from: soldar on April 16, 2019, 11:49:37 am ---For a while I had a car which would kill any 12V to USB adapter which was plugged in when cranking the engine. I had to remember to unplug before cranking and I killed quite a few when I forgot....
--- End quote ---

I have an old car, that lacks a central suppressor, which may see up to 80V load dumps if you respect the original equipment requirements.

I used an off the shelf, 12V USB car charger, and designed and placed a circuit upstream that disconnects the charger around 24V, well below its 36V absolute limit. I can dig up and post the design if there is interest.

I decided that was less work than designing and making an extra robust charger.
Charybdis:
Thank you for all your replies.
--- Quote from: madires on April 16, 2019, 01:09:23 pm ---
--- Quote from: Charybdis on April 16, 2019, 12:56:50 am ---Hi there I was wondering about F1 specs.
Is it ok to put a 3A fuse?
does it have to be fast, or slow?

--- End quote ---

The things to consider are the inrush current (mostly by filter caps), the maximum load current and over-voltage situations (suppressed by TVS). I'd suggest to go for a medium blow fuse rated slightly higher than the maximum load current to be able to deal with short overloads by inrush current or over-voltage. BTW, you could add a power resistor in series with the fuse to help with the power dissipation (less for the LM339) and to limit current. In this case a fast blow fuse could be used.

--- End quote ---

I'll go for a medium fuse then. I would take the fuse 3 times greater than the amount delivered to the usb. Am I right ?
Also, is it safe to put the fuse AFTER the diode 1.5KE diode ?
Thank you  :)
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