| 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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