I want to connect my modem directly to a 12volt battery so that I dont need a adapter. My modem’s specs says that it need 12 volt 1 ampere as input.Now the voltage of 12 volts can be directly given by the battery and I think its not an issue to connect it directly.Still I want to use a resistor to limit current supply to 1 ampere.What value of resistor should I use.
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1\$\begingroup\$ The modem will take whatever current it needs. \$\endgroup\$– Andy akaCommented Jun 18, 2020 at 13:27
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1\$\begingroup\$ i.e. You do NOT need a resistor. Now the voltage of 12 volts can be directly given by the battery and I think its not an issue to connect it directly Indeed it is not. To limit the current to 1 A an option would be to add a 1A fuse in series with the battery. But honestly: it isn't really needed. You're worrying far too much about the 1 A. The 1 A means that the modem needs a supply that can deliver at least 1 A. As Andy says: the modem will take whatever it needs and that need can go up to 1 A. The modem limits the current, no need for you to mess with that. \$\endgroup\$– BimpelrekkieCommented Jun 18, 2020 at 13:33
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2\$\begingroup\$ Have you thought about what will happen when the battery voltage level gradually drops below 12V ? \$\endgroup\$– AJNCommented Jun 18, 2020 at 13:38
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1\$\begingroup\$ You do NOT need to limit current. OTOH : what voltage is your "12V" battery? Is your modem happy at that voltage? A fully charged "12V" car battery can be up to 14.4V (more if the alternator is connected and running) : your modem may not like that (check its manual). And a drained 12V battery may be well below 12V. So you may need a voltage regulator that can cope with both these eventualities; called a buck-boost convertor. (With a lead-acid battery, you don't want to discharge it below 12V, so a buck converter may be adequate). \$\endgroup\$– user16324Commented Jun 18, 2020 at 13:45
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\$\begingroup\$ Thanks. I searched for this buck boost convertor and all I was able to find was 2 different items one buck and one boost.Can I connect both in my circuit.Connecting the battery first to buck converter and then to boost convertor.Also if I set the buck converter to make 12 volts while the input will be 14 volt(suppose the battery is charging).Once the input gets to 12 volts to buck convert, wont it pass 10 volts around as it will set to decrease the voltage.I am not sure if it will always pass 12 volts whether the input is 12 volts or 14 volts or will it change as per the input. \$\endgroup\$– saranshCommented Jun 18, 2020 at 15:03
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A 12 V source will deliver whatever current to the modem it wants to draw. There is not only no requirement for a current limiting resistor, but any resistor large enough to limit current will be positively harmful for the modem's operation.
A more serious problem is the tolerances on the voltage. Does the modem need 12 V +/- some tight tolerance which is provided by the power supply that came with it, or will it tolerate the sort of variations that you could get from a '12 V' battery? A lead-acid battery could be 14 V on a float charge, and 11 V when fully depleted.
In practice, there's a good chance that the first module in the modem is a buck converter, and will accept quite a wide variation in input voltage. How lucky do you feel?
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\$\begingroup\$ the vast majority of modems will be fine with a quite wide range of input voltages around spec; they step it down internally to 3.3 or 5v anyway. \$\endgroup\$– dandavisCommented Jun 18, 2020 at 16:43