Hi, I have a 6v 12AH battery. I have a 6v electric device, that will take as much amperage i give it. I want to limit the amperage to only 2 amps I understand i need some kind of high Wattage resistor, with low value. the question is, what is the calculation for resistor's value, and wattage ? again, i have 12A, and want to use only 2A at a specific time for the device. Thanks a lot ! Av
Nope, you have a 12 Ampere Hour battery, that means that if you draw 1A it will last for 12 Hours. You need the formula Voltage = Current x Resistance 12/2 = resistance = 6 Ohms Then you need to look at power = Current^2 * Resistance P = 2^2 * 6 = 24W That is not a small resistor
A circuit with 6ohms of resistive load and a six volt supply is only one amp. This is calculated by using ohm's law(voltage=currentXresistance). You are looking at creating a circuit with total resistance of 3ohms. 6v/2amp = 3ohms This means that you need to know the resistance of this 6volt load you are limiting. Is the load a relay coil? Just trying to guess what you are powering. If it is a relay, you should have specs on the coil impedance.
Thanks. actually it is a camera flash unit. so i guess it is not that easy to check. sometimes it charges, other times it just waits.... i've been told to use 0.5 Ohm or 1 Ohm, won't that be suitable ? the idea is to limit the current to the flash. this flash works with 2000MAH batts (2AH), when i connected to it my 12AH battery, it got charging too fast, and causing overheating. i want to give it only 2A, like with AA batts, thats the idea any decision ? Thanks !
Right.... I think your a little confused as to what you actually want.. a 2AH battery doesn't need to provide 2 amps.... likewise a 12AH battery doesn't necesserily provide 12A. Your problem lies in voltage, not ampage. I'd guess that your 12AH battery is pushing what 14V? and the battery of how many AA's are providing ~12V? 8?.. they should be ablout the same if so... but you can always throw a multimeter across the terminals and find out. If you sort out the voltage the amapge will sort itself out.
Inferno, I think i was very clear. if not enough clear for you, here it is again, I have 6v 12AH batt. I have 6v flash unit device, (which will draw a lot of current if i let it) when i connected the flash unit to the big battery, it charged VERY fast, which caused overheating. I want it to charge normally, as it would do with 2AH 6v batt. (4x1.5v2AH) therefore i need to block the high current flowing to the flash, and make the current lower. i succeeded to do that using a thin wire, which caused the flash to charge slowly with the 12AH batt. i just want to use a resistor instead. Is that more clear now ?
You were clear but your science is wrong. The capacity of a battery (Ah) is not an indicator of the current it can supply. That depends on the internal resistance of the battery plus the load resistance, so is a function of the battery chemistry. Looks like your four 2Ah batteries have a much higher internal resistance than the 12Ah battery and your flash device is not really rated for 6V. At reduced current it doesn't get the full 6V, simple Ohms Law. A bit like the key-ring torches that run a 3.5V white LED from 6V lithium cells with no LED resistor. They rely on the high internal resistance of lithium cells to prevent the LED blowing.
Measure the voltage at the flash device when it's charging from the 2Ah pack and measure the current flowing.
To boil things down a bit you have a flash module. In essence it's just a capacitive load. You need something that will at like a pre-charge circuit. When the flash module is complete discharged, it acts as a short. As it charges, the current will slow. I would use a 3ohm resistor as calculated before. This will limit your highest current to 2amps. If the module does not have a visual indicator that it is charged, use a light in-line with the module power feed. The light will operate while module charges and dim or go out when the charge is complete.
it's closer to a 12W resistor. You will only be pulling 2amps for a very short time. You might be able to cheat one at less of a rating.
