Stopping a battery from shorting (while driving a magnetic coil)

[burntoastt]

Can anyone tell me how to determine the minimum resistance a battery needs to stop it from shorting? for example I'm making an electro magnet, with a 1.5v dc battery cell, but when i turn it on it just drains the battery.

 

[jbriggs444]

Can anyone tell me how to determine the minimum resistance a battery needs to stop it from shorting? for example I'm making an electro magnet, with a 1.5v dc battery cell, but when i turn it on it just drains the battery.

An electromagnet amounts to an inductor. An DC current through an inductor amounts to a short circuit. Any resistor will prevent it from being a "short circuit".

However, that's not the relevant question. The relevant questions are how much current you want, how much time you want it for and how much charge you have to expend.

Edit: I am no expert on electromagnet design, but it seems to me that any power dissipated in a resistor is completely wasted. If you are draining power too quickly, the answer is not to add a resistor. It is to use a thinner gauge wire and add more turns.

 

[kuruman]

It's not a matter of shorting as much as it is the current requirements of your electromagnet. Start by considering how much current you need for your application. Then you need to consider the rating of the battery you plan to use. Read about battery ratings here https://www.allaboutcircuits.com/textbook/direct-current/chpt-11/battery-ratings/. Then calculate how long your battery will last. In short, the battery has a limited amount of charge (Amp$\cdot$hr rating) it can provide. Once the charge is gone, it's gone. You can prolong battery life by reducing the current in the electromagnet, but that will of course reduce the magnetic field.

 

[berkeman]

it seems to me that any power dissipated in a resistor is completely wasted. If you are draining power too quickly, the answer is not to add a resistor. It is to use a thinner gauge wire and add more turns.

+1.0

You either figure out how much wire you need to give you the resistance (and therefore the current) that you want, or you put a DC-DC converter between your power source and your coil to have an efficient way of transforming the source voltage down.

You can find out the resistance per length for different AWG wires using Wire Tables, like the one on this page:

https://www.powerstream.com/Wire_Size.htm

 

[jrmichler]

The pull of an electromagnet is a function of ampere-turns (and other variables). That's total current times number of turns. One thousand turns at one ampere will pull the same as ten turns at one hundred amperes.

Connect a voltmeter to your battery. Then connect your electromagnet, and observe the voltage drop. If the voltage drops a lot, most of the energy is wasted heating the battery, and you need more turns. If you run out of room to add more turns, use smaller wire. As a very rough guess, for an electromagnet driven by a 1.5 volt battery, you will end up with 30 gauge wire or a little smaller. That assumes enamel covered wire, not wire with thick rubber or plastic insulation.

 

[anorlunda]

I'm making an electro magnet, with a 1.5v dc battery cell, but when i turn it on it just drains the battery.

Before you put too much effort on doing it better, it is a good time to think about what you are trying to accomplish. How do you define success in your project? Can you share that with us.

 

[Khashishi]

Edit: I am no expert on electromagnet design, but it seems to me that any power dissipated in a resistor is completely wasted. If you are draining power too quickly, the answer is not to add a resistor. It is to use a thinner gauge wire and add more turns.

Any power lost through resistance in the wire is also wasted. The strength of the magnetic field just depends on the current and the number of turns. You probably need to limit the current to keep the battery from (literally) blowing up, but it doesn't really matter if the power is dissipated in a resistor element or the wire itself, as long as the series resistance is the same, and as long as the resistor and wire don't overheat.

The main benefit of a thinner gauge wire is that you can fit more turns in, so you can get the same magnetic field with less drain on the battery.