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I have been learning about electronics as a hobby. And I'm still confused about current and voltage. I've read many sites and watched many videos. But still have questions.

Here is what I learned so far.

Voltage is how much force the electrons are being pushed. Current is how many are flowing. So If I increase the voltage, means I increase the pressure and current will increase too. There are different kind of power supplies, those supply different currents in same voltage. They reduce current using resistance and increase using transistors. Right ?

Now the confusion part, what I think is that how much current will be pulled from power supply is depend on the device that need the electricity. And if device pulls more current than power supply can push, power supply will heat up and blow.

Now lets say, I have an air conditioner, I think it takes around 10A. While a normal fan takes 1A. That seems to prove, it depends on device how much current it will take. So if I put 100 air conditioner which will take 1000A, it will blow up the power supply. Means power supply was unable to push that much of current.

Now lets take a phone, that comes with a charge that supplies 200mA and good for phone. But if I use a phone charger than can output 1500mA, why my phone will charge fast ? It is suppose to only use 200mA(or enough) only, not 1500mA. And it might blow up the phone charging circuit or battery.

My actual question is, can we force an electronic device to use more current if more current is being pushed from power supply ? If yes, then how come a fan only takes 1A while air conditioner takes 10A on same power supply. Why fan is not blowing up ?

Please if possible, try to explain in simple words. Thank you :)

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  • \$\begingroup\$ It's not so much the power supply pushes current rather than the load (your fan/aircon) drawing current - a power supply pushes more current (generally) by increasing the voltage. An aircon will (presumably) use more power so it will draw more current from the power supply while a fan will use less power so it draws less current. \$\endgroup\$ – tangrs Mar 8 '15 at 3:29
  • \$\begingroup\$ electronics.stackexchange.com/questions/107569/… \$\endgroup\$ – GR Tech Mar 8 '15 at 5:31
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Think as power supplys as a constant voltage, rather than the current they can provide. So a constant voltage supply will try to maintain the same voltage independent of the load you put on it (until, as you said, it blows up). So, a fan is designed to "pull" or "let pass" 1A for a given voltage while an air conditioning device is designed to "pull" 10A for the same given voltage. Thats why they pull different currents. And, while you can "force" more current with more voltage, some devices are smart enough that they will try to compensate for that using their regulators (switching or linear) by having their own constant voltage supplies on the inside, thus maintining about the same current consumption up to a given voltage. Normaly supplys fail not because they fail to push the current, but because they fail to provide the current that is being "pulled". If you have a constant current supply, when you try to "push" more current to a given resistive load, the voltage will rise.

About the phone, battery charging ICs will often have a limit to the current they can charge (as will the batteries). Often on cellphones that limit is close to 1A. Hence you can charge it faster on 1500mA. The 200mA rating is probably based on the USB standard max current, and is obviously easier for the manufacturer to supply you with the phone because its cheaper than a 1500mA supply.

p.s.: to better understand different current draws for same voltages: http://en.wikipedia.org/wiki/Ohm%27s_law also remember that not all loads are resistive(most arent)

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Voltage is how much force the electrons are being pushed. Current is how many are flowing. So If I increase the voltage, means I increase the pressure and current will increase too.

Exactly, if it helps my analogies are :

  • I think of electrons as a gas.
  • I think of a large resistance as a thin pipe, and a small resistance as a thick pipe, which makes it easier or harder for the current to flow.
  • I think of a capacitor as a membrane in a pipe; if pressure (voltage) is increased on one side the membrane bulges and the pressure must also change on the other side. A larger capacitor means a larger area membrane or a softer material. A smaller capacitor means a smaller area membrane or a harder material.
  • I think of an inductor as lots of fans in a pipe. If current flows the fans spin, and if no more current is forced through, the fans continue to spin for a while.

There are different kind of power supplies, those supply different currents in same voltage. They reduce current using resistance and increase using transistors. Right ?

Almost. Power supplies are normally regulated to supply either a fixed voltage or a fixed current. The regulating circuit would then adjust the other unknown quantity to whatever is necessary. What type of components do the regulation might vary but for a voltage source it is often the case that transistors supply more current when neccessary and a resistance draws a smaller current to get a stable regulation circuit.

  • I would think of a transistor as an adjustable resistor. In the pipe/membrane analogy, think of a hose that you can squeeze to make the pipe thinner.

Now the confusion part, what I think is that how much current will be pulled from power supply is depend on the device that need the electricity. And if device pulls more current than power supply can push, power supply will heat up and blow.

Right. That can happen. It doesn't have to happen, but it could. And often the heat causes something else than an explosion.

My actual question is, can we force an electronic device to use more current if more current is being pushed from power supply ? If yes, then how come a fan only takes 1A while air conditioner takes 10A on same power supply. Why fan is not blowing up ?

The answer is yes or no, depending on the circuit. Like the voltage sources, most electronic circuits are designed to operate on either fixed conditions for voltage, or fixed conditions for current. If you increase the quantitity that was seen as fixed in the design the device will usually draw more power and heat up until it (...whatever happens...). Now some electronics include their own power supply regulation, usually in forms of a transformer that converts an expected input to an output, and it can provide some overload protection if the input exceeds maximum values. In the case of overload protection you would just shunt the extra power and the actual part that does something useful would hardly see any change in the available power... at least until the overload protection itself melts.

In the case of your USB phone charger you are looking at a (more or less) fixed voltage source. In one case it is limited to supplying 0-200mA, and in the other case it is limited to supply 0-1500mA. If the phone is designed in a way so it can detect how many amps are available, then it can regulate itself how many amps are used for charging (at a safe rate for the proprietary battery, and ratings of internal components). Keeping things safe for the charger might be more tricky, but usually (I imagine) by sensing the input temperature, and the voltage vs current draw, you would have a pretty good idea if you are within safe limits. Anyway, if you draw too much current and the supply couldn't handle it people would normally not think "it was the phone's fault", the would say that was the charger, and get a new one with the right specs. If you were to swap the charger with one that supplies more voltage, overload protection in the phone would likely just shunt that until something melts.

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Current is drawn...meaning it is pulled from the supply by the device attached to it. An extremely crude model of a device that's plugged into a power supply is a resistor...it completes the path from higher to lower voltage potential and will restrict the flow of current. A phone that draws 200mA of current has a higher resistance than a phone that draws 500mA of current for example in the same way that a 2kOhm resistor has more resistance than a 1kOhm resistor.

The fact that the power supply can source 1500mA of current means that it can handle having seven 200mA phones plugged into it. In other words, it can charge MORE phones but it will not charge them any faster because each phone is still only going to draw 200mA max.

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  • \$\begingroup\$ But it does charge faster, I have experienced it many times. \$\endgroup\$ – xmen Mar 8 '15 at 3:41
  • \$\begingroup\$ Btw, how do you know that? Did you measure the current and voltage? \$\endgroup\$ – tangrs Mar 8 '15 at 3:44
  • \$\begingroup\$ I did not measure it with any tool. However, it is faster. Assume a full charge takes 8 hours with 200mA charger(came with phone), while 2 hours with 1500mA. \$\endgroup\$ – xmen Mar 8 '15 at 3:52
  • \$\begingroup\$ @tangrs, the USB charger actually "reports" power capacity to the phone, which then controls the charge current. Check it out. \$\endgroup\$ – Sean Boddy Mar 8 '15 at 4:36
  • \$\begingroup\$ And resistance doesn't have much to do with setting the charge rate of a mobile device. \$\endgroup\$ – Sean Boddy Mar 8 '15 at 4:38
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The only way to force an electronic device to use more current is to increase the voltage. The resistance of the device determines the maximum current that can flow through it at a given voltage. If the resistance is very low, then it's possible that the power source won't be able to provide that much current (or will overheat and explode in the attempt), but it can never push more current than voltage divided by resistance.

With your 200mA phone charger, assuming 5V USB power, the phone should provide at least 25 ohms resistance to avoid overloading it (0.2A = 5V/25Ohms). For the 1500mA supply, the phone only needs to provide 3.33 ohms resistance. And if the phone can communicate with the power supply, then it can switch its resistance depending on how much current the power supply can safely provide.

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  • \$\begingroup\$ Talk about power and you will surely get more up-votes. \$\endgroup\$ – qualebs Feb 28 at 8:35

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