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Author Topic: Stupid Resistor Question  (Read 605 times)

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Offline sherbyTopic starter

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Stupid Resistor Question
« on: May 25, 2013, 11:26:30 PM »
Hey Guys I had a stupid question.
If we have a resistor in a circuit of 10 ohm, and power supply of 5V, and a load is connected in series with the resistor.
Then wouldnt the resistor behave as a fuse i.e.. I=V/R , so I throught resistor & Load = 0.5A, as both in series.
So doesn't it act as a Current limiter.
Why do we use a fuse then?
Its a conceptual question, and i found myself to be confused.
Any comments will be appreciated.

Thank You!

Offline jkerns

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Re: Stupid Resistor Question
« Reply #1 on: May 26, 2013, 06:20:48 AM »
In a series circuit, there is a voltage drop across each resistance (your resistor and the load). The current will be the supply voltage divided by the sum of the resistances (your resistor and the load). Since you are adding resistance to the circuit, the current will be reduced.

A fuse has a very low resistance but limits current by melting and opening the circuit in the event of a short circuit or other malfunction - it only works once. Resistors are not supposed to do that. They will do that if you overload them enough, but that's not a very reliable way to limit current.

You would use a resistor to limit current in a circuit during normal operation. You use a fuse to limit damage in the event that something goes wrong.
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Offline jwatte

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Re: Stupid Resistor Question
« Reply #2 on: May 26, 2013, 11:17:57 AM »
The maximum current through a 10 Ohm resistor with 5 V voltage is going to be 0.5 A, this is true!

What happens if the resistor happens to short out? Or if someone hooks up a voltage greater than 5 V? That's what the fuse is for.

Offline nanotechopolis_gr

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Re: Stupid Resistor Question
« Reply #3 on: May 29, 2013, 01:48:22 PM »
hi ! i am not good to physics so how i can convert 9v to 5v?   its possible to do this with 20kΩ ressistors and if yes how i should place them? 9v to 7v what i will need?

thanks
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Offline obiwanjacobi

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Re: Stupid Resistor Question
« Reply #4 on: May 29, 2013, 02:06:19 PM »

Offline jwatte

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Re: Stupid Resistor Question
« Reply #5 on: May 29, 2013, 02:48:23 PM »
i am not good to physics so how i can convert 9v to 5v?   its possible to do this with 20kΩ ressistors and if yes how i should place them? 9v to 7v what i will need?


A reistor can only drop voltage in proportion to current. If your current draw is fixed (a LED or lamp, say) then you can calculate a resistance that will drop the right amount of voltage. For example, if your load is 10 mA, and you want a 4V voltage drop, then you can use a 400 Ohm resistor (4V / 0.010 A)

Input voltages are seldom constant, though, and only the most boring loads are even -- an amplifier, or a digital circuit, or really anything beyond a fixed light or heater, will have variations in current draw.

The simple solution is to use a linear voltage regulator, which can act to vary their resistance to generate the output voltage you want. The 7805, shown above, can take 7V to 25V in, and generate 5V out. Another linear regulator is the LF50AB, which can take betweren 5.4V and 16V in, and generate 5V out. (Note the much lower minimum drop-out of 0.4V instead of 2V!)

Resistors and linear regulators burn off the additional energy as heat. This is inefficient. If you are using a battery, and want it to last longer, you want to be able to deplete the battery from 9V to 3V and still get the output voltage you need. To do this, use a boost/buck or SEPIC switching DC DC converter circuit. For example:
http://www.pololu.com/catalog/product/2119

 


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