Hi all,

For the past week, I have been trying to get a grasp on the basics of circuitry, but with no luck. I understand OHM's law, and understand how V, I, and R relate to one another mathematically. However, in a practical viewpoint, I just cant seem to wrap my head around this. To clarify my problem, I have an example:

Lets say you have an LED in series with a 3 Volt battery. You dont know how much resistance there is between both ends of the LED, and you dont know the current going through the circuit either. You then add a resistor with a known and constant resistor to this circuit. The light from the LED dims. Is this because by adding the resistance, we are limiting the amount of charge that can pass, (V/R =I), or does it dim because if the voltage drop provided by the resistor(V=IR)?

Additionally, this question is related to the above. If we put four LEDs in series with a battery, will each LED be brighter than the next because of the sum of the voltage drops from the previous LEDs?

Lastly, I've read somewhere that a load only takes the amount of current that it needs. So does that mean that if you have two LEDs that require a different amount of current to run, and you put both in a series with a battery, will the amount of current be equal (because "I" doesn't change with with resistance if they are in a series)? And if so, what effects will there be on the LED that requires less current to run?

As you can see, I am really confused about this. Any feedback would be greatly appreciated.

Thanks,

Sean

 

Hi all,

For the past week, I have been trying to get a grasp on the basics of circuitry, but with no luck. I understand OHM's law, and understand how V, I, and R relate to one another mathematically. However, in a practical viewpoint, I just cant seem to wrap my head around this. To clarify my problem, I have an example:

Lets say you have an LED in series with a 3 Volt battery.

If you have just a plain LED connected across a 3v battery, you'll likely either kill the LED due to overcurrent (if it's a low voltage LED) or the LED will be very dim if it's blue or white.

LEDs need to have their current limited, whether passively by a resistor, or actively by a current regulation scheme.

You dont know how much resistance there is between both ends of the LED

LEDs come with specifications for typical Vf @ If, which means forward voltage at a given forward current. Many modern LEDs are rated for an If of 20mA or 25mA; check your datasheet for certain.

, and you dont know the current going through the circuit either.

Not unless you use a current limiting resistor, or a current regulator circuit.

You then add a resistor with a known and constant (value of resistance in Ohms) to this circuit. The light from the LED dims. Is this because by adding the resistance, we are limiting the amount of charge that can pass, (V/R =I), or does it dim because if the voltage drop provided by the resistor(V=IR)?

Limiting the current flow.

You calculate the resistor by:
Rlimit >= (Vsupply - Vf_LED) / DesiredCurrent
If you don't have at least 1v or 10% (whichever is greater) left when you subtract the Vf_LED from Vsupply, then you really need more "headroom" (higher voltage source) or an LED with a lower Vf.

Some really cheap flashlights don't use current limiting resistors. I don't recommend trying this, as your LEDs will likely have a very short life.

Semiconductors don't generally have a "resistance" per se; they have "conductance", which is more or less the reciprocal of resistance.

Here is a chart I made from measurements of a typical 1N4148 computer switching diode over various forward currents, with all of the accuracy that a couple of $2.99 Harbor Freight DMM's could offer:

Notice that as the current through the diode increases, the effective resistance decreases, and the Vf increases.

LEDs will have a somewhat different looking Vf vs If plot (higher voltage for one) however, it will bear a resemblance to this plot.

Additionally, this question is related to the above. If we put four LEDs in series with a battery, will each LED be brighter than the next because of the sum of the voltage drops from the previous LEDs?

No. Current through a series circuit is the same at any point in the circuit. If -20mA is coming out of the power source on one terminal, then 20mA will be going in on the other. Several LEDs wired in series will have the same current flowing through them. Now, it's quite possible for the Vf across each LED to be different, and therefore the power expended in each LED will be different. But, that by itself wouldn't make one LED brighter than another.

Lastly, I've read somewhere that a load only takes the amount of current that it needs. So does that mean that if you have two LEDs that require a different amount of current to run, and you put both in a series with a battery, will the amount of current be equal (because "I" doesn't change with with resistance if they are in a series)?

If the LEDs are rated for different amounts of current, then they should not be operated in series. You should operate them in separate strings.

And if so, what effects will there be on the LED that requires less current to run?

If you are going to operate two LEDs or more in series that require different amounts of current, the lowest-rated LED will define the maximum current that you can put through the series string.

 

Thanks a lot SgtWookie for replying so quickly. I have yet another situation, where I am unsure of what is going on. So I recently got my hands on an arduni microcontroller, and as a test, I wanted to hook it up with a DC motor, and see if I am able to control it. I connected one end of the motor to one of the digital pins of the microcontroller, and connected the other to the ground pin, and I programmed the pin to be high.

digitalWrite(pinnumber,HIGH);

It didn't work. However, I asked around, and I found out that I could have damaged it by doing it this way. Instead, I used a transistor as a switch for a separate circuit to get this to work. What I want to understand, is why the digital pin that I initially hooked the motor on to could have been damaged by my initial configuration.

Thanks in advanced,

Sean

 

Microcontroller I/O pins are usually limited to around 20mA source or sink current at an absolute maximum. Depending on your motor's rated voltage and current, you may or may not have fried your I/O pin.

Unless you can give us some specifications to work with, we're not going to be much help.

 

Alright, I actually do not have any specifications for the motor because I took the motor from an old portable fan that I no longer used. About what you said though:

Microcontroller I/O pins are usually limited to around 20mA source or sink current at an absolute maximum.

How can something have a limit to the amount of current that it can provide if the amount of current is dependent on the Vf and resistance that you use when making the circuit?

Thanks again,

Sean

 

Microcontrollers (uCs)have current limits because they cannot have lots of current flowing through them, otherwise they silicon inside breaks down, aka you fried it. Hopefully, the uC you are using hit a thermal limit and shutdown before the high current fried it.

Yes there is V=IR, but you cannot have a uC drive a motor, because the uC is not possible of having that much current go through it. If you want to drive a motor using a uC, I would suggest that you use a rated H-Bridge or motor controller. Basically, you can have the uC send a high or low digital signal (so 3.3V or 5V) at a low current (<20mA) and you can controller your motors. Check out H-bridge on Wikipedia, as that can probably explain it better than I can: http://en.wikipedia.org/wiki/H_bridge . Does that make sense?

 

Hi aegistalons,

Thanks a lot for the clarity of your explanation, it makes perfect sense to me. But just to reiterate, in case I did miss something:

The reason why the uC cannot supply a current of greater than 20mA is because the pins themselves cant physically handle it. Not because there just isn't enough of it to pass through.

fingers crossed*

Thanks again for the information,

Sean

 

That's correct. The uC cannot handle great amounts of current going through it. You can drive a LED from a uC, but I would recommend to use a resistor just for protection. Additionally, anything that draws more current than a LED, I would not use the uC to directly power it, a la the motor. So for motors, use a H-bridge or motor controller.

If you want to drive a lot of LEDs, you may want to look in transistors rated to the Voltage and Amperage you are looking at, or LED Drivers (though they are more expensive). If you still have any questions, let us know.

 

Aegistalons, I think I'm good with respects to V, I, and R and microcontrollers now. Thanks again for the quality help!