Hey guys
I wired up this circuit:

(A DMM series with an LED to know how much current it draws)

At first when I connect the LED and turn on the circuit it starts to draw 30mA and it goes down. IIRC I have the same situation measuring the current other devices draw. is it normal? why does it happen?

 

What is your power source?
If it's a battery, it may just be weak.
Measure the battery voltage when you do the test.

 

What is your power source?
If it's a battery, it may just be weak.
Measure the battery voltage when you do the test.

Yes, sounds like you are right.
I measured the pole of that 9v battery you are seeing in the picture and it was 3.38v and it starts to go down 3.37v,3.36v,3.35v,... about each 2-3second.

 

Have you got a resistor in series with your LED? Or did you simply connect the LED directly across the battery?

 

Have you got a resistor in series with your LED? Or did you simply connect the LED directly across the battery?

No, there is no resistor in series with the LED.

 

Hello,

No, there is no resistor in series with the LED.

Then you are using the internal resistance of the battery.
It is not good to use leds directly on batteries.
A current limiting resistor is a must, as leds are current driven elements.

Read this page for more info on the leds:
http://electronicsclub.info/leds.htm

Bertus

 

So the LED current is being controlled by the internal resistance of the battery. This resistance quickly rises as the battery discharges. As the resistance rises, the current falls.

Normally, you would connect a resistor in series with the LED. This would set the LED operating current to be much lower, extending the life of the battery.
Also, as this resistance is in series with the internal resistance of the battery (and hopefully much larger) the internal resistance has much less effect on the current.

Check out Ohms Law!

 

Several question:
1- The battery is 9v but when the led was connected to the battery and I measured the voltage, it showed me 3.37v. why? shouldn't it be 9v?
maybe because the battery is a current source. is it correct? if not, then are all of the supplies like this?
2- Do all supplies have internal resistor? I take a glance here but it was about batteries.

 

No, there is no resistor in series with the LED.

Then what is there to limit the current through the LED and meter? Nothing, except the ability of the battery to deliver current-- which, in this case, is pretty weak (and is probably all that prevented you from destroying the LED and/or blowing the meter's internal fuse).

ALWAYS put a resistor in series with an LED when driving it from a voltage source such as a battery or a power supply. The value of the resistor should be chosen to pass an appropriate amount of current (10 mA is a good figure) given the power supply voltage and LED forward voltage drop.

 

Several question:
1- The battery is 9v but when the led was connected to the battery and I measured the voltage, it showed me 3.37v. why? shouldn't it be 9v?
maybe because the battery is a current source. is it correct? if not, then are all of the supplies like this?
2- Do all supplies have internal resistor? I take a glance here but it was about batteries.

1. Because the LED is an electronic device. It is not a constant resistance. As the voltage across it increases above 3.37V the current it conducts increases very rapidly with only a small an increase in voltage. In other words, it's resistance becomes very low above 3.3V. Below 3.3V it's resistance is much higher.

2. All power supplies (of any type) have an internal resistance. Your small 9V battery has quite a high internal resistance. (Which was lucky - otherwise your LED would have gone POP very quickly!). For example: a 12V car battery has a much lower internal resistance. Good thing you did not connect your LED across that as it would have been instantly destroyed due to the very large current that would have flowed.

 

Hello,

The batteries will have internal resistance.

Battery | Type | Internal Resistance (Ohms)
9V | Zinc-Carbon | 35
9V | Lithium | 16-18
9V | Alkaline | 1-2
AA | Alkaline | 0.15
AA | NiMH | 0.03
D | Alkaline | 0.10
D | NiCad | 0.009
D | Lead-Acid | 0.006

Note: internal resistances shown above are at full charge and room temperature.

The table is from this page on leds:
http://www.gizmology.net/LEDs.htm

Bertus

 

A picture is better than words. I have drawn a diagram to try and explain to the thread starter why he only measured 3.37V across his "9V" battery. All to do with the internal resistance of the battery. And good old Ohm's Law!

Think of the battery as a perfect 9V supply in series with its 35 ohm internal resistance:-

 

Hello,

The batteries will have internal resistance.

Battery | Type | Internal Resistance (Ohms)
9V | Zinc-Carbon | 35
9V | Lithium | 16-18
9V | Alkaline | 1-2
AA | Alkaline | 0.15
AA | NiMH | 0.03
D | Alkaline | 0.10
D | NiCad | 0.009
D | Lead-Acid | 0.006

Note: internal resistances shown above are at full charge and room temperature.

The table is from this page on leds:
http://www.gizmology.net/LEDs.htm

Bertus

???
9 V (alkaline?)
Measures 3.7 V at 30 mA. Internal resistance is about 170 ohms in his case.

 

Two question:
1- Why don't we a resistor in series to supply other devices? transistors, Op-Amps, microcontrollers,...
2- Should I use resistor in series to other devices while I'm measuring its current?

 

Hello,

@hp1729
The table shows fresh charged batteries at room temperature.
The values will likely be typical values.
There will also be differences between brands and status of the batteries.

Two question:
1- Why don't we a resistor in series to supply other devices? transistors, Op-Amps, microcontrollers,...
2- Should I use resistor in series to other devices while I'm measuring its current?

1) transistor DO need resistors in the circuits.
Opamps and microcontrollers are active devices that will have a rated power supply voltage.
The datasheets of the devices will tell you the range.

2) It depends on the device.
As said opamps and microcontrollers will have a rated power supply range.

Bertus

 

1. Because some things have a sufficiently high resistance that the they don't need extra resistance to control the current.
2. No, because adding resistance will change the value of the current.

Current is a difficult thing for beginners to understand. The value of the current depends on the resistance of the circuit (and the driving voltage). You can't say that a battery (for example) has a "current" in itself - It always depends on what is connected to it.

Think of current like the amount of water flowing in a pipe. The flow depends on many things: diameter of the pipe, how much the valve at the end of the pipe has been opened and the pressure (the head of water) driving the flow along.

 

1) transistor DO need resistors in the circuits...

Ops! I think I made a ridiculous mistake.

 

...Current is a difficult thing for beginners to understand...

Do you believe if I tell you I have about 4 years experience in electronic and this is the first time I'm asking this question!

 

No probs. The person that never made a mistake never learned anything!

 

Another question,
LED is a diode, right? it has a voltage drop, doesn't it?(between 0.3 - 0.7) What does drop voltage across a diode? resistor? if so, then why we use a resistor in series with it?