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Topic: What are the typical current drawn by these components? (Read 645 times) previous topic - next topic

kurtselva

Apr 19, 2013, 07:35 am Last Edit: Apr 19, 2013, 07:40 am by kurtselva Reason: 1
Hi guys,

As the title says, I need to know what are the typical current drawn by the following components:

1) Red LED  (1.5mA?)
2) Small analogue clock running on 1.5V battery
3) Servo motor
4) LCD display with no backlight (1mA?? measured the current at the Vcc)

I have measured some of the components (those with values in bracket) and would like to check if they are normal, while the other components I am not able to measure.

kurtselva

And when I measure the backlight alone for the LCD, it is ard 6mA.. acceptable value?

Grumpy_Mike

They all look a little light. Typical values would be.
1) 20mA
2) naff all
3) 500mA
4) 20 mA
A back light from an LCD screen is about 40mA

kurtselva

Hmm.

For the led, used a multimeter to measure the current thru the anode leg of the led. Held the probes in series to check. To confirm the value, I connected a resistor to the led and measured the voltage between the resistor. And used V=RI to calculate. Both, I got the same value.

For the LCD, I connected the probe in series to the power pin of the LCD.

Anyway, in the multimeter itself, I connected one probe to the ground and 1 to a pin labelled "300mA".
So the current value read in the display is taken as it is? Or do I have to do further calculations?

kurtselva

When I measure the current thru a LED alone, I get ard 20mA.

But when I measure it on a circuit that I built, it gives me 0.48mA now... Is that reasonable?
There is a resistor connected to the LED..

Grumpy_Mike

It is hard to know what you are doing but an LED with only 0.48mA through it would be very dim indeed. So no your measurements on the face of it do not sound reasonable.
When you measure 20mA through the LED, is there a resistor and what voltage are you applying to it?

What sort of meter do you have?
Does it have a battery and is it in good condition?

kurtselva


It is hard to know what you are doing but an LED with only 0.48mA through it would be very dim indeed. So no your measurements on the face of it do not sound reasonable.
When you measure 20mA through the LED, is there a resistor and what voltage are you applying to it?

What sort of meter do you have?
Does it have a battery and is it in good condition?


when measuring 20mA, there is no resistor and voltage applied is around 2V.

I am using this meter: http://www.myflukestore.com/p1348/fluke_73-iii.php

The meter looks like it should be able to read current properly, as I am able to get 20mA when I connect it in series with the LED alone.
But I am confused with the low readings I get for the LED in connection to a resistor and the LCD..

oric_dan

You have to use a resistor in series with the Led to set the current.

Also, the newer high intensity Leds will be quite bright even at low currents, below
5 mA. I just tried an RGB Led that puts out about 3000-5000 mcd, and the Leds are
well lit with just 1 mA of current, 2.7K series-R from 5V.

tack

#8
Apr 20, 2013, 10:42 am Last Edit: Apr 20, 2013, 10:44 am by tack Reason: 1
Current for an LED can vary widely. Full current would usually be around 20mA but you don't always have to use full current, in fact lower current is usually perfectly adequate to suit the brightness you require.

Certainly more modern LEDe can give adequate brightness with quite low currents.

For example, I have a sentry gun controller board I made where I use Blue, Yellow and Red 3mm LEDs for the mode indicators (fire, comms and reload). With 5V I use 4k7 resistors, providing for around 0.4-0.5mA on the various colours (they have different Vfd's).

On the same board I have 5v and 9v outputs to servos, with power health indicator LEDs on he regulator outputs. I don't want these as bright, just enough to indicate they are on. With 5V year are on. 10k resistor. With 9V they are on a 22k resistor. This give about 0.25-0.3mA through 3mm Green diffused LEDs.

These give me the brightness I want and, as you can see, are well below the 20mA maximum the data sheets quote.

So, typical LED current for an SMD or 3mm or 5mm could be anything from about 0.25mA to 20mA.

Grumpy_Mike

Quote
when measuring 20mA, there is no resistor and voltage applied is around 2V.

Sorry but all that proves is that your meter will display 20mA when something flows through it. Having no resistor means that the current through the LED could be anything. See this on why you need a resistor with an LED.
http://www.thebox.myzen.co.uk/Tutorial/LEDs.html

The meter needs to be in seriese with the circuit. Try this.
300mA socket on the meter to 5V. COM socket on the meter to a 1K resistor. Other end of the resistor to ground.
Meter knob on the 300mA setting. You should then see a reading of 5mA if you do not then replace the battery in your meter.

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