LED help

[biferi]

I have a 24 VDC that will give 4,000 mAh.

And I will be powering a LED that just takes 5 VDC.

I know I will need a Resister in Series with the LED.

Now If I am right the Math to figureout the type of Resister is
E Divided by I = R

Now first I convert mA to Amp by moving the point 3 places to the Left.

So this will make 4,000 mA to 4.000 Amp right?

So my math will be 24 Divided by 4.000 = 166 Ohm Resister am I right?

 

[Ian Rogers]

Nope..... To light an LED you need to give it around 25mA at 3v... So 24v -3v = 21v... Then work out the voltage drop using the current required.. 21v / 25mA = 840.... Therefore a 840 ohm resistor is needed...

Unless the LED has a inbuilt resistor!! as you have specified 5v for the LED..

If the LED is indeed a 5v LED then you'll need to give us the current consumption of said device.

 

[biferi]

I gave wrong figures.

My Battery Pack will give 24 VDC at 4,000mAh
My LED needs 5 VDC.

I am making a device for fun that will powr my Tablet witch needs 5 VDC to run.

So I am going to take a Battery Pack that holds 16 D Cell Batteries in Series so I will get 24VDC at 4,000mAh.

Then I am going to cut of the end of a USB cable that would go to the PC. and connect the Black and Red wires in it to the Black and Red wires on the Battery Pack.

OK so I know my Battery Pack will be giving more power then I need so I was going to conect a Variable Resistor in Series with the Switch.

I would use the Variable Resistor to get the right power.
Now if I want to conect a LED it would have to be in Paralle with the Batter Pack but after the Switch?

 

[JimB]

STOP!!

You will destroy your Tablet.
This is not the way to convery 24v to 5v.
The current drawn from the supply by the tablet will vary, and so the voltage dropped across the resistor will vary. The voltage at the tablet will vary in an undefined manner.

If you really want to do this, you should use a switching regulator to provide a controlled 5v from the 24v supply.
A liniear regulator such as a 7805 or a 317 will be very inefficient and waste most of your battery power as heat.

JimB

 

[Ian Rogers]

To add to Jim's post.... You need quite a large linear regulator.. Standard 7805 in the TO92 packages are only 100mA so you would be better off getting hold of a purpose built adapter.... If you google for a 24v to 5v convertor ( Used in lorries ) this would be a better solution...

 

[alec_t]

Why on earth are you starting off with a 24V stack of D-cells? Five D-cells in series will give you ~7.5V, and you could parallel-connect three strings of five to boost capacity. The 7.5V will allow some working headroom for a linear regulator to get you down to 5V.