Thread: OHM's Law and LED's

Yes.
Current should increase...
Validates Ohm's Law.
(You've got it ronng... )

V= RxI

I = V/R

R=V/I

Or just cheat and go here... Ohm's Law Calculator

V = Voltage

I = Current

R = Resistance

Some folk are more used to "E" instead of "V". - depends on your age.....

Currant = watts divided voltage. Raise the voltage and the currant should drop. A 100 watt globe at 12v uses 8.3 amps. At 14v it should use 7.1 amps.

Dave.

Originally Posted by Mr Whippy

I have installed a strip of LED lights in the rear of my Disco. When I measured the currant draw I found that with the engine running and the voltage higher, the amp draw also went up.

Engine off and battery voltage around 12.65v the draw was 440 mil amps
Engine running, voltage 13.9 and the currant was 570 m.amps.

This defies OHM's law. As the voltage increases the currant should decrease.
Can anyone explain this?

Dave.

Not at all. You are assuming things! Increase the voltage across any ordinary resistance and the current increases too.

Ohms Law determines the relationship between DC current, voltage and resistance at any given moment. It does not therefore follow that a resistance as calculated remains constant for different voltages applied. Your LED lights exhibit non linear resistance depending on the forward voltage, therefore your current goes up faster than the voltage (percent wise).

Now if you had a smart LED driver circuit the input current of your light assembly would indeed have a lower current draw at a higher voltage as the "switch mode" circuit reduces input current at higher voltages to maintain a constant output current to the LED's. That is how all multi-volt circuits operate.

Originally Posted by Mr Whippy

Currant = watts divided voltage. Raise the voltage and the currant should drop. A 100 watt globe at 12v uses 8.3 amps. At 14v it should use 7.1 amps.

Dave.

Another one! A 100W globe draws 100W power only at it's rated input voltage. Raise its input voltage and the power input rises too. That is why you get brighter lights with relays, high output alternators etc. by reducing voltage loss to the bulb circuit.

Um... lets use a water analogy.

For Voltage, lets use 'Pressure' ... psi, kPa - whatever. its the 'Push'.-factor or 'how Strong' is the pressure

For Current, instead of AMPS or MilliAMps, use 'Flow', litres per minute or drops of water per hour, whatever! - Its the number of, over a period of time.

For Resistance, imagine a 'Flow Restrictor', one of those $15+ inserts you put in the HOt water line of a teen-ager's bathroom.... so they don't use it all... - (I just bash the pipe flatter.... makes a SMALLER pipe...harder to get through so it slows down..)

Now, the LEDs have a set resistance or 'restriction of Flow' This is not going to change, (for the purpose of our example) unless we do Nasty Things to them...

So, when the 'push' gets a bit more enthusiastic - engine/alternator running - then with this extra strength... its gunna shove MORE water through the SAME 'restrictor' - in this case the LEDs.

= more 'litres per minute' = more milliamps in your case.

Simple.

Don't believe me? Just try out every on-line calculator on the net...

Naturally, we're talking about DC, electrons flowing in ONE direction, into a simple flow-restrictor. (Resitance)

Or looking at it another way, you have assumed that the wattage is the fixed value. But it is not. It is the resistance that is fixed (well, it is not exactly correct, but that is the closest to fixed and is near enough to solve your confusion).

As indicated above, once we agree that the resistance is what is fixed, increasing voltage will increase current, and increase watts consumed, as well as brightness of the LEDs, at least until they go 'pop'. The wattage of the LEDs is only correct at the design voltage (which should be 13.8).

John

In reality, an LED behaves as a fixed voltage opposing the applied voltage, plus a fixed resistance, unless it has a switch mode driver, in which case it probably does exhibit constant power use, which yours clearly does not. In contrast to this, and incandescent bulb exhibits only resistance, but the resistance (for a tungsten filament, universal these days) increases with brightness, also apparently failing to conform to Ohm's law.

Apologies, but I can't resist this...

"...This defies OHM's law. As the voltage increases the currAnt should decrease. ..."

- maybe too many sultanas...

sorry

Originally Posted by Mr Whippy

I have installed a strip of LED lights in the rear of my Disco......

Engine off and battery voltage around 12.65v the draw was 440 mil amps
Engine running, voltage 13.9 and the currant was 570 m.amps.


Dave.

Dave, Are they still ok ? Are they designed for a 12 volt system ? If they're running cool, and the smoke is not escaping... then All Is Well.!