Multimeter reads higher current at LED than at battery terminal

Thread Starter

elizjohansen

Joined Sep 12, 2018
5
Dear forum,

I am trying to characterize the battery requirements for an existing battery-powered lighting product. Specifically, I am trying to figure out how much current the LED uses when at maximum setting, v. the rest of the electronics.

I inserted the multimeter in series at the positive battery terminal and got a result of 1.3 A. Then I inserted the multimeter in series at the red wire exiting the LED and got a result of 1.45A.

I am primarily a mechanical engineer and product designer with a very limited electronics background. Based on my limited understanding the current at the LED should have been less than or equal to the current at the positive battery terminal.

Can anyone suggest what I might be doing wrong?
 

Thread Starter

elizjohansen

Joined Sep 12, 2018
5
I should also mention, I put the multimeter in voltage measurement mode and measured voltage across the LED. This changed from 1.5 to 3.5 V as I slid the linear potentiometer to change the LED illuminance. However, the LED forward voltage according to its spec sheet is 3.0 - 3.5V. What am I misunderstanding? Is this a measurement error? A good measurement but concept misunderstanding? Or a poorly-designed circuit?
 

paulktreg

Joined Jun 2, 2008
833
I don't know your battery voltage but let's say 12VDC so at 1.3A that's 12V x 1.3A=15.6W.

There will be a LED driver circuit that drops the output voltage and as long as the input power available is 15.6W at 3.5V that will be, without losses, 15.6W/3.5V=4.4A. The voltage can be stepped down whilst stepping up the current so your readings are feasible.
 

KeithWalker

Joined Jul 10, 2017
3,063
What you describe does not make any sense.
Can you post a photo of what you are doing?
Your voltage measurements across the led suggest that the intensity is being controlled by pulse width modulation which varies the duty cycle of of pulses which are applied to the led. This could explain the strange current readings you are getting at the led terminal. Your ammeter is calibrated to read steady DC current rather than pulses of current.
 
Last edited:

ebp

Joined Feb 8, 2018
2,332
A photo of the circuit would help. The current difference suggests a "buck" switchmode converter. A buck converter produces output at lower voltage and higher current than the input.
 

Thread Starter

elizjohansen

Joined Sep 12, 2018
5
I don't know your battery voltage but let's say 12VDC so at 1.3A that's 12V x 1.3A=15.6W.

There will be a LED driver circuit that drops the output voltage and as long as the input power available is 15.6W at 3.5V that will be, without losses, 15.6W/3.5V=4.4A. The voltage can be stepped down whilst stepping up the current so your readings are feasible.
I
 

Thread Starter

elizjohansen

Joined Sep 12, 2018
5
I don't know your battery voltage but let's say 12VDC so at 1.3A that's 12V x 1.3A=15.6W.

There will be a LED driver circuit that drops the output voltage and as long as the input power available is 15.6W at 3.5V that will be, without losses, 15.6W/3.5V=4.4A. The voltage can be stepped down whilst stepping up the current so your readings are feasible.
Thank you Paul - this really helps. I think the current (A) measurements are correct.

The battery is 8.1V based on multimeter measurements (spec sheet says 7.5V). Now I am only mystified by the voltage across the LED varying from 1.5-3.5V as the current at LED varies from 0.3 - 1.45A. Any thoughts on whether this should be possible?
 

BR-549

Joined Sep 22, 2013
4,928
You can trade voltage for current, and current for voltage. As has been asked several times.....show your circuit. Are you using a LED driver of some sort?
 

Thread Starter

elizjohansen

Joined Sep 12, 2018
5
Your voltage measurements across the led suggest that the intensity is being controlled by pulse width modulation which varies the duty cycle of of pulses which are applied to the led. This could explain the strange current readings you are getting at the led terminal. Your ammeter is calibrated to read steady DC current rather than pulses of current.
Great thought. On highest intensity setting when we would assume the PWM is 100%, we get the expected forward voltage of 3.5V per spec sheet. At lower intensity settings, voltage dips below the spec sheet’s lowest forward voltage of 3.0 because of PWM duty cycle less than 100%.
 

ebp

Joined Feb 8, 2018
2,332
LED driver circuits often employ two control methods. Conversion from the supply voltage to a constant current to drive the LED using a high-frequency (tens of kilohertz) switchmode circuit is more or less "universal" in high-efficiency drivers. Constant current is used because it is best suited to the nature of the LED which acts as a not-very-stable constant voltage drop, which makes regulation by controlling the voltage directly difficult. Sometimes dimming is accomplished by changing the current, however this is sometimes a bit difficult, depending on the circuit design. Dimming is often done with low frequency (typically low hundreds of hertz) pulse width modulation (PWM). The driver is either fully on or fully off. If it is on for 25% of the time (25% or 0.25 duty cycle), for example, the average light output will be about 1/2 (one 'stop" in photography terms) what it would be if on for 50% of the time which is about half of what it would be if on continuously. As long as the PWM frequency is high enough, the light is perceived to be continuous. The average voltage across the LED is then proportional to the duty cycle. For example, a white LED would typically be about 3 V when on. At 50% duty cycle a meter which responds to the average voltage would show about 1.5 V.
 
There may also be problems with "metering". There are TRUE RMS meters which typically have some limitations. Most meters are "average responding", RMS reading for low frequency sine waves only.

The RMS value is the same DC value that would dissipate the same power across some resistor. So, 120 VAC and 120 DC across a 10 ohm resistor would dissipate the same power.

When the waveforms are not withing the measuring capability of the meter, you have problems.
There is a mathematical definition of RMS or Root Mean Square.
 

KeithWalker

Joined Jul 10, 2017
3,063
Thank you Paul - this really helps. I think the current (A) measurements are correct.

The battery is 8.1V based on multimeter measurements (spec sheet says 7.5V). Now I am only mystified by the voltage across the LED varying from 1.5-3.5V as the current at LED varies from 0.3 - 1.45A. Any thoughts on whether this should be possible?
As I mentioned above, the led is probably being driven by a pulse width modulated signal. At full intensity the on period of the duty cycle will be almost 100% which gives you 3.5 volts. The intensity is lowered by reducing the "on" time of the modulated signal. Your voltmeter is too slow to follow the pulses so it give you an average value.
PWM.jpg PWM.jpg
 

BobTPH

Joined Jun 5, 2013
8,813
As an aside, if the battery is 8.1V and the device is drawing 1.3A and the LED is 3.5V at 1.45A, the the efficiency is:

1.45 * 3.5 / 1.3 * 8.1 = 0.48 or 48%

It this device is only running the LED that is pretty bad efficiency. If it is doing something else requiring about the same amount of power, it is OK.

Bob
 
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