Hi everyone,

I am currently building a small device for my internship, and part of it's functionality has to include a means to measure the current that's passing through a PCB mounted LED.

Since I cannot break down the PCB to connect an ammeter in series with the LED, I tried finding some workarounds. I thought about using two probes, to measure the internal resistance of the LED while the pcb is not hooked up to a source. Then power the PCB, and measure the voltage across the LED and use ohm's law to calculate the current, but this seems like too much of a hassle, and would take quite a bit of time.

Secondly, I tought about using a hall effect sensor, but the current that's drawn by the whole pcb(not just the LED) varies between 100mA and 1.2A, so I am worried that the value of the field will be too small for the sensor to pick up.

Any ideas about how to solve this?

 

Hi G,
Welcome to AAC.
Is there a current limiting resistor in series with the LED.?
E

 

Hi G,
Welcome to AAC.
Is there a current limiting resistor in series with the LED.?
E

Not sure, I'll have to check when I get back to the office tomorrow. But, if there is a current limiting resistor in series with it, how would that help?

 

hi,
Knowing the value of the resistor and the voltage drop across that resistor , you calculate the LED current.
E

I=V/R

 

You cannot determine the current through an LED by measuring the voltage across it.

It's totally non-linear, ohms law does not apply.

 

hi,
Knowing the value of the resistor and the voltage drop across that resistor , you calculate the LED current.
E

I=V/R

So, I checked the pcb, and it does not have a current limiting resistor. The current is limited through an intrgrated circuit on the pcb, that keeps the current value through the LED constant no matter the voltage applied to it.

 

What is the marking on this IC?

 

There's no marking per se, it's just the schmatics of the circuit. And to be honest I don't think I can give you any details about it, since it's marked as confidential, and I don't want to get in trouble.

 

You need to get a datasheet for the IC to understand how it sets the current.

 

The current is limited through an intrgrated circuit on the pcb, that keeps the current value through the LED constant no matter the voltage applied to it.

If the current is constant, what is the point of monitoring it?

Bob

 

There's no marking per se, it's just the schmatics of the circuit. And to be honest I don't think I can give you any details about it, since it's marked as confidential, and I don't want to get in trouble.

It’s time for this approach...

 

It’s time for this approach...

Like mine? It really helps.

 

Those last 2 answers are spot on!
Another TS who cannot share info, but wants answers.

 

I am still wondering why he needs to monitor a constant current.

Bob

 

How about desoldering one of the LED terminals from the pcb and then use a multimeeter between pcb pad and the open terminal to measure the current?

 

hi jipristt,
Unfortunately the TS posted.

Since I cannot break down the PCB to connect an ammeter in series with the LED

E

 

So the only rational answer is that the intern is being pranked. There is no way to measure current and you don't need to do it.

 

Sorry, I skipped this part.. But still, this is more like temporarily altering the board rather than breaking. After measuring, the led can be placed back like it was before.

But besides that, since the led is powered through a constant current source wouldn't mean that bypassing the led (with one of lower Vf or just multimeter probes across its terminals) and measuring the current passing through would still give a correct current value?

 

Sorry, I skipped this part.. But still, this is more like temporarily altering the board rather than breaking. After measuring, the led can be placed back like it was before.

But besides that, since the led is powered through a constant current source wouldn't mean that bypassing the led (with one of lower Vf or just multimeter probes across its terminals) and measuring the current passing through would still give a correct current value?

No. Every led is different, even if it comes from the same lot in the manufacturing process. The current probes themselves are low impedance and will not provide any significant Vf. The IC could do anything with that condition including fry the meter. Instead of throwing stuff at the wall to see what sticks it might be a good idea to actually learn a thing or two. The variation in Vf, leads to small changes in the current draw. This is why the resistor makes such a good current limiter. I'm dubious about the ability of the IC to provide a constant current; normally you would do this with a resistor and those can be difficult to fabricate. It's too bad we can't help you on that one.

 

Yeah, good point with frying the meter!

Then what about having a few small shunt resistors (1, 5 and 10 ohmd or something like that) put paralel with the led one at a time instead of just the probes.
Then measure the voltage accross that and calculate the current as I = V/R. If the current matches all cases the assumption of a constant current source is valid and we have a current value.
If is not a cc source things can go bad for then circuit but with the info provided is all I can think of.