hi
how do you or can you easily measure the actual forward voltage and amps right at the led emitter using a standard DMM?
Forward voltage is easy. Put your DMM Leads on the two connection points of the LED with your DMM in DC Volts mode. It will display the actual Vf of the LED while in operation.
Actual amps, you must get your DMM in-circuit with the LED - and then your DMM’s resistance is going to effect the reading. If you can get your DMM in series with the actual LED emitter though - you’ll be able to put your DMM in DC Current mode and get an actual reading of what the LED is consuming for current.
PPtk
Just curious, is there any way to measure DC current using its electromagnetic inductance?
I have the inductance current meter for AC, works ok (just need to get general idea of the amperage, nothing accurate). I wonder why there's none of these for DC.
I mean, both AC and DC current induce EM... why can't we read the DC one?
We can. Using a device called a current clamp. Not quite as accurate as shunt-based measurement, but they have the benefit of not interfering with the operation of the circuit.
Here’s an example.
NICE!!
Thanks PPtk. I'll keep a lookout for these next time I go to the electronics store.
They are not very good at low currents (Below a few A).
Thanks for all the info
I am not sure what you mean by getting the DMM in-series with the led?
They’re actually not bad… The fluke I linked to, for instance, is at it’s most accurate in the lower current ranges…
Basic Accuracy (DC to 1kHz):
100mV/A setting: 50mA to 10A /- 3% of reading 50mA
10mV/A setting: 50mA to 40A /- 4% of reading 50mA
40A to 80A /- 12% of reading 50mA
80 to 100A +/- 15% of reading
at 1 Amps of real current, it’s 3% and 50mA of max error; meaning it will measure between .92A and 1.08A
at 3 Amps of real current, between 2.86A and 3.14A
Those numbers are probably far more accurate than shunt based measurement in this hobby since the small amount of additional resistance introduced by the shunt(DMM+probes) likely interferes with the driver output levels by far more than 3%
And, there are clamps available with far higher accuracy at low current levels. They’re not inexpensive, but they are available.
PPtk
Meaning you need to chop off the wire to the LED, bare the wire, and connect the DMM in between. That's how you measure current. It's destructive, you have to repair the connection back after you're done.
Another way is estimating the current using the electromagnetic field revolving around the wire, which we're discussing. It's supposedly not as accurate, but you don't even have to strip the wire, just put the sensor clamp on it. (I never found ammeter clamp for DC current around here which is why I was asking about it.)
I have 5 currents clamps, 3 designed for lower currents and I am not impressed with any of them. Just turning them slightly can change the reading.
They are very convenient for some measurements, when you want a low voltage drop and for higher currents they are often the best solution.
Actually using the DMM in serie is a wrong way to measure the current. The DMM cables will behave as a resistor in the circuit and lower the current a lot.
What you have to do is take a very little value resistor, like < 1 ohm. Put it in serie in the circuit with the LED, and use the DMM in Volt mode to measure the voltage at the resistor.
Then using U = RI, I = U/R, you will determine precisely the current.
“<1 Ohm” still sounds far too much 
At 3A that would drop 3V…
Go for 0.01 Ohm. Difficult to find the with good precision and for low budget.
OR, just use thick fat cable as lead, as most of us do when measuring current. That's a simpler solution to that problem.
There’s a lot of good and bad info in this thread…
First, if your driver does PWM, you may not get a good reading. The pulsing current is easily mis-interpreted by a lot of volt/current meters.
It looks like that Fluke current clamp is spec’d to 1 kHz… some PWM’s are beyond this value. I use a Tektronix current probe rated DC to 50 MHz. New price on a probe/amplifier/power module can be several kilobucks though. They are also meant to be used with an oscilloscope. If you want the best possible current reading, this is probably the best way to go.
For most people, forget using a 0.01 ohm resistor. The resistor lead resistance and connection resistance can easily be much more than that value. You will get a bad reading. You need a multi thousand dollar meter and Kelvin clips to find where on the resistor leads you get 0.01 ohms. And 3 amps across 0.01 ohms is a 30 millivolt reading. Again, you need a high dollar meter to measure that accurately.
At low voltages and/or high currents, the voltage drop across the resistance of a multimeter’s internal current shunt can cause gross errors in the reading. Cheaper meters are usually pretty bad about this, but even high-dollar ones can easily mess up reading a low-voltage, high current signal.
The meter lead resistance itself is usually not a problem. The current is the same everywhere in a series connected circuit. The way cheap leads mess up the current reading is the same as how the meter current shunt does.
Very true. Some DMM’s are really good at displaying average current and some really (REALLY) not good.
Not sure where you got 1khz. That Fluke clamp is (from spec page):
Useable frequency: DC - 100 kHz
Also very true. Unless you understand what a kelvin connection is, stick to a DMM in current mode with thick test leads.
True, true and more true. Resistance is resistance to the circuit - whether it be in the Shunt or the test leads is completely irrelevant.
Here’s the run down:
- LOW ohm current shunt: Probably not do-able for the average DIYer who doesn’t have big dollar equipment and a good understanding of electronics
- DMM in current mode: From moderate to horrible for an accurate reading - Every meter will ‘mess with’ the circuit; some a little, some a lot. Different driver’s will be effected differently as well.
- Current Clamp: Accuracy is low at very low currents - doesn’t become acceptable until 100mA or more (depends on the clamp). Clamps can be quite expensive; The fluke I linked to is north of $500, and that’s a ‘budget’ clamp in comparison to some.
So, the real answer is that measuring current is almost pointless. No matter how you do it, you’re going to introduce error by the very nature of the current measurement device being in the circuit. The ‘tailcap’ current measurements that we see being flung around are just barely more than worthless, really… Would that same light still be 3.083763 Amps at the tail if the DMM wasn’t there? Don’t know - maybe… Probably not… Likely not…
WOW - now it looks undoable to measure amps at the led without expensive equipment.
so. is voltage at the led then reasonable to measure with a DMM?
thanks gang
Absolutely. Measuring voltage with a DMM doesn’t interfere with the circuit. A DMM Reading of voltage at the emitter will be quite accurate.
PPtk
P.S. Stay tuned - uploading a video showing some interesting things right now - should be up on youtube in about 15 minutes and I’ll post the link.
This was supposed to be a short video demonstrating the differences between DMM current measurement and Current-Clamp current measurement, but it turned into something not-so-short. Still, I think a lot of you will find it very informative - Give it a look!
PPtk
SUPER VIDEO, THANKS SO MUCH, SHEDS ALOT OF LIGHT !!! ON THE TOPIC!!!