electronic rookie here.
i driving LEDs with PAM2861 pulse width modulated chip.
how can i measure actual current going through LEDs?
i know what it says is supposed to be going through according to current set resistor but i want to be sure.
thanks

 

Ohm's law. V/R=I. Whatever voltage you are supplying is divided by the current limiting resistor value in ohms in order to get ~20 mA in most cases.

 

 

electronic rookie here.
i driving LEDs with PAM2861 pulse width modulated chip.
how can i measure actual current going through LEDs?
i know what it says is supposed to be going through according to current set resistor but i want to be sure.
thanks

You need to know a few values… the resistor value and the voltage drop across the resistor.

If you can see the “current set resistor”, read the color bands to get it’s value.

Using a DVM place the probes on the resistor leads. With the power on and the LED illuminated, read the voltage. This is the voltage drop.

The rest is just ohms law.
Current = voltage drop divided by resistance
(I = V/R)

 

electronic rookie here.
i driving LEDs with PAM2861 pulse width modulated chip.
how can i measure actual current going through LEDs?
i know what it says is supposed to be going through according to current set resistor but i want to be sure.
thanks

how can i measure actual current going through LEDs?

The current flowing through a circuit is the same no matter where you test for it. While there are methods for measuring the current such as examples above, one other way to test current is to use a multi-meter. Simply set it to read current and place it in series with the circuit. You will get a readout of what the current through the LED, through the resistor and even through the battery (if battery powered). IF you're flowing 20mA through the circuit it will be 20mA no matter where you test for the current.

12VDC at 20mA means you have a resistance value of 2.4KΩ. 12 / 0.02 = 2,400.

 

how can i measure actual current going through LEDs?

Since the LEDs are being driven with PWM, it's going to change.

You didn't post a schematic so we're being asked to make educated guesses. If there's a current limiting resistor, you can measure the drop across it with a DVM and that'll give you some average current reading. If you want a better answer, post a schematic.

 

Post #3, RS is the current sense resistor and the IC will try to keep 0.1V across it.

 

The current flowing through a circuit is the same no matter where you test for it. While there are methods for measuring the current such as examples above, one other way to test current is to use a multi-meter. Simply set it to read current and place it in series with the circuit. You will get a readout of what the current through the LED, through the resistor and even through the battery (if battery powered). IF you're flowing 20mA through the circuit it will be 20mA no matter where you test for the current.

12VDC at 20mA means you have a resistance value of 2.4KΩ. 12 / 0.02 = 2,400.

This is only valid for a series circuit. That's not the case here -- the TS is using a PWM driver circuit. He needs to be careful to measure the current using a component that is in series with the LED and to do so in a way that does not significantly impact the circuit.

 

Just put an amp meter in series with the power supply or battery for pete's sake.

 

electronic rookie here.
i driving LEDs with PAM2861 pulse width modulated chip.
how can i measure actual current going through LEDs?
i know what it says is supposed to be going through according to current set resistor but i want to be sure.
thanks

What, exactly, do you mean by measure the actual current through the LEDs? Do you mean the average current? Or are you wanting to see the instantaneous current, which is pulsing due to the PWM and inductive kick nature of the circuit.

What current level are you setting things for? The current sense resistor tends to be pretty small, so it can be difficult to get a good reading on it (the voltage across it is only going to be about 100 mV on average, but the variations could be significant, particularly for low duty cycles). Measuring voltage across Rs should not disturb the circuit, but be careful to NOT try to insert a current meter in series with Rs -- the resistance of the current meter will almost certainly completely change the circuit's behavior. You could put the meter in series with the LEDs and this probably won't affect the behavior too much. You can also put a larger sense resistor in series with the LEDs and as long as it isn't too large, the current will likely be about the same.

If you use a voltmeter or ammeter, be sure to use a DC setting, otherwise it will likely be trying to display the RMS value of the waveform based on some assumptions about the shape of the waveform. But, in either case, there can be an issue with the bandwidth of the meter in use.

The best way to make the measurement is with an oscilloscope.

 

What, exactly, do you mean by measure the actual current through the LEDs? Do you mean the average current? Or are you wanting to see the instantaneous current, which is pulsing due to the PWM and inductive kick nature of the circuit.

What current level are you setting things for? The current sense resistor tends to be pretty small, so it can be difficult to get a good reading on it (the voltage across it is only going to be about 100 mV on average, but the variations could be significant, particularly for low duty cycles). Measuring voltage across Rs should not disturb the circuit, but be careful to NOT try to insert a current meter in series with Rs -- the resistance of the current meter will almost certainly completely change the circuit's behavior. You could put the meter in series with the LEDs and this probably won't affect the behavior too much. You can also put a larger sense resistor in series with the LEDs and as long as it isn't too large, the current will likely be about the same.

If you use a voltmeter or ammeter, be sure to use a DC setting, otherwise it will likely be trying to display the RMS value of the waveform based on some assumptions about the shape of the waveform. But, in either case, there can be an issue with the bandwidth of the meter in use.

The best way to make the measurement is with an oscilloscope.

average current.
i'm using 0.1 ohm which should equal 1 amp through the LEDs.
the power supply shows 0.6 amp being used. Does that sound right for PWM circuit?
so i need to cut the circuit to measure current?
thanks

 

Just put an amp meter in series with the power supply or battery for pete's sake.

pete, i don't think that works for pwm circuits? it does work for linear drivers.

 

pete, i don't think that works for pwm circuits? it does work for linear drivers.

Your amp meter in the power supply is showing the average current the same if you installed an external amp meter

 

Your amp meter in the power supply is showing the average current the same if you installed an external amp meter

the current sense resistor is set to run 1 amp through the circuit. the power supply is only showing 0.6 amp. that's why i want to verify what's actually going to LEDs.

 

Install an external amp meter and see if the reading is the same as the power supply.

 

the power supply is only showing 0.6 amp. that's why i want to verify what's actually going to LEDs.

Where else can the current go (you haven't provided a schematic of your circuit)?

 

the current sense resistor is set to run 1 amp through the circuit. the power supply is only showing 0.6 amp.

Actually that's probably correct. One amp peak current with an average reading of .6 amps

 

Actually that's probably correct. One amp peak current with an average reading of .6 amps

But the intent of the driver is to produce an average current of 1 A when the the sense resistor is 0.1 Ω.

From the device data sheet: "The PAM2861 includes the output switch and a high-side output current sensing circuit, which uses an external resistor to set the nominal average output current."

 

pete, i don't think that works for pwm circuits? it does work for linear drivers.

Same principle as measuring the voltage across a current limiting resistor. You'll get some average current reading, but a current meter will likely perturb the and give bogus readings.

 

the current sense resistor is set to run 1 amp through the circuit. the power supply is only showing 0.6 amp. that's why i want to verify what's actually going to LEDs.

A quick sanity check is to take your power supply and connect it to a resistor (or several resistors in parallel to spread the heat) such that it will draw about 1 A and see what the supply's meter shows.

If it shows about 600 mA, then you know the problem. If it shows 1 A, then it might still be a problem with the supply and/or meter.

One thing that might be causing a problem would be if the supply can't deliver the peak current that the circuit needs. The LED driver works by pumping up the voltage using an inductive kick from the inductor and the result is that you get pulses of current that are well above the 1 A target and that are spaced out so that they average out to 1 A. If the supply can't provide those peak currents, then the driver can't meet the target and it's just doing the best it can.

What voltage is your supply set for and how much current is it supposed to be able to deliver?

Do you have a suitable capacitor across the supply terminals at the driver chip? What size and kind? It needs to be a low-ESR capacitor.

What value inductor are you using?