Hello, I am working on an LED project where I am driving 25 LEDs (in series) with full-wave bridge rectified line voltage for a B+ (with current limiting 2k 1W resistor). The circuit's output uses SCRs, so I can't filter the pulsing DC without the SCR latching in an on state.
When I measure the series current with my Fluke 77 in 300mA mode, it reads app 15 mA in DC amps mode and app 12mA in AC amps mode. Again, the B+ is pulsing DC (120 Hz). Is the DC current on the meter accurate, or do I need to do some sort of conversion to get the true current reading?
DC range should give the average _linearly_ integrated current, while AC should give you the average _RMS_ integrated current. The latter is correct only for sinusoidal waveforms as long as you have no true RMS multimeter. Since the performance of the DMM is not specified for non-continuous DC measurements that read values may be more or less inaccurate.
In order to completely characterize the measurement it is necessary to know the Vf of the LEDs. Are these high brightness types with 3.5V/LED making the 25x string voltage 88V for a conduction angle of
180-2*arcsin(88/170)=120o, or are they say more like 1.5V/LED for a conduction angle of 180-2*arcsin(38/180)=150o? This particular type of pulsating current and its frequency should support accurate readings for DC and RMS if the meter displays a steady number, and this is due to the fact that the waveforms are such that the waveform averages sought are identical to the averages obtained by randomly sampling (digitizing) readings across waveform cycles. Regardless of the signal processing used internally by the Fluke, the end result is designed to be in agreement with the waveform average value sought, be that an arithmetic mean corresponding to DC or the square root of the mean of squares corresponding to AC,RMS of the waveform with DC component stripped off. The simplest case is for full wave rectified DC current with full 180o conduction. The extension to partial cycle conduction angle is easily done based on the same method.
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