Measuring pulsing DC current with multimeter

[Dave97]

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?

 

[Thomas Gallenkamp]

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 latchcontinuous 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.

Thomas

 

[Fred Bloggs]

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?

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.

View in a fixed-width font such as Courier.