|
Home > Archive > Electrical Engineering > May 2006 > How should I measure half-wave rectified current?
You are viewing an archived Text-only version of the thread.
To view this thread in it's original format and/or if you want to reply to
this thread please [click here]
| Author |
How should I measure half-wave rectified current?
|
|
| itsme.ultimate@gmail.com 2006-05-05, 8:21 am |
| X-No-Archive: Yes
If I'd like to monitor the actual rms current going into the battery
from a battery charger using a series resistor and a single diode, how
should I approach it?
I've tried my generic true RMS DMM in both DC and AC modes and both
settings registers different readings and neither is consistent with
what I expect. I place a known, accurate shunt and a resistive load in
series with the diode. By measuring the peak voltage at shunt, I know
that Vrms should be 1/sqrt2 x peak, but my DMM doesn't reflect that.
The DMM actually doesn't register anything in true RMS AC mode with a
true DC, yet DC mode wouldn't register correctly uni-directional,
rippled voltage.
| |
|
| itsme.ultimate@gmail.com wrote:
> X-No-Archive: Yes
>
> If I'd like to monitor the actual rms current going into the battery
> from a battery charger using a series resistor and a single diode, how
> should I approach it?
>
> I've tried my generic true RMS DMM in both DC and AC modes and both
> settings registers different readings and neither is consistent with
> what I expect. I place a known, accurate shunt and a resistive load in
> series with the diode. By measuring the peak voltage at shunt, I know
> that Vrms should be 1/sqrt2 x peak, but my DMM doesn't reflect that.
The RMS value of a sine wave is 0.707 x peak, but you're looking at a
half-wave rectified waveform: one hump instead of two per period ... lower
RMS value.
The charging current waveform will be even less sinusoidal: zero until
the source voltage exceeds battery terminal voltage by a diode drop, then
a pulse of current that tails off to zero when source voltage drops below
battery voltage again.
>
> The DMM actually doesn't register anything in true RMS AC mode with a
> true DC, yet DC mode wouldn't register correctly uni-directional,
> rippled voltage.
"True DC" has zero AC component, so its true RMS AC value is zero.
A "unidirectional rippled voltage" has a non-zero DC component as well
as AC harmonics. I would expect the meter to show (probably different)
non-zero RMS values for "DC" and "AC" modes.
| |
| AverageGuy 2006-05-12, 12:21 am |
| If you know the maximum voltage, then the RMS current for that waveform
is one half of the sine wave.
The half wave rectifier output current looks like a sine wave, but only
half the time, the other half of the sine wave is set to zero -
therefore the RMS current is one half of what you would expect from the
full sine wave.
| |
| stevenal 2006-05-12, 1:21 pm |
|
"AverageGuy" <tangee333@yahoo.com> wrote in message
news:1147402394.788652.167040@q12g2000cwa.googlegroups.com...
> If you know the maximum voltage, then the RMS current for that waveform
> is one half of the sine wave.
>
> The half wave rectifier output current looks like a sine wave, but only
> half the time, the other half of the sine wave is set to zero -
> therefore the RMS current is one half of what you would expect from the
> full sine wave.
>
Try again, this time square the two currents in the period before averaging
and taking the square root.
|
|
|
|
|