|
|
This is only a preview of the April 2021 issue of Silicon Chip. You can view 0 of the 112 pages in the full issue. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "":
Articles in this series:
Items relevant to "Digital FX (Effects) Pedal - Part 1":
Items relevant to "Refined Full-Wave Motor Speed Controller":
Articles in this series:
Items relevant to "High-Current Four Battery/Cell Balancer - Part 2":
Items relevant to "Arduino-based MIDI Soundboard - Part 1":
Purchase a printed copy of this issue for $10.00. |
Follow-up to ‘constant’ AC source
This circuit develops my ideas on
the “infinite impedance” alternating
current source concept, previously described in the December 2020 Circuit
Notebook section (siliconchip.com.au/
Article/14681).
That circuit used a direct digital synthesis (DDS) sinewave generator and
standard op amp to drive the resonant
network. The result is a sinewave at
the output that delivers an essentially constant magnitude alternating current into a resistive load.
To simplify the circuit, I have
ditched the DDS sinewave generator
and I am instead using an LM3900 dual
Norton (current input) amplifier chip.
The circuit snippet below is cribbed
from my October 2019 Circuit Notebook submission (siliconchip.com.
au/Article/12027) describing how to
build a stable sinewave oscillator using a Norton amp, and also gives the
formulas (in the blue box) for the oscillation condition and to derive the
frequency.
For the output resonant circuit, I
had a 0.7mH inductor available. Using the inductance vs capacitance and
frequency charts published in the December 2020 issue, that sets the capacitance required as 70nF (eg, 68nF,
siliconchip.com.au
1.8nF & 200pF in parallel) for a frequency of 22.66kHz.
The oscillator circuit achieves this
frequency with the values shown. VR2
is used to fine-tune the frequency, with
a nominal value of 4kW giving 11kW +
4kW = 15kW to set the frequency close
to 22.66kHz. VR1 sets the amplitude of
the input voltage to the resonant circuit and hence the value of the ‘constant’ current.
IC1b buffers the oscillator’s signal
and then drives a current booster circuit
using NPN and PNP emitter-followers
Q1 & Q2, with their base voltages biased around 0.7V above and below the
oscillator signal by diodes D1 & D2. The
output at the emitter junctions of Q1 &
Q2 drives the resonant circuit that, in
turn, drives the load resistance.
I built this circuit and tested it, and
the results are shown in scope grabs
Scope 1-3. Scope 1 was with a load
resistance of 100W, Scope 2 with 50W
and Scope 3 with 200W. In each case,
the oscillator’s output is the trace plotted in yellow while the voltage across
the load resistance is shown in cyan.
The current waveform leads the
voltage waveform by 90° in all three
test cases, and as expected, the voltage amplitude adjusts to supply the
same current to the load. So in Scope
2, the voltage is halved as the load resistance is halved, while in Scope 3,
it is doubled as the load resistance is
doubled. This is not obvious from the
sinewaves since the channel scaling
changes in each plot; check the scale
values at the bottom.
Mauri Lampi,
Glenroy, Vic. ($75)
Australia’s electronics magazine
April 2021 63
|
