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Interesting circuit ideas which we have checked but not built and tested. Contributions from
readers are welcome and will be paid for at standard rates.
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Cheap AC current measurement
The easy way to measure high AC currents is to use a clamp
meter but these are generally quite expensive and cost several hundred dollars
at a minimum. Add-on clampmeter adaptors can work well but they only work with
digital multimeters which have millivolt AC resolution. This is because the
output of most clamp adaptors is quite low, 0.1A = 1mV, for example. This is no
good for typical cheap DMMs which have a lowest AC voltage range of 200V.
This circuit can be built into a low cost clamp meter such as
the Digitech QM-1565 from Jaycar Electronics (see 2002 catalog, page 189). When
dismantling this clamp adaptor, remove the label which has the AC range
conversion factors and then undo the two screws gain access to the inside.
The two cross-connected transistors act like low voltage drop
diodes to generate a DC voltage which is proportional to the current in the
primary of clamp adaptor (ie, the circuit under test). The recommended
transistors are power germanium types such as ADZ16, AD162, AD149, ADY16,
2SD471, OC16 and OC28. This approach gives lowest voltage drop and good
linearity, from 10 to 300A. Schottky power diodes can also be used but the
result will not be as linear.
To calibrate, wind 10 turns through the clamp adaptor's jaws
and feed a current of 20A through the winding. This is equivalent to a single
turn carrying 200A. Set the trimpot to suit your multimeter, normally set to the
2V DC range. Do not calibrate for a low current otherwise accuracy at high
currents will be poor.
Gerard La Rooy,
Christchurch, NZ.
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Low-cost dual power supply
This circuit shows how to symmetrically split a supply voltage
using a minimum of parts - one LM380 power amplifier plus two 10μF capacitors.
It was originally published in National Semiconductor's AN69 and provides more
output power than a conventional general-purpose op amp split power supply.
Unlike the normal power zener diode technique, the LM380
circuit does not require a high standby current to maintain regulation. In
addition, with a 20V input voltage (ie, for ± 10V outputs), the circuit exhibits
a change in output voltage of only about 2% per 100mA of unbalanced load change.
Any balanced load change will reflect only the regulation of the source voltage,
Vin.
The theoretical plus and minus output tracking ability is 100%
since the device will provide an output voltage at one-half of the instantaneous
supply voltage in the absence of a capacitor on the bypass terminal. The actual
error in tracking will be directly proportional to the unbalance in the
quiescent output voltage.
An optional 1MΩ
potentiometer may be installed with its wiper
connected to pin 1 of the LM380 IC to null any output offset. The unbalanced
current output is limited by the power dissipation of the package.
In the case of sustained unbalanced excess loads, the device
will go into thermal limiting as the internal temperature sensing circuit begins
to function. And for instantaneous high current loads or short circuits, the
device limits the output current to approximately 1.3A until thermal shutdown
takes over or the fault is removed.
For maximum output power (2.5W), all ground pins (3-5 &
10-12) should be soldered to a large copper area (the LM380 data sheet contains
more details).
National Semiconductor.
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Quick counter for young children
This circuit is a toy to encourage young children to count.
Power is turned on by switch S1, then S2 is closed. This makes nine LEDs flash
slowly. S2 is then opened and the LEDs go out. Pressing pushbutton PB1 turns on
a random number of LEDs - briefly - during which time they are to be counted.
The number counted can be checked by pressing PB2 which turns the same LEDs on
for as long as needed. Then repeat.
The circuit works as follows: IC3 is a 4049 hex inverter
connected as three oscillators running at different rates. It is turned on by
closing switch S2a. The clock pulses from IC3 drive both halves of IC1 and one
half of IC2, both being 4015 dual 4-stage shift registers. Each shift register
has four outputs which go high in order: 1, 1 and 2; 1 and 2 and 3; 1 and 2 and
3 and 4. However as output 4 is connected to the reset line of its own half -
the shift register resets to zero. Outputs 1, 2 & 3 of all three shift
registers are connected to nine LEDs, the cathodes of which go to a common rail.
This rail is connected to ground via S2b when switch S2 is closed.
When S2 is opened the three oscillators stop but a random
number of LEDs is still connected to the high outputs of the 4015s. That number
can be viewed briefly by pressing PB1 which pulses the 7555 timer in monostable
mode, to give a short duration output which drives Q1 and connects the LED
cathodes to 0V. The viewing time is adjustable by VR1.
Checking a count is done by pressing PB2 which holds the same
LEDs on as long as desired.
The LEDs are set in a 3 x 3 grid with the connection scattered,
ie, the first row is not the three LEDs from the first half of IC1. Note that,
unlike the usual dice, a number such as 5 can appear in many formats, so pattern
recognition is no help. Also note that this is not a nine output true dice -
because the numbers do not come up with equal frequency.
A. Lowe,
Bardon, Qld. ($50)
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Switchmode constant current source
As pointed out in the "Stepper Motor Controller" article in the
May 2002 issue of SILICON CHIP, operating a stepper motor using a fixed (constant) voltage supply
results in poor torque at high speeds. In fact, stepper motors tend to stall at
fairly low speeds under such conditions.
Several approaches can be used to overcome this problem, one of
which is to use a constant current supply in place of the more conventional
constant voltage supply. A disadvantage of many constant current supplies is
that simple circuits are inefficient but that doesn't apply to switchmode
supplies such as the circuit shown here.
Basically, this circuit is a conventional switchmode regulator
adapted for constant current output and is specially designed for stepper motor
drivers - although it could be used for other applications as well. The circuit
works as follows: IC1 (LM2575T) and its associated components (D1, L1, C1, etc)
operate as a switchmode power supply. Normally, for constant voltage operation,
the output is connected - either directly or via a resistive divider - back to
the feedback input (pin 4) of IC1.
In this circuit, however, Q1 senses the current flowing through
R1 and produces a corresponding voltage across R3. This voltage is then fed to
pin 4 of IC1. As a result, the the circuit regulates the current into a load
rather than the voltage across the load.
Only one adjustment is needed: you have to adjust VR1 for
optimum stepper motor performance over the desired speed range. The simplest way
to do this is to measure the motor current at its rated voltage at zero stepping
speed and then adjust VR1 for this current.
The prototype worked well with a stepper motor rated at 80Ω per
winding and a 12V nominal input voltage. Some components might have to be
modified for motors having different characteristics.
H. Nacinovich,
Gulgong, NSW.($35)
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Passive RIAA preamplifier
There are two types of preamplifiers for magnetic phono
cartridges. An example of the most common type is the one described in the March
2002 issue of SILICON CHIP. It
has the RIAA equalisation network in the feedback loop. The second type was
previously used in valve circuits which typically had no feedback loop and used
passive RC networks to provide the phono equalisation.
This experimental preamp was put together using inexpensive
FETs to compare the performance of these two types of preamp. The first stage,
consisting of Q1 and Q2, is a simple FET audio amplifier, where the FETs are
connected in parallel to reduce noise.
This is followed by a passive RIAA network consisting of 240kΩ
and 15kΩ resistors and the associated 0.1ΩF .022ΩF and .0047ΩF capacitors. Some
of the gain loss in the passive network is then made up by FET Q3. It also has a
51kΩ drain
resistor and is buffered by bipolar transistor Q4 which is connected as an
emitter-follower stage.
All resistors are 1% tolerance metal film type while the
capacitors for equalisation are MKT polyester types. Ideally, the Idss of all
FETs should be matched for both channels. Resistors R3 and R8 should be adjusted
so that the drain voltage in each stage is between 13V and 14V, to give
symmetrical signal clipping.
The power supply can be three 9V batteries connected in series.
Current consumption is only 3mA for the stereo circuit.
Sam Yoshioka,
Kahibah, NSW. ($35)
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Up/down timer for a power antenna
This up/down timer was designed to control a power antenna on a
late-model vehicle. Normally, this vehicle uses a body computer to control the
antenna. However, the person who owned the vehicle wanted to install his own
high-powered audio stereo system.
The original stereo system was tied in with the body computer
and this meant that a separate antenna controller was required for the
after-market sound system. Also, the power antenna fitted did not have limit
switches inside, hence the need for a timed control circuit.
Here's how the circuit works. first, assume that the radio
antenna control output is not switched on - ie, the radio is switched off. In
that case, relay RLYC will be off and so relay RLYA will also be off, as is the
motor.
Conversely, when the radio is switched on, the radio antenna
control output line switches to +12V. And when that happens, RLYC closes its
contacts and applies power to the circuit.
As a result, C2 (330ΩF) quickly charges via D4, while Q4 is
biased on via D5 and R5. This ensures that Q3 and relay RLYB remain off. At the
same time, Q2 is is turned on, thus turning on RLYA and applying power to the
motor. This drives the antenna in the up direction.
During this time, C1 charges via R2. When the voltage across
the capacitor reaches +8.1V, Q1 turns on via ZD1 and so Q2 turns off and
switches off the relay - ie, this gives the "up" timeout.
Using the values shown for C1, R2 and ZD1 gives an "up"
duration of approximately 6 seconds - long enough to fully extend the antenna.
D1 discharges C1 (via resistor R1) when the +12V supply is later removed.
When the radio is switched off (or a CD placed into the stereo
unit), the radio antenna control output switches back to 0V. This does several
things: first, it turns Q4 off and this allows Q3 to turn on due to the stored
charge in C2. Q3 and RLYB now turn on for about six seconds - ie, while C2
discharges via R4 - and this switches power to the motor in the opposite
direction to drive the antenna down.
Diodes D4 and D5 are there to prevent C2 from discharging back
via the circuitry around on Q1 and Q2.
Peter Howarth,
Gunnedah, NSW. ($40)
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