Friday, April 3, 2009

Constant-Current Circuit

Booster Constant-Current Circuit
Constant-current circuits can be configured to take
advantage of the fact that CMOS regulators consume
a very low current.

If you wish to set constant current value io to a
larger value, PNP transistor Tr1 and resistor R1 can be
added, as seen in Figure 5. This will improve the
input/output voltage characteristics, allowing you to
increase the said current value as a result.
Consequently, under the same conditions as stated
above, that is, an input voltage VIN of 3 V and a
device voltage VO higher than 1V, for example, the
drive capacity will rise from 10 mA (typ.) to 100 mA

Figure 4. Booster Constant-Current Circuit

Constant Current Battery Charger Circuit

The resistors R1 and R2 determine the final charging voltage
and RSC the initial charging current. D1 prevents discharge
of the battery throught the regulator.
The resistor RL limits the reverse currents through ther
regulator (which should be 100 mA max) when the battery
is accidentally reverse connected. If RL is in series
with a bulb of 12 V/50 mA rating this will indicate incorrect

Transistor constant current Circuit
This is a schematic of the smoke circuit regulator. This is a
constant current regulator. The two 1.5 ohm resistors sense
the current in the smoke elements. If the smoke element
current increases, the base voltage of the 2N3904 increases
and it in turn drags down the base of the TIP122 pass transistor
which reduces the voltage and therefore the current of the
smoke elements. The circuit regulates the current to about 1 amp.
The resulting voltage across the elements is about 6 volts. The
most significant internal voltages are shown in red referenced to
the (-) side of the bridge.

Constant Current for Sensor Excitation Circuit
The MAX1464 can be easily configured to generate constant
current excitation for sensors that is ratiometric to the power supply
voltage for resistive transducer applications. Applications utilizing
sensing elements with high temperature coefficients, TCR, such as
piezo resistive bridges, RTDs, etc. are typically implemented with
constant current excitation. This application note suggests a simple
resistive network that can be implemented to provide a ratiometric
current source for sensor excitation.

Transistor constant current source Circuit
The current source shared by the two transistors is also shown in the
figure. Due to the fact that the forward biased diodes have fixed
voltageVd = 0.7V, the base voltage of the transistor is also fixed at 2.1V
, so is the current Icc, i.e., the circuit can be used as a constant current source

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