Application Report
SLVA650 – April 2014
Design of a Low Cost, 45W Flasher with Short Circuit
Protection Using LM2902 (Quad Op-Amp) and CSD18534
Gitesh Bhagwat
ABSTRACT
This Application note presents the design of a low cost, flasher circuit with short circuit protection. The
design incorporates the entire recommended design feature set for two wheeler flashers and includes -
low/high voltage operation, half load frequency doubling, and short circuit protection.
A circuit simulation and reference schematic are also provided.
Topic
...........................................................................................................................
Page
1
2
3
4
5
6
7
Introduction
........................................................................................................
2
Flasher Specifications
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2
Design Using LM2902 (Quad Op-Amp) and CSD18534
..............................................
2
Operation
............................................................................................................
3
Test Results
........................................................................................................
5
Conclusion
........................................................................................................
10
References
........................................................................................................
10
SLVA650 – April 2014
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Design of a Low Cost, 45W Flasher with Short Circuit Protection Using
LM2902 (Quad Op-Amp) and CSD18534
Copyright © 2014, Texas Instruments Incorporated
1
Introduction
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1
Introduction
Automobiles utilize flashers for turn indications. Traditionally such control has been through
electromechanical devices. However, the same functionality can be implemented using electronics at a
fraction of the cost.
This design proposes such a scheme, which can be used for a load of 45 W.
2
Flasher Specifications
Table 1. Specifications
SI. No.
1
2
3
4
5
Operating Voltage
Flash Rate
On time Duty Ratio
Starting Time
Indication of single Bulb Failure
SPECIFICATIONS
9 V to 16 V
85 c/m ± 10 c/m at 12.8 V
35% to 60% at 12.8 V
<1 sec.
Flashing Frequency Doubles at 21 W (single
bulb load)
3
Design Using LM2902 (Quad Op-Amp) and CSD18534
Figure 1
shows the schematic for the flasher design.
Figure 1. Flasher Schematic
2
Design of a Low Cost, 45W Flasher with Short Circuit Protection Using
LM2902 (Quad Op-Amp) and CSD18534
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SLVA650 – April 2014
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Operation
4
Operation
The design of the flasher can be divided into three main segments:
4.1
Ocillator
The Op-amp U2-A acts as an oscillator triggering transistor Q5. This in turn triggers MOSFET Q4. The
frequency of oscillation is set to about 85 c/m as required by the specifications.
4.2
Frequency Doubling Circuit
The Op-amps U2-C (Current Sensing Amplifier) and U2-D (Comparator) form a frequency doubling circuit.
U2-C senses the load current across resistor R12, and amplifies it by a factor of 6.
U2-D then compares this amplified voltage with a threshold set by resistors R17 and R23.
4.2.1
Full Load Operation
As long as the amplified sensed voltage is higher than the threshold level, output of the comparator is
driven high which keeps the transistor Q3 turned on. This transistor, when turned on, brings the capacitor
C2 into action by connecting it to ground.
Thus the effective capacitance seen by the U2-A oscillator is approximately a parallel combination of C2
and C3. It is for this combination that the oscillation frequency is set to 85c/m.
4.2.2
Half Load Operation
When the load is halved (failure of one of the bulbs), the sensed and amplified voltage provided by U2-C
falls below the threshold set for U2-D. The output of U2-D comparator goes low thereby turning off the
transistor Q3. This disconnects capacitor C2.
Now, the U2-A oscillator sees only C3 which is approximately half of the parallel combination of C2 and
C3. This results in doubling of the oscillation frequency.
4.3
Short Circuit Protection
The U2-B is another comparator which is used for implementing short circuit protection.
When the output of U2-C exceeds the threshold set by R9 and R15, the output of U2-B goes high. The
transistor Q2 latches this high output. The diode D1 provides a unidirectional path for this high output to
latch Q6 which in turn latches off MOSFET Q4. This latching action is reversed only by resetting the circuit
(disconnecting V
CC
).
Capacitor C5 connected on the positive terminal of U2-B, introduces a delay in the triggering of U2-B. This
is used to avoid false triggering due to high inrush current during cold start.
Figure 2
shows the worst case
cold start condition with 12.8V battery voltage. The circuit starts up within 1 second and avoids false
detection of the initial current spike as a short circuit. The capacitor C5 takes care of the same.
The start up time is measured to be 658 ms.
SLVA650 – April 2014
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Design of a Low Cost, 45W Flasher with Short Circuit Protection Using
LM2902 (Quad Op-Amp) and CSD18534
Copyright © 2014, Texas Instruments Incorporated
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Operation
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Startup time: 658 ms
Figure 2. Worst Case Cold Start Condition
The design is first simulated in TINA software to fine tune the oscillator, frequency doubler and short
circuit protection components.
Figure 3.
shows a simulation of the same using LM324 as the quad op-
amp.
Figure 3. Circuit Simulation in TINA
4
Design of a Low Cost, 45W Flasher with Short Circuit Protection Using
LM2902 (Quad Op-Amp) and CSD18534
Copyright © 2014, Texas Instruments Incorporated
SLVA650 – April 2014
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Test Results
5
Test Results
Test results for the circuit are divided into two categories:
Operating Voltage
and
Short Circuit Test.
5.1
Operating Voltage
Operating Voltage
12.8 V
12.8 V
9V
9V
16 V
16 V
Load
21 W
45 W
21 W
45 W
21 W
45 W
5.2
Short Circuit Test
Figure 12
shows the waveform of load current in the event of a short circuit.
Note: In all the measured waveforms that follow, Ch1 (Yellow) is the load current and Ch2 (Pink) is the
voltage across the load.
V
IN
= 12.8 V
Load = 45 W
Time Period = 687 ms
Figure 4.
Frequency = 87 Cyles/min
SLVA650 – April 2014
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Design of a Low Cost, 45W Flasher with Short Circuit Protection Using
LM2902 (Quad Op-Amp) and CSD18534
Copyright © 2014, Texas Instruments Incorporated
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