The invention relates to a flasher unit having a comparator, the output signal of which feeds a pulse generator in order to produce the flash frequency signal and a reference voltage is supplied to one input and a voltage, which depends on the current through the flasher lamps, is supplied to the other input.
As is known a failure check has to be carried out for flasher units for use in motorised vehicles. In the known unit, the flasher lamps together with a precision resistor are connected between the positive pole and the negative pole of a battery. The path comprising the precision resistor and the flasher lamps can be switched and in fact the switch is switched by frequency switching in rhythm with the flash frequency by means of a pulse generator.
The voltage drop which takes place across the precision resistor, which is proportional to the current through the flash lamps is evaluated and supplied to the pulse generator. The pulse generator is switched in frequency by the voltage across precision resistor. In order to be able to detect whether all of the flasher lamps which have to light up on actuation of the flash switch are flashing and therefore whether the flash unit is operating (correctly), there must be a clear jump in flash frequency. This means that when operating the flash unit flash lamps have to flash at a substantially lower frequency than is the case when a lamp fails or the flash unit fails.
In a practical example of a flash unit with fault indication, besides the precision resistor a relay contact which can be switched in time with the flash frequency, and the flasher lamps, there is a voltage divider which comprises two resistors and is connected in parallel with the voltage source. A comparator is provided, a reference voltage U.sub.ref being supplied to one input. This reference voltage is tapped at the connecting point between resistors of the voltage divider. The voltage across precision resistor is supplied to the other input of the comparator. The voltage across the precision resistor, which is proportional to the current through the flasher lamps delivers the measured signal which is compared in the comparator with the reference voltage.
In operation the measured voltage (voltage drop across precision resistor) depends on the filament temperature of the flasher lamps. As is known, the current through an incandescent lamp is substantially greater at the moment when it switched on, than is the case during ordinary running. Therefore the voltage drop across precision resistor is also at its greatest at the moment of switching on. After switching on (the lamp), the voltage across precision resistor falls. The voltage drop across precision resistor at low frequency (flash unit OK-) is considerably larger than the voltage drop across precision resistor when there was a fairly high (doubled) frequency--(flash unit not OK-). When the relay contact is open, the i.e. when the lamp is not on, the voltage drop across precision resistor returns to zero.
The reference voltage is equal to the voltage drop across the first of the dividor resistors and remains almost constant both during the light up time and when the lamp is not lit, however, it remains within a certain tolerance range.
If a fixed reference voltage is used, then only very small tolerances are permitted for optimum operation of the pulse generator which is fed by the output signal of the comparator. The tolerance range available means that, depending on the circumstances, either a comparator signal is provided for control of the pulse generator or is not. However, perfect generation of the flash frequency presupposes that a comparator output signal has to be provided for the whole of the light up time (light up time equals the time in which the flasher lamps are illuminated).
The invention is based on the knowledge that a tolerance range for the comparator output signal cannot be avoided with a fixed reference voltage and therefore it is not possible to produce a stable frequency. In practice, the negative effect of the tolerance is compensated by individual adjustment of the flasher unit, however it is desirable to provide a circuit which no longer requires such adjustment.