A flasher is used to switch lamps on and off rhythmically. In the best-known case, flasher units are part of the direction indicator of motor vehicles and switch the direction indicators there, whereby for this application the exact function of the flasher unit is specified in the relevant laws through detailed equipment regulations, such as for Germany in the StVZO or for Austria in the KFG.

Basics

The most common application for a flasher is the switching of signal lamps, for example in railway or automotive engineering. The circuit is often set up in such a way that the switching processes can be heard as a clear click or by another signal. In the motor vehicle, it reminds the driver to reset the turn signal if this does not happen automatically; in other applications, it makes troubleshooting easier if a flashing light fails. The flashing frequency is between 60 and 120 cycles per minute, depending on the flasher unit. Flasher units are designed for a specific voltage and a specific current. When replacing it, make sure that the power and number of lamps match the data of the flasher unit. In vehicle technology, it must be taken into account that in very old motor vehicles and some two-wheeled vehicles, less powerful indicators of 18 or 15 watts (normally 21 watts) or other on-board voltages (6 volts) than are common today (12 volts, sometimes 24 volts) were installed. This must be taken into account when exchanging. However, there are also LED-compatible flasher units that can handle the lower power consumption of light-emitting diode retrofits (LED lamps).

vehicle technology

Layout and function

Each flasher unit consists of a pulse generator and a circuit breaker. The circuit breaker can either be a relay or a transistor (solid state relay). The flasher unit is connected to terminal 15, the so-called ignition plus, and is ready for operation after switching on the ignition switch. The travel direction switch is switched between the flasher unit and the flasher lights. This is a changeover switch that is interrupted in the middle position. In the middle position, the flasher does not work - it only becomes active when the switch is switched in one of the two directions of travel (left or right). When the hazard warning switch is activated, the flasher unit is supplied with power regardless of the position of the ignition lock and the direction switch, and all of the vehicle's indicators are activated.

Some manufacturers deviate from this standard structure. In these flashing systems, the interconnection initially runs from the ignition plus to the direction switch, from its two outgoing contacts to a two-circuit flashing device and then to the respective flashing lamps. There are also flasher units that are signaled via serial digital data transmission what they should do. Combination devices for hazard warning lights / direction indicators generate the signals for hazard warning lights and the direction indicators in one device.

While bimetal flasher units get by with two connection contacts, other flasher units require three or more connection contacts. There are flasher units - especially combined flasher units with two-circuit control - which have up to eight connection contacts.

What all flashers have in common is that they can detect defective lamps. This was already done with the first bimetal and hot wire flasher devices based on the thermal time constants and the heating power, which is proportional to the square of the load current. A faster flashing sequence was the hallmark of a lamp failure. Electronic flasher units require a current measurement (shunt) for this function and simulate the failure signal in the same way.

Fully electronic flasher units often generate the typical blinking sound of the (no longer present) relay.

Flashing rhythms

A distinction is made between two flashing cards on motor vehicles:

• Direction indicators
• Hazard warning lights

Direction indicators are the synchronous blinking of the indicators on one side of the vehicle; When the hazard warning lights are flashing, all of the vehicle's direction indicators light up. Hazard warning lights were approved in motor vehicles from 1963. In the older vehicles equipped with bimetal and hot wire flasher units, an additional flasher unit was required for this, which had to be retrofitted. Both flasher units were connected to a special hazard warning flasher switch, with the hazard warning lights having priority. From the end of the 1960s, all new cars were equipped with combined direction and hazard warning flasher units. From January 1, 1973, old cars had to have hazard warning lights.

Dedicated hazard warning flasher units differ from flasher units partly in terms of frequency, but generally in terms of duty cycle. In the case of the hazard warning lights, the lamps are switched on for a shorter period of time and switched off longer in order to protect the vehicle longer with one battery charge. From the 1970s onwards, a flasher relay was switched to all flasher cards using a switch. In today's motor vehicles, the indicators are controlled via bus systems and the on-board power supply control unit; Here the hazard warning mode - when the engine is switched off - is often equipped with different phase times (shorter lighting phase) to protect the vehicle battery, as this can be done cost-neutrally using software.

The lamp failure is indicated by an increased flashing frequency, see section Structure and function.

Flasher types

Various types of flasher have been designed and used over the years. These are among others:

• Thermal flasher
  • Bimetal flasher
  • Hot wire flasher
• Pneumatic flasher
• Electronic flasher
  • Conventionally connected flasher units with electronic timer and lamp failure control
  • Digital flasher units that get their signal via a serial bus connection

Bimetal flasher

The first flasher units used a bimetallic strip to delay suit and fall, which is periodically heated by a heating coil. The bimetal contact is designed as a break contact and connected in series with the heating coil. This is why these flasher units only need two connections. When the travel direction switch is switched off, the bimetal is cold and the normally closed contact is closed.

When the travel direction switch is activated, a current flows through the bimetal strip, the heating coil and the flashing lamps. The heating coil heats the bimetal strip. The normally closed contact is opened after a few seconds by the bending bimetal, whereby both the indicator lamps and the heating coil lying in series are de-energized. The open switch contact now interrupts the flow of current; the bimetal strip cools down, whereupon the switching contact closes again. This process is repeated periodically.

This type of flasher unit has two special design-related features: on the one hand, the first flashing interval lasts longer than the following (because the bimetal strip must first heat up sufficiently), on the other hand, the failure of a flashing lamp results in a significant increase in the flashing frequency (because the current flow is lower and the bimetallic strip therefore heats up less, so it bends less and cools down again more quickly). Today the bimetal flasher is only used in historic motor vehicles (oldtimers).

Hot wire flasher

Hot wire flasher units were used in vehicles until the 1990s. These flasher units work as follows: In the idle state, a hot wire (see also hot wire instrument), an electromagnet and the light bulbs to be switched are connected in series. The resistance of the hot wire and a series resistor is so great that the current flowing is too small to make the lamps glow. In front of the magnetic coil, similar to a relay, there is an armature which is kept away from the coil by the tensioned, cold hot wire.

The wire is now heated by the current flow, expands and enables the armature to be attracted more and more by the magnetic field of the coil. At the end of this movement, the armature reaches a switching contact that short-circuits the hot wire. The current now flows through the coil directly to the light bulb. The incandescent lamp lights up because the voltage drop across the coil is only slight. While the magnetic force of the coil, through which the strong lamp current flows, attracts the armature all the more, the heating wire cools down and tries to shorten itself again. After cooling down sufficiently, the hot wire contracts so much that it can pull the armature away from the coil against the magnetic force and open the contact. The lamp goes out and the process starts again.

The advantage of the principle is the support of the contact force and the release of the contact (feedback effect), so that the contact is spared and lives longer.

There are hot wire flasher units that start with the light phase and those that start with the dark phase. In the case of flasher units that begin with a dark phase, this is shortened by a property of the light bulb. Incandescent lamps are PTC thermistors and have a resistance that is around ten times lower when switched off than when they are in operation. The light bulb, which is cold when the flasher is switched on, causes a significantly higher current to flow, which leads to a faster expansion of the hot wire, so that the flasher switches to the light phase in the legally prescribed time. During the regular flashing operation, the incandescent lamp never cools down completely, which means that its PTC behavior is less important.

The disadvantage of these flasher units is that they are very sensitive to voltage fluctuations. In the event of undervoltage, the flashing frequency of the flasher unit decreases. In addition, the principle is subject to aging: the wire must never reach the plastic range when heated, but should be allowed to become as hot as possible because of the stroke. This is why some hot wire flasher units have an adjusting screw for the heating wire. After setting the correct flashing frequency using the adjusting screw is not easy and should be left to a specialist.

Pneumatic flasher

In the 1960s there were also flasher units with pneumatic switching delay. With this type of flasher type, a small piston is pulled into a cylinder by a coil through which the lamp current flows, until it causes an armature to switch over and thus interrupts the lamp and coil current. A spring then pushes the piston out of the cylinder again until the power is turned on again. The armature is alternately attracted and released by the solenoid.

The armature controls the contacts for the flashing lamp current. In principle, such a flasher works like a horn. However, the frequency of the oscillating armature is much too high for the direction indicator control. For this reason, the armature movement of this flasher is slowed down by an air nozzle. The air nozzle decelerates the air displaced by the piston and dampens the movement of the piston. The term "pneumatic" is derived from this so-called air brake.

Pneumatic flasher units are largely independent of the load. The special feature of this type was the universal applicability with different electrical system voltages, e.g. B. with Pneutron type AB 16: 2 to 6 × 26 W at 6 or 12 V. However, they were only used to a limited extent - mainly as hazard warning lights. However, this technology was quickly replaced by electronic flasher units.

Electronic flasher

From the late 1960s, flasher units with an electronic pulse generator were used. This initially consisted of discrete components. For this purpose, resistors, capacitors and transistors were connected to form a so-called astable multivibrator. Relays and occasionally also transistors were used as circuit breakers. From around the mid-1970s, the discrete components were increasingly replaced by integrated circuits (e.g. timer IC type NE555). Since 1981 there have been special indicator ICs which also take over the failure control based on a current measurement. Widely used indicator ICs are z. B. the U643B and the U6043B from Atmel.

If one of the bulbs fails, the current flowing through the flasher unit is reduced. The built-in control circuit recognizes this and doubles the flashing frequency - imitating the system-related behavior of the historically used bimetal flashers. Flasher units for trailer operation have an additional output (C2) to control the special trailer indicator light, which enables the vehicle driver to check the direction indicators of the trailer as required. Electronic flasher units have a very constant frequency and are insensitive to temperature and voltage. Flasher units for 12 volts also work properly at 9 volts. This means that these flasher units have a constant flashing rhythm even in the event of a breakdown when the on-board voltage has dropped significantly. From around the end of the 1990s, these separate flasher units were replaced by solutions integrated into the central vehicle electrical system.

Integrated flasher

In modern vehicles, there is no longer a separate flasher unit installed. The flashing (switching on and off) is generated in a central control unit, usually the so-called on-board power supply control unit (Body Control Module, BCM; at Ford also called Generic Electronic Module, GEM). This is controlled by the indicator switch, through which only a small control current flows. The current for the indicator lamps is switched via the electronics of the control unit. With this technology, as with separate flashers, it is possible to detect a lamp failure by internally monitoring the lamp current in the control unit. As with the older technology, the driver then receives feedback via the higher flashing frequency, possibly also via a text message in the instrument cluster (e.g. "Front right turn signal light - please check!"). The acoustic control of the blinking process is usually implemented by a small (piezo) loudspeaker behind the dashboard (usually installed in the instrument cluster), with some instrument clusters even using very realistic audio samples of clicking blinker relays. In addition to the failure of a lamp, control units also detect short circuits and switch off the affected lamp circuits without triggering the fuse. As soon as the damage has been repaired, the corresponding lamp is activated again. Some vehicles also have a small relay installed that is used exclusively to generate a click in rhythm with the flashing frequency.

Error control

As a special feature, as already mentioned, flasher units have a monitoring function for the flasher lamps. This error control can be done optically or acoustically. If a bulb fails, the flow of current in the flasher circuit is reduced. The flasher unit then operates at twice the switching frequency and indicates to the driver that there is a fault in the flasher system: audible by the faster ticking and visible by the faster flashing of the green indicator light. In addition to doubling the flashing frequency, it is also possible to implement failure control by not activating the control lamp. There are basically two options for checking the function of the indicators on towing vehicles with trailers, the single-circuit control and the dual-circuit control.

With single-circuit control, both the trailer and the towing vehicle have a common control circuit. Two indicator lights are controlled via the control circuit in the rhythm of the flashing frequency. If one or more flashing lights on the towing vehicle or trailer fail, both control lights remain dark. With this system, it is not possible to assign a lamp fault to the trailer or the towing vehicle. The flashing frequency remains unchanged.

With the two-circuit control, there are separate control circuits with separate control lights for the trailer and for the towing vehicle. If a control lamp remains dark, a clear assignment of the lamp fault is possible. Depending on which indicator light does not light up, there is a lamp fault either on the towing vehicle or on the trailer. The flashing frequency remains unchanged in this system as well.

Other uses

Flasher units are also used for error messages in electronic controls and in some household appliances.

Flasher units are also used in surveillance systems, traffic lights, St. Andrew's crosses and the like. A. Signal systems.