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Location:
Northwestern Sydney
Registered:
August 2002
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Re: 2 pin flasher unit
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Fri, 12 March 2004 02:05
 
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Somthing I found a while back when I was aking the same question
Hope it helps
The Flasher Relay
A flasher relay is a self-actuating switch. It turns itself on and off as current flows through it.
A relay is just a electrically controlled switch. Instead of manually flipping the switch on or off, the switch is controlled by applying voltage to it. A relay is most commonly implemented using an electro-magnet to move the switch mechanism.
A flasher relay is just a special kind of relay: it's a relay that opens and closes itself automatically. Most automotive flasher relays have a electro-magnet that controls current flow. It's this electro-magnet opening and closing the switch that makes the clicking noise inside the relay. What makes each type of flasher relay different is the switching technology used to operate the relay.
There are a few types of flasher relays:
Thermal (Thermo-Couple)
Electro-Mechanical
Electronic (IC controlled)
A Thermo-Couple Flasher is packaged in an heat-tolerant aluminum "can". It uses a thermal-mechanical switching mechanism to control current flow to the bulb directly, unlike electro-mecanical or electronic flasher relays which use an electro-magnet.
The thermal switch is designed with two paths for current to flow. We'll refer to the initial-position of the switch as "open". In the initial position, the switch allows current to flow, but not fast enough to light the bulb. In this "open" position, the switch actually looks like a resistor. (e.g. The Tridon Stant 552 thermal flasher I disected has a resistance of 42.9 Ohms across the 2 terminals, X and L.)
When voltage is applied to the switch, current flows through it heating it up and causing it to bend and "close". In its closed position, the switch creates a less restrictive path for current to flow--enough current to cause the bulbs to flash on.
The bulb continues to draw current until the switch cools enough to bend back to its initial "open" position. And the cycle continues...
An Electo-Mechanical Flasher uses a different switching mechanism. Typically it uses what's called a "tank circuit." A tank circuit uses a coil or inductor and a capacitor togeather. In the case of a relay, the coil is working double duty because we're using the magnetic field it creates not only to store a charge, but to operate the relay itself.
You have to know a little about how inductors (e.g. a coil) and capacitors work and how they interact to understand how a tank circuit works. The capacitor and coil both are able to store a "charge". A coil stores a charge in a magnetic field, whereas a capacitor stores a charge in an electric field. A coil tends to resist change in current, whereas a capacitor tends to resist change in voltage. When you connect a properly selected coil and capacitor in parallel and apply voltage, the circuit will essentially store a charge by "bouncing" it back and forth between the coil and capacitor.
Here are some links that describe how a tank circuit works:
From the "Electrical Engineering Training Series" chapter on "Tuned Circuits". How the Parallel-LC Circuit Stores Energy
An Electronic Flasher typically incorporates an intigrated circuit (IC) and other components to control the relay. The IC controls precicely the rate in which the relay opens and closes and isn't dependant on the load like a thermal or electro-mecanical flasher relay, but many implement this behavior in the IC itself. That is, the IC is smart enough to know when one bulb is burnt out and adjusts the speed of the relay to "fake" the behavior of the thermal flasher relay.
I don't have a schematic of these, nor do I know how they work internally. They are the norm in automotive applications today. In any case, the important part is to know what the job of the relay is, and what the terminals are for. In the computer science arena we'd say "look at it as a black box. Abstract away from the internal details, and work with the interface." In this case and ironically enough, we're literally working with a black box, and the interface is the terminals sticking out of it. 
Depending on the implementation of the flasher relay circuit, it may have between 2 and 4 terminals. Depending on the origen of the relay (where and by whom it's manufactured) the terminals may be labeled differently.
Minimally the relay is going to have 2 terminals. One is the input from the battery and the other is the output to the blinker bulbs through the turn signal or hazard switch.
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