Flasher Cans

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Page created by T Nov 9th 2007:

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2 Pin Flasher Units. Electrical type (left), thermal type (right). Click to Enlargen. Photo by HQ_SS.

Flasher Cans:

Thermal Type:

These contain a bimetal strip heated by an heater element.  The bimetal strip closes 2 contacts when it becomes hot and opens the contacts when it becomes cold.When first activated, the bimetal strip is cold and the contacts are open. When the contacts are open the heater element is placed in series with the Flasher Lights. Because the heater element has a much higher resistance than the Flasher Lights it heats up. The Element then heats a bi-metal strip. During this time the Flasher Lights are not glowing because the Heater Element takes all the power applied to the circuit.After a short time the Heater Element causes the bimetal strip to bend and close 2 contacts. Once the 2 contacts close the Flasher Lights come on. While the Flasher Lights are on, the Heater Element is shorted out so it cools down. At this time the Bimetal Strip cools down too. Once the Bimetal Strip has cooled enough it opens the 2 contacts again. This starts the cycle again by causing the Heater Element to begin heating again.Because the Heater Element is in series with the Flasher Lights, the resistance of the Flasher lights influences the heat up time of the Heater Element. The more Flasher Lights the shorter the heatup time because the total resistance is lower. The fewer Flasher Lights the longer the heatup time because the resistance is higher.

Thermal Type Flasher Unit. Animation by T. Click to Enlargen.

Electrical Type:

These are often called electronic though they are really electrical. The difference is that they don't have any Transistors in them. The Electrical type works by using a Resistor and a Capacitor to create the on/off time. There is no Heater Element inside them.The unit is an ordinary Relay with a Second Winding wrapped around the Coil. The Second Winding is wound the opposite way to the main winding.When the Flasher Unit is first activated the Contacts are open which allows current to flow through both Windings. The Secondary Winding is in series with a Capacitor and a high value Resistor. While the Capacitor is charging an opposing Magnetic Field is generated in the Relay to hold it open. Once the Capacitor is charged the Relay pulls the Contacts together. At this point the Flasher Lights come on and the Capacitor begins to discharge through the Resistor. When the Capacitor is sufficiently discharged the Contacts open, the Flasher Lights go out and the Capacitor charge cycle starts again.The on/off time of the Flasher Lights is determined by the total resistance of the Flasher Lights and the internal Resistor inside the Flasher Can. Because the internal resistance is much larger than the Flasher Lights resistance the number of Bulbs doesn't have a noticable effect on the flash rate. Example: If the internal resistance in the Electrical Flasher Can is 1000 ohms and each Bulb has a resistance of 6 ohms. 2 Bulbs in parallel will have a resistance of 3 ohms. The Electrical Flasher Can would see (1000 ohms plus 3 ohms) 1003 ohms. The increase in resistance is .03 % and would have no peceptible change in the flash rate that the eye could see. If there are 4 Bulbs in parallel the Electrical Flasher Can  would see a resistance of 1000 plus 1.5 ohms or 1001.5 ohms.  The increase in resistance is .015 % and would have no perceptible change in the flash rate that the eye could see. These are sometimes called Universal Flasher Relays because they can be used for both Flasher and Hazard Lights.

Electrical Type Flasher Unit. Animation by T. Click to Enlargen.

Electronic Type:

WB Flasher Units. Left hand side is the Hazard Unit Right Hand Side is the Flasher Unit. Photo by 81WB. Click to Enlargen.
The Electronic Type Flasher Unit usually has 3 Connections. One Connection is Power in, another Connection is Ground and the third Connection is Output.Internally there will be an Electronic Timing Circuit and possibly a Relay as well. These were introduced to provide an accurate Flash Rate and will work with a variety of loads. 
You need to match the load to the Flasher Unit.You can buy universal Flasher Units that will work with anything. These tend to be electronic and are not affected by the number of globes connected to them because they use an internal resistance that is significantly higher in value than the bulbs total resistances.The original thermal type Flasher Unit is dependant on the number of globes connected to determine the flash rate. Not enough bulbs and they flash fast. Too many bulbs and they flash slow and/or burn out.There was a time when you used an 80 watt flasher for hazard lights and a 40 watt flasher for blinkers.A universal electronic type Flasher can be used for either
blinkers or hazard.There are also 2 and 3 pin types which must be matched. Most Holdens up to about black motors used the 2 pin type.

VK Commodore:

This Car uses one Flasher Unit to perform both Flasher and Hazard functions. You must use a Universal unit in these Cars.The VK Commodore uses a 2 Pin Universal Flasher Unit.




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