Electronic turn-signal flashers

[Number_35]

The old turn-signal flashers used a bi-metallic strip which curved as it heated up, broke contact, cooled, and remade contact.

It seems obvious that the flasher rate would vary with load. Lower resistance, higher current, bimetallic strip heats up faster ...

I first saw the modern Triton electronic flashers in the 1970s. There was a capacitor inside, and I presumed they operated on the R-C time-constant principle.

For whatever reason, I assumed that the resistance was internal, and that the flasher rate would be independent of external load.

However, it looks like the rate still changes when a bulb is out (and the load therefore changes).

Can anyone shed light on this for me? (Arg, pun not intended!)

 

[97prizm]

If I remember correctly it has to do with the way the bulbs are wired i.e. series vs parallel can't remember which.

 

[brianl703]

I thought that flashers were required by some DOT specification to indicate when a bulb is out.

It was probably purposefully designed to do that to comply with said DOT spec.

 

[mk378]

There are two kinds of two-terminal flashers: current operated and voltage operated. A current-operated flasher is usually stock equipment. It has the advantage of detecting bad bulbs but the disadvantage that plugging in a trailer will generally make it blink very rapidly or burn out.

A voltage operated flasher (most "electronic" and also "heavy duty" thermal) will flow a small current through the bulb circuit during the "off" period to drive the timer. As long as there is any path (one good bulb) the flash rate will be constant.

 

[A_User55555]

You're correct about an RC circuit determining the flash rate, However, this RC circuit doesn't directly power the bulbs. Turn signal circuits like relay control, relay flash speed, and burned bulb detection were consolidated into this common L9686 "Turn Signal Flasher IC. This was actually used by the flasher on my old 1999 Deville

Check out the datasheet for it. It should answer all your technical details about car flashers.

https://www.mouser.com/datasheet/2/389/stmicroelectronics_cd00000105-329709-1206584.pdf

The normal flash rate is set by the RC circuit connected to the flash rate pins.

For burnt bulb detection, there's a "fault" pin on the IC. This pin is connected with a resistor to the bulb load. When a bulb burns out, the voltage drop decreases across the resistor. The IC detects this and then doubles the flash rate of the relay out pin.


Nowadays, each signal bulb is computer controlled. So if a bulb burns out, an internal current sensor detects that, and the flash rate is switched programmatically.

 

[Number_35]

You're correct about an RC circuit determining the flash rate, However, this RC circuit doesn't directly power the bulbs. Turn signal circuits like relay control, relay flash speed, and burned bulb detection were consolidated into this common L9686 "Turn Signal Flasher IC. This was actually used by the flasher on my old 1999 Deville

Check out the datasheet for it. It should answer all your technical details about car flashers.

https://www.mouser.com/datasheet/2/389/stmicroelectronics_cd00000105-329709-1206584.pdf

The normal flash rate is set by the RC circuit connected to the flash rate pins.

For burnt bulb detection, there's a "fault" pin on the IC. This pin is connected with a resistor to the bulb load. When a bulb burns out, the voltage drop decreases across the resistor. The IC detects this and then doubles the flash rate of the relay out pin.


Nowadays, each signal bulb is computer controlled. So if a bulb burns out, an internal current sensor detects that, and the flash rate is switched programmatically.

In summary, it sounds like modern flashers have the capability of maintaining a constant frequency, but deliberately change the frequency to indicate that there's a problem.

 

[97prizm]

Back in the 99-2000 ish era there was a "hyperflasher mod" if you will that was popular with the Honda/Acura crowd that made the turn signals blink really fast. From what I can tell it got a number of the import crowd pulled over.

 

[Number_35]

Back in the 99-2000 ish era there was a "hyperflasher mod" if you will that was popular with the Honda/Acura crowd that made the turn signals blink really fast. From what I can tell it got a number of the import crowd pulled over.

Just because it can be done ...

 

[meep]

In summary, it sounds like modern flashers have the capability of maintaining a constant frequency, but deliberately change the frequency to indicate that there's a problem.

Yes, this is indeed correct. the quick-flash is a required feature as feedback to the driver that there’s a problem. there are still current shunts in the controllers today to check load current to trigger hyperflash if less power is being pulled than expected. I know it’s actually a switchable parameter in newer fords, for instance, via forscan, for such cases like factory led lamps. Some cars more than just the turns. How many cars will tell you if a headlamp is out, or Volvo for instance, turns, brakes, headlamps, and then a single indicator for “exterior lamp”, meaning marker lights.

 

[Asterix]

I don't know when they appeared, but definitely by 1986 the turn signal flasher was electronic. The mid-1980s Toyota flasher used a Denso SE051 chip, which is exactly equivalent to the Microchip/Atmel U643B. (Datasheet: https://media.digikey.com/pdf/Data Sheets/Atmel PDFs/U643B_Rev2005.pdf)

It has a slow or normal speed flash and a fast speed flash, depending on the voltage across a current-sense resistor. If the voltage is low, it flashes quickly, because that means a bulb is burned out.

When I changed the signal lamps to LEDs in my 1988 Supra, I took a flasher apart and changed the current sense resistor to work with the lower-current LED lamps. The fast flash feature is really nice to have.