Category: Digital Electronics

We had turn signal flashers that that would misbehave from time to time, varying wildly, even though the math said it should work properly.  I investigated.

It turned out that the supply to the main timing IC, a bipolar 555 timer, was reliant on the drop across the turn signal flasher… which itself varied as the turn signal flasher turned “on” and “off”!  It was shorting out its own supply, essentially!  Oh boy.

I developed a circuit that would allow the 555 timer to still drive the monster output transistor, the TO-3 cased 2N5301, but hold the IC’s supply during the “on” time.  There were two problems with this – the circuit consumed too much current to be reasonably held up during the “on” time, and the output turned out to be referred to the wrong rail, in order to drive the output transistor using the same mechanism as before.

I addressed the current consumption by switching from the bipolar 555 timer to the then-relatively-new Intersil CMOS ICM7555.  What a joy that it ran on almost no current!

Then, I developed a 2-transistor output drive to replace the 1-transistor stage.  It worked well, until we ran it over temperature.  It was then that I learned one of the fundamental lessons that I’ve taken through my entire career: do not rely on one transistor to “short out” second transistor’s base drive over temperature!  It turned out that the Vce(sat) of the first transistor could exceed the Vbe(th) of the second transistor, sometimes, at low temperatures.

I found that you can short out the second transistor’s base drive, but then you need to put a resistor to the second transistor’s base, just to make it that much harder for the second transistor to turn on with leakage.

Well, that was a lesson learned, but then I just redesigned the stages to avoid that problem altogether.  Once bitten, twice shy!

I did do some preliminary work on using a MOSFET output on the flasher, but I couldn’t make the short-circuit protection work, and decided to stick with the bipolar output.  Later, a colleague, James White, took up that effort and successfully developed the “FET flasher”, which was another step forward turn signal flasher design.

James White redesigned the “standard” 12 Volt turn signal flasher to use a MOSFET output, then he modified the design to successfully run on 24 Volts for bus & truck applications.

At about the same time that James was doing this, Can-Car Rail approached us to develop a 36 Volt flasher for their rail coaches.  I thought it would be easy!  Whoops, not so fast…

Well, oh my Lord, 36 Volts DC is nasty.  My first attempt to just use my modified 12 Volt design was a complete disaster.  The three lead-acid car batteries that we used for development just provided so much power that the 2N5301 output transistor overheated and failed on the first flash into a short.  The 12V and 24V designs could flash into a short indefinitely (Ed Van Humbeck drove around with a flasher wired directly across his car battery for about 6 months in 1980/1981 to prove that it could easily handle it).

The FET Flasher didn’t perform any better.

Instead, I had to develop a completely different design, right from scratch.  I used some of the same concepts, although I had to change components – the 2N5301, for instance, is only rated for Vceo to 40V, so it had to be changed.  The supply to the 555 timer had to be modified to handle the high voltage.  The drive stages had to be changed to handle the voltage translation and the drive requirements of the new output transistor.

The biggest change was that the 36V flasher got a “ground return connection”, which allowed it to be constantly powered, instead of having to scavenge the operating current from the bulb current.

We had to change the mechanicals as well – the original 12V and 24V flashers were simply placed into a plastic cap along with a heatsink/mount made of a piece of bent & punched aluminum, then the cap was filled with epoxy.  The But, like all challenging designs, it was 36V flasher needed much better heat dissipation for several devices, so it was designed as a flange mount device, a circuit board mounted on top of the “W” shaped metal mount, then covered with a lid.

The design was a challenge, the result was a success, but we didn’t make many units – perhaps a few hundred.  But, that’s the difficulty – you can’t tell ahead of time, what will be a winner, and what will be a loser.  You just have to do your best, and take on the ones that look reasonable.

Ford New Holland had purchased Versatile Farm Equipment, brought some new products to the plant, and Vansco was providing much of the electronics that went into all of its tractors.

A new tractor was being made, which at the time was called “P53”.   It needed a power distribution panel / fuse panel and turn signal flasher.  I suggested that it could be a monster printed circuit board, and was asked to make it happen.

To accommodate differing standards worldwide, the turn signal flasher was a separate, small circuit board that plugged into a connector in the middle of the board.

Meanwhile, the main board was developed to fit inside one of the vertical window pillars.  It was physically large surface area, and had a large metal bus bar down the middle to distribute power to the fuses.   Of course, it had monster copper on it, 2 oz, and wide tracks, to ensure long life.