I'm rebuilding an MGA and i'm at the point of revising the electronics. Today I was making a DIY flasher with a simple RC circuit. This will replace the old thermal flasher which depends on the right load to flash at the correct interval.(it's also broken by the way.) Hence the question arose in me;
Why did those old thermal flashers remain in use for so long?

In comparison with a flasher built with a few semi-conductors: They're bigger in size, more prone to failure, and probably more expensive, considering their relatively complex and precise mechanical construction.

Second, they require the right wattage bulbs or they'll flash too fast/slow, and for this reason you also need a second one for hazard lights use. A last argument against thermal flashers is that they waste more energy, although this is probably negligible.

The only argument in advantage of the thermal flasher I can think of, is that sometimes it is required by law to give an indication when one of your blinkers is burnt out. The thermal flasher's speed automatically increases in this case, something you'd have to make a special arrangement for with the modern type of flasher.

NB: Obviously semi-conductors and the like weren't really available at the production of the MGA, but the thermal flasher remained in use for decades after.

  • 1
    the thermal flasher is cheap and reliable. I wouldn't be surprise if its one of the few electrical bits on your 50+yr old car that still works. You can buy off the shelf solid state flasher units you know.
    – agentp
    Commented May 2, 2017 at 17:47
  • @agentp They're still for sale, but not used in modern cars anymore. (And it's just for the sake of hobbying that i'm diy'ing one myself.:p) I'm doubting about the price and reliability. Prices of both vary much so you can't really compare them, but thermal flashers are more complex to produce. Second, their reliability isn't any better than modern methods of flashing IMO. Heat and stress makes material age, something that doesn't happen with semiconductors when loaded with a few milliamperes for controlling a switch. If they were really better that nowadays flashers, they'd still be in use.
    – Bart
    Commented May 2, 2017 at 17:57
  • I guess the timeline part of your question is ambiguous. Can you give example of what era vehicles used thermal flashers where you think they should have transitioned?
    – agentp
    Commented May 2, 2017 at 18:58
  • @agentp I can't really pinpoint a concrete example, but I have found thermal flashers in vehicles as 'new' as the 90s era. Electronics were readily available at that moment and cheap. The surprising thing is, the original clock and even the headlight lift construction in my 70s Triumph do incorporate semiconductors.
    – Bart
    Commented May 2, 2017 at 19:55
  • 2
    "Heat and stress makes material age, something that doesn't happen with semiconductors" - have you noticed that most consumer-grade electronic equipment only guarantees operation from about -10C to +50C, and with a limit on relative humidity? A vehicle that you can't drive outside those limits is not much use to many people! Of course you could use mil-spec electronics, but that raises the cost. (Temperature behind the dashboard can easily exceed 50C if the vehicle is left in the sun for a few hours)
    – alephzero
    Commented May 2, 2017 at 21:15

4 Answers 4


Three major reasons why they were used so long:

1. Cost - Even today a thermal flasher units cost about $2-2.50. The cheapest equivalent electronic flasher unit, which happens to be a direct replacement, costs about $6.50 (NOTE: Prices shown are USD found through a quick internet search).

2. Simplicity - They work until they don't.

3. Longevity - I can tell you throughout my many years of working on cars, I don't ever remember replacing one of these. These things just continue to function.

These thermal units also make a sound loud enough to tell most drivers the blinker is on. All-in-all you give good reasons why to switch to an electronic unit, but remember, when it costs a few dollars more for each car produced, you'll still look at economy of scale and go with the cheaper version, especially when you know they work and work well.

  • The costs are a simple and strong argument to choose the thermal flashers, but i'm really surprised they're so cheap. I've had to repair them in both the vehicles I have restored, due too oxidated internals. I Guess I was just in bad luck. (Repairing them was mainly for fun btw) They seemed to have a materials and construction that is at least as expensive to produce as a relay. And they are usually more expensive. But if they are indeed cheaper, then it makes sense to keep using them, If they're so reliable and such, then why did they stop using them?
    – Bart
    Commented May 3, 2017 at 6:10
  • Perhaps they stopped using them to go to LED Bulbs? I'm sure with those the circutry is less expensive and simpler and the low power draw may not be enough to change the flashing rate and indicate a burnt-out bulb.
    – Bill K
    Commented Sep 21, 2022 at 23:59

At the time your mg was originally made, the thermal flasher unit was cheap and effective. The use of electronics in most cars came along later, such as electronic ignition only became common place in cars in the 80's and those were with ballast resistors which was a common fail point.

And as for wasting energy - well they may do, but it is a fraction of the energy wasted in turning fuel into motion...

  • A included a note under my question that obviously alternatives weren't available at production time of the MG, but thermal flashers have remained in use for much longer. I have found them in much newer 90s vehicles when electronics were readily available. They're even still for sale, and not only for nostalgic reasons. And obviously energy consumption is a really minor detail, I should've left it out of the question.
    – Bart
    Commented May 2, 2017 at 19:52

In comparison with a flasher built with a few semi-conductors: They're bigger in size,

No, not really. Electronics were pretty big until recently. In 70's a transistor that could hold enough current for a blinker alone was bigger than the entire flasher, not even talking about cost.

and probably more expensive, considering their relatively complex and precise mechanical construction.

But you forget about capital costs. The factories building thermal flashers were already up and running. Even if electronic flasher is £2 cheaper, how many do you have to install to cover the cost of setting up new supply?

more prone to failure

A car is a very bad environment for electronics. It can become very hot (every 10 degrees cuts lifetime of electronics in half), very cold (most semiconductors are not rated for operation below -20), the electrical grid is unstable (11-14 volts is typical range - just typical because values outside it WILL happen) and the noise from ignition system is horrible.

Plus, you haven't specified what "more prone to failure" means. If, let's say, the thermal flasher failure rate is 100% in 10 years and electronics is 30% in same timeframe, it still doesn't paint the whole picture. Flashers continuously wear down, so they can simply be engineered to withstand about 5 years, and most units fail after that. But - who cares, the warranty period is over. Electronics, on the other hand, don't wear down gradually, let's say it just has a random chance by being killed by voltage spike from the starter. So even if flashers are overall less reliable, they won't show up in warranty claims while electronics will. Also, things breaking down in an old car are simply expected, but in a new car - that's very bad press.

The movement from flashers to electronics isn't simply "engineering a drop-in replacement". It's a consequence of design trend that resulted in more electronic cars, that have stabilized circuits to run electronics in an electronics-friendly environment in the first place. It's not just flashers that changed, the whole car changed.

If you really want to know how "electronification" of cars went, read about the pioneers. Like Aston Martin Lagonda, it's electrical problems are legendary. When you try to introduce cutting edge before it's ready - this is what you get. That's why old, tried and true tech remains around so long.

Second, they require the right wattage bulbs or they'll flash too fast/slow, and for this reason you also need a second one for hazard lights use.

This argument is moot. The fixture requires correct wattage, it's stated in the manual, which is probably mandated by traffic code compliance. There is absolutely no reason to put any other wattage.

Today, we have other light sources, like LEDs. But their radiation patterns are vastly different, so you can't actually substitute a LED "bulb" for an incandescent bulb. (Sellers say that you can, but you'll never know if you got a proper beam until someone measures it.)

A last argument against thermal flashers is that they waste more energy, although this is probably negligible.

Yes, it is. Compared to the power draw of the bulbs it's blinking, it doesn't matter.


First of all, thermal flashers do NOT blink faster when a bulb blows - they flash slower. Bulb-out indication by "hyper-flashing" is a function of electronic relay flashers and some solid state flashers. Thermal flashers and electronic relay flashers can both suffer from corroded contacts. Electronic flashers can function with no solid state devices - just a simple R/C network or LC network - often using the coil of the relay as the inductance, or the lamps as the resistance. These are DIRT CHEAP to produce and basically bomb-proof as well as being non-polarity-sensitive.(and load sensitive to one degree or another)

Solid state flashers, up until a short time ago, were much more expensive to produce and polarity sensitive and totally incompatible with high current or momentary short circuits. A simple 2 terminal solid state flasher can now be purchased for as little as $2 out of China on Flea-Bay or Amazon - much less than the component cost in quantities of less than 100 - but many of them are still sensitive to overload and polarity. For a little more you can buy "overload protected" and "polarity agnostic" solid state flashers that work with LEDs - not even requiring a separate ground wire - which do not clock or buzz to indicate they are on. These require 2 flashers to provide emergency flashers. More common today are "integrated" electronic flashers that combine 2 flashers in one unit - and now "indicator lamp controllers" that do ALL of the switching in the unit - with low current control switches grounding inputs for left, right, or Hazard functions (generally used only with separate brake lights) - or even the flashers integrated with the "Body Control Unit" computer and even run on the CANBUS system.

Necessity being the mother of invention, these technologies FINALLY made the thermal flasher totally obsolete.

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