This is about replacing a LUCAS FL5 3-prong flasher unit with a plug-and-play replacement circuit that will work with a much lower current than the FL5.

The FL5 uses a bi-metallic strip and a heater coil to provide the flashing mechanism, see Diagram. LUCAS FL5 Schematic

When the load is applied (there is in fact the indicator turn switch between the load and the FL5) the heater warms. This causes the bi-metallic strip to bend and the contacts close. This shorts out the heater, and applies full current to both the load and the test light. As the strip cools the contacts open again. This repeats, and the lights flash.

The unit is NOT polarity sensitive and was used in both Positive and Negative Earth Cars.

Using LED replacement bulbs reduces the current through the heater to a level where the unit will not function.

So we need to replace the heater with an electronic equivalent.

Issues here include no permanent connection to both +VE and -VE (depending on the car polarity) and the need to make the circuit polarity insensitive.

Any ideas?

You should note that there are commercially available units BUT these either are only for -VE cars or they have another flying lead that needs to be earthed. Being a purist I really want the replacement unit to be exactly the same as the original, only electronic and not electro-mechanical.

  • I had this big write up about LED flashers, but did not take into account the need for it to be polarity insensitive. I'll see what I can figure out. You can still get the Tung-Sol flashers as @DwayneReid states below, but you have to order them off of eBay and they are not that cheap. – Pᴀᴜʟsᴛᴇʀ2 Feb 5 '15 at 21:57

Use an electronic flasher. I have no idea if Tung Sol is still in business, but they made a very nice electronic flasher that simply replaced the old bi-metal unit. True plug-and-play operation: remove the old flasher, plug in the new.

Note that even though this was classified as an electronic flasher, the only electronics inside was a really cool relay coil having two windings and a large-value bi-polar electrolytic capacitor. No transistors or chips or even resistors. They work very well.

The Tung-sol electronic flashers are not sensitive to polarity and were available in 2 or 3 different styles: SPST, SPDT (alternating flashing for emergency vehicles) and, I think, the version you are looking for with two simultaneous single-pole outputs.

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