Quote:
Originally Posted by utopia
That's the bit I've never really understood.
I've always reasoned that if the system behaves as if a bulb is out, that would tally with an ultra high or infinite resistance, not the ultra low resistance of an LED.
In other words, it would behave as if the bulb had been short circuited (low resistance path) rather than blown (infinite resistance path).
|
Good lord, I'm too young to understand all that analog stuff!
Heh. As-if...
https://auto.howstuffworks.com/turn-...he%20resistor.
Quote:
When you push the turn-signal stalk down, the thermal flasher connects to the turn-signal bulbs by way of the turn-signal switch. This completes the circuit, allowing current to flow. Initially, the spring steel does not touch the contact, so the only thing that draws power is the resistor. Current flows through the resistive wire, heating up the smaller piece of spring steel and then continuing on to the turn-signal lights. At this point, the current is so small that the lights won't even glow dimly.
After less than a second, the small piece of spring steel heats up enough that it expands and straightens out the larger, curved piece of spring steel. This forces the curved spring steel into the contact so that current flows to the signal lights unimpeded by the resistor. With almost no current passing through the resistor, the spring steel quickly cools, bending back away from the contact and breaking the circuit. The cycle then starts over. This happens at a rate of one to two times per second.
|
Typical spec for these relays is 90 +/- 5 cycles/minute - when the load is 2x 21W bulbs wired in parallel, as per this example diagram:
So the resistance from the bulbs in parallel is lower than two bulbs in series (parallel resistance is calculated as 1/Rt = 1/R1 + 1/R2 + 1/R3 + ...).
A modern one will still operate with one bulb out - but if you've blown a 21W bulb, it'll hyperflash - because a single bulb means more resistance, so with less current flowing, bi-metallic strip doesn't get as hot - and cools down faster.
LEDs draw so little current without load resistors that an electro-mechanical relay might not operate at all - but if it does, it hyperflashes, same as if you've got a bulb out.