Thread: Basic electrics question

Most warning lights are triggered by an earth return, your test light would provide the earth

can you please explain how that differs from a normal circuit?

Because you using a test light has high impedence, the warning light lights up easier with low current flow, DC can be wierd stuff

so power is flowing thru the fuse in the test lamp,but not enough to light it up, just following the path of least resistance. But its still a 12 volt circuit, I dont get it.

Originally Posted by crump

so power is flowing thru the fuse in the test lamp,but not enough to light it up, just following the path of least resistance. But its still a 12 volt circuit, I dont get it.

Just like I tell everyone when they ask me questions about working on DC control systems, the correct answer is...........Because its DC, it just does

I'm rethinking my original answer, I'm thinking, you have a low impedence test lamp, high impedence warning lamp, not enough current flow after heading through the warning lamp to illuminate the test lamp, I need to set it up and check with a meter, which I can;t 3 hours from home

Unnecessary post

Hi Crump, are the dash lights, globes or LED?

While the principal is the same, it’s just easier to explain the set up when it’s LEDs.

An LED will light up with as little as 2v and draw only 20ma. but because they are being used in a 12 volt circuit, a resistor is used as a very crude voltage and current regulator.

When you put your test light in the circuit, the test light’s globe needs about 300ma to light up BUT the resistor is still in the circuit and limits the total current of the whole circuit to around the 20ma mark.

While the 20ma is not enough to light up your test light globe to a point where you can see light, the globe is actually lighting up but at such a low luminous that it is not visible.

How ever, there is still 20ma in the circuit which is enough to light up the LED.

If you have small dash light globes, the principal is basically the same except the small dash light globe act as both the LED and resistor combined in one device.

This set up has been used for many years in other devices and as a safety device.

A perfect example is that used in old Triang train controllers.

While the loco stayed on the tracks, it run around powered by current flowing through a large light bulb mounted in the controller.

BUT, if the loco derailed and shorted the neg and pos rails, the bulb lit up till the loco was lifted off the rails.

Exact same principal as the small dash light globe ( the loco ) and the large test light globe ( the bulb in the controller ).