idiot lights

I need to wire some basic idiot lights for my homebuilt. Some are simple
independent switched light circuits but others indicate motors, such as a
fuel pump is active.

Are there any basic schematics/resources available on the web to show how to
run an indicator in parallel with something like a pump motor. My guess
would be that a diode needs to be included someplace. BTW I will probably
be using LEDs with drop down resistors

Thanks, Doug

The "D" in LED means Diode, so you will have a diode there whether or not
you want it. The simple answer is that a resistor in series with an LED
across an active circuit (like a motor) will light up. THe problem is the
reverse voltage spike (back EMF) of an inductive circuit like a motor when
it is shut off. You may wish (and probably should) break the current limit
resistor to the LED into two parts and put a fairly hefty snubber diode in
the circuit.


Like so:


+ bus ----- switch ----- motor --- ground
|
|
resistor
|
|------resistor---LEDA-LEDK---ground
|
diodeK
diodeA
|
|
ground

Jim


"Doug Palmer" wrote in message
ink.net...
I need to wire some basic idiot lights for my homebuilt. Some are simple
independent switched light circuits but others indicate motors, such as a
fuel pump is active.

Are there any basic schematics/resources available on the web to show how
to
run an indicator in parallel with something like a pump motor. My guess
would be that a diode needs to be included someplace. BTW I will probably
be using LEDs with drop down resistors

Thanks, Doug

Hi Doug,
I'm planning on doing this kind of thing also.

Can you give some example part numbers for a snubber diode. Are you
referring to zener diodes?

RST Engineering wrote:
The "D" in LED means Diode, so you will have a diode there whether or not
you want it. The simple answer is that a resistor in series with an LED
across an active circuit (like a motor) will light up. THe problem is the
reverse voltage spike (back EMF) of an inductive circuit like a motor when
it is shut off. You may wish (and probably should) break the current limit
resistor to the LED into two parts and put a fairly hefty snubber diode in
the circuit.


Like so:


+ bus ----- switch ----- motor --- ground
|
|
resistor
|
|------resistor---LEDA-LEDK---ground
|
diodeK
diodeA
|
|
ground

Jim


"Doug Palmer" wrote in message
ink.net...
I need to wire some basic idiot lights for my homebuilt. Some are simple
independent switched light circuits but others indicate motors, such as a
fuel pump is active.

Are there any basic schematics/resources available on the web to show how
to
run an indicator in parallel with something like a pump motor. My guess
would be that a diode needs to be included someplace. BTW I will probably
be using LEDs with drop down resistors

Thanks, Doug

The 1N400x series are generally hefty enough to use for snubbers. For a
small motor like a fuel pump, I'd probably use a 1N4004 or above.

Jim



"ejb" wrote in message
ups.com...
Hi Doug,
I'm planning on doing this kind of thing also.

Can you give some example part numbers for a snubber diode. Are you
referring to zener diodes?

Great stuff. I'm very glad to have this forewarning.
There's no substitute for this usnet group :-)

Thanks All, Very Helpful

Doug