Some call them "clamping diodes".

I have a situation with some of the solenoid circuits in our trucks.

Where I have switch-panels that have multiple LED indicators that share a common GROUND, when I turn OFF power that triggers a solenoid, all of the LED indicators will instantly flash.

This makes no sense at all to me; as, once the switch breaks, there is no electrical connection whatsoever between the LED indicators and the solenoid, and the LEDs are GROUNDed to GROUND bars in the cab and the solenoids are GROUNDed elsewhere, wherever they are mounted.



I will describe one group of four LED indicators in particular; one is low vacuum warning for the truck vacuum system, one is low vacuum warning for the trailer vacuum system, one indicates the air-compressor system is ON regardless of whether the compressor is running, and the fourth indicates the pressure-switch has turned ON the compressor.

The pressure-switch is installed in the air system about two inches from a solenoid that blows a set of air-horns; although both are plumbed into the air-line, they share no electrical connections in common.

BUT, when I energize the horn solenoid and blow the horns, upon release of the horn-button, all four of these LEDs will brightly flicker.

I have similar situations in other circuits and in other vehicles.

This has killed a couple of the indicators.


On reading about the use of "quenching diodes" in the book "Automotive Wiring and Electrical Systems" by Tony Candela, I understand that current can enter a diode on the ANODE side and exit on the CATHODE side, right ???

He goes on to state and illustrate to attach the CATHODE to the HOT side of the relay/solenoid energizer and the ANODE to the GROUND side; thus, when the power is cut and the magnetic field collapses, the excess voltage will be shunted to GROUND.

I have read and examined his illustrations over and over; and, to my way of thinking, there is already a GROUND connection on the GROUND side; and, he has the diode oriented such that current can only pass from the GROUND side to the HOT side through the diode, and the HOT side is already electrically connected to the GROUND side via the coil through the magnet and can go nowhere else other than back to GROUND since the switch is now open; I can really see no useful purpose that these quenching diodes are actually performing.

But then, I can also not see how it is possible for this stray current to flicker LEDs that are OPEN on their HOT side, when I can not even get a flicker from one when I purposefully wire one backwards with a test-wire.


None-the-less, my electrical ignorance not withstanding, will it cure my flickering LED problem if I install these diodes as illustrated in the book ??


Thanks.

 

I know this may sound stupid, but why not wire all of your grounds through a rectifier. Current can only pass one way through one so if grounded thru a rectifier, will it not stop the transient voltage from flowing thru the diode??

Gary

 

The LED's are 12v? And do you have a 100-400 ohm resistors installed inline with each of the diodes? The resistors is for current limiting so it doesn't burn up the LED's. I usually run power to the anode and cathode to ground.

Can also try installing a 8.2k-10k ohm resistor as pull down on the input to the LED's but upstream of the series resistor.

MikeyB