Diode help needed

[Smokinyoda]

Ok electrical gurus I need your help again. I have 2 different scenarios where I need a diode, please reply if you can help.

1. I have a 3 way toggle switch that operates reverse lights and back up alarm in 1 position. Next position is back up lights only. Last position is everything off. What diode do I need to keep from back flowing alarm when lights only is selected. Lights and back up alarm are 12v, lights are 55w each total of 2.

2. I want the park lights on my m35a2 to come on when I run my headlights, but I still want to use the park only function on the 3 way switch. I know what wires to jumper, but not sure which diode I need to keep from back flowing to the headlights when park is selected. Standard 24v lights.

I'm not to bright on diodes so a part # or link would be appreciated.

 

[FullSpecial]

1. DPDT on-off-on switch would be easiest thing to do.
2. 3amp 50 volt diode. Two front "Parking lights" are different circuit from all the other running lights.
I used RadioShack 276-1141 Epoxy rectifier diodes for that very propose on my 5T.
*yeah I know, good luck with RadioShack*

 

[tim292stro]

I'd recommend relays... Diodes that support more than 1Amp are going to be expensive and generate a lot of waste heat.



With the above circuit, the supply for the relay coils is from your lighting circuit, the relays take the big load straight to the battery through a fuse. Neither relay can activate if your transmission switch is not closed. This way, your master light switch can control the availability of the backup lights and beeper through the simple controls - your SPDT toggle switch gives you control of Light+beeper, Light-only, and Off. A diode with the anode to the beeper side of the switch and cathode (side with the line on it) towards the light side of the switch, means that power will only flow from the beeper to the light side (only flow to "lights" when "beeper" is selected).

For diodes you can use the simple and very common 1N4001 diode.

If you have a pushbutton Master Light Switch:


You'll want to make sure you snub the relays so they don't spike your electronics... (it's hooked up cathode to positive to return the pulse from the collapsing coil's field back to the coil to use it as heat)



Again for the Park+Service or Park-Only lights, I'd recommend using a relay for the parking light circuit, and using diodes (again 1N4001) from the service light and parking light circuits to power the relay coil (other side to ground). Power the parking lights from a circuit breaker to the battery. This way you are only using diodes for very low current relays, not the potentially full 30-40 amp loaded circuit...

 

[dmetalmiki]

I wish I had picked your brains when (before) I wired up my headlights and the rear spots on 530C sequentially. Interesting 2 or 3 hours! but a nightmare in figuring out. (not a diode in sight).

https://youtu.be/d9ET3P_K0XE

 

[papakb]

removed

 

[tim292stro]

Plenty for relays, which is still my advise. Diodes drop (consume) a specific voltage, volts * amps = watts, the desire should be to reduce loads that pass through lossy componets IMHO...

 

[papakb]

removed

 

[tim292stro]

Wait... re-read the OP's questions and take another look at my proposed schematic (also he's talking about controlling 12V lights, so that volt drop is no longer "negligible").

Again, it's the voltage * current I'm more bothered by in a power circuit. That voltage drop is constant (and for both the 1N4000 and 1N5400 series, it's 1V), so whatever current goes across that diode goes towards its power rating. 2 Amps across 1V is 2watts - if he's talking about putting the entire backup light load or parking light circuit load through a single diode, he's going to need a diode with 10Amps plus rating. 10Amps * 1V = 10Watts, and now we're talking heatsink territory too (parking light circuit is a constant use load, so we can't get away with short duration heating).

This is the primary reason that I (and I would argue the majority of the automotive industry) use low current for control and logic, but relays and MOSFETs/IGBTs/BJTs for ultimate load switching.