I need a little electrical help

[JoshH]

I’m working on some wiring for a swap I’m going to be starting on here pretty soon, and I’m going over the factory fan control wiring. The show a couple of diodes, but I’m having trouble figuring out what function they perform in the circuit. I’m guessing maybe it is some sort of damping or something for when it switches from high to low, but I’m really not an expert on this at all. I’ll post a picture of the diagram, and hopefully someone smarter than me on this stuff can tell me what they do.

 

[dndj]

Those are snubber diodes aka flyback diodes. Commonly used in DC motor circuits to suppress voltage spikes created from back EMF when the fan motor is switched off.

 

[JoshH]

Thanks for your help. I suspected it was something along those lines. Can you tell enough from the diagram to tell me if these diodes would be ok to use there? I ended up with a bag of them from something, and I figure if they are up to the task, I’ll use them for this too.

 

[2004LB7]

I would not use those as they are only good for up to 45 volts. I would look for a 1N400X type diode in the fast recovery verity and maybe 200 or more volts.

A 1N4007 is probably more than suitable and cheap

 

[DAVe3283]

Those are rated for 45 volts reverse voltage, and you should only see ~14V. During transient surge suppression, they aren't reverse biased. 15 amps forward current continuous with 275 amp surge. Very low forward voltage. Not seeing much info on their RF characteristics, which isn't surprising since they are for solar panels.

I suspect they'll do fine. I'd run them.

 

[2004LB7]

Brushed motors can induce several hundred volts or more into the circuit. They will likely be fine for relays but I would use something with much higher voltage rating for brushed motors

 

[dndj]

Brushed motors can induce several hundred volts or more into the circuit. They will likely be fine for relays but I would use something with much higher voltage rating for brushed motors

That is true about brushed motors, but when used as a flyback diode, it becomes forward biased in the presence of these large back EMF voltages. So reverse voltage breakdown spec only needs to meet ~14v (with some margin). Most important diode spec in this application is peak forward surge current. I would use a minimum of a 1N5408 with 200A surge. 275A from the schottky diodes in the picture is even better than that.

 

[2004LB7]

I know schottky diodes are really good for this purpose so if you think the voltage specs are suitable then good. Give them a try Josh

 

[JoshH]

Thanks guys. I guess worse case, if the diodes aren't big enough, it'll just burn out the contacts on the relays quicker than normal?

 

[2004LB7]

Typically yes. But also remember, before that arc is broken, some of that higher voltage may make it back to the source or other electronics

 

[DAVe3283]

It's trying to pull the voltage negative, not send power back to the truck. That's why the diode allows current to flow from ground to positive, to snub the negative spike that can keep/start an arc in the relay contacts.

The inductance of the motors is like a long water pipe. When things are running, you have positive pressure at the start of the pile. Then you slam the valve shut (relay opens). The pipe will actually create a suction, and on long enough pipes they can collapse. So you run a vacuum breaker allowing air (0 PSI) in to the pipe. Under normal use, it never does anything, but when you slam the valve, it opens and lets in air to prevent the pipe from going to a suction. A diode across the relay is like the vacuum breaker.

Not a perfect analogy, but I find the water comparison for inductance to be a helpful way to mentally model what is happening.