Should a diode be used with a relay from upfitter switch?

[GordoJay]

I think we probably only need to add a diode when wiring into an existing circuit in which we don't want to backfeed as in the example of adding reverse lights and not only wiring them up to an uplifter switch but also tying them into the reverse lights ... it prevents the path to the OEM reverse lights when operating the uplifter switch...

Bingo. That's it. I knew there had to be a reason. Thanks

 

[2020Raptor]

Afternoon.
Thanks to everyone for all of the info.

More to it than meets the eye, as usual.
The diode favorited by Just_in_time has a 48-week lead time.

"The diode should have a voltage rating >12 Volts and instantaneous current rating = the load current.
Most diodes current rating is much much less than their instantaneous current rating."

This seems to be the trickier spec to find.
Has anyone identified a diode that meets all of the requirements ?

I you have, it would be greatly appreciated if you would share that info.
Thanks very much !

 

[bryanb]

"The diode should have a voltage rating >12 Volts and instantaneous current rating = the load current.
Most diodes current rating is much much less than their instantaneous current rating."

This seems to be the trickier spec to find.
Has anyone identified a diode that meets all of the requirements ?

Hey,

I think maybe two different diode use cases are being mixed in this thread. Is your question about:

1) Using a diode to protect against damage from the transient voltage created when power is turned off to a relay coil; OR

2) Using a diode to ensure current only flows in one direction

The thing you quoted doesn't have anything to do with case 1 and a diode doesn't have a "voltage rating" in the sense it is being used in that sentence. A diode has a forward and reverse bias voltage and the former is not a "rating" bur rather a characteristic of the semiconductor type and the latter is the breakdown voltage when the diode is reverse biased (the voltage at which the diode stops behaving like a one-way street and becomes two-way). The forward bias voltage drop (usually ~0.7V for silicon) has nothing to do with either case above (in terms of selecting which diode to use). The reverse bias (or peak inverse voltage) rating is relevant to the first case, but not in a way that is likely to matter for case 1 in automotive since the peak inverse voltage is pretty high for silicon diodes in general (above 12 V). And if you google this topic, you need to understand that there are different diodes not used in either of the two cases above where what I said won't apply in the same way (e.g., Zener diodes).

So tell me which case you are interested in and I'll tell you which diode to use (that doesn't have a 48 week lead time!).

 

[The Car Stereo Company]

almost a year wait time for a diode? what laser mind control device you building? as stated above, whats the purpose of this diode?

 

[GordoJay]

Afternoon.
Thanks to everyone for all of the info.

More to it than meets the eye, as usual.
The diode favorited by Just_in_time has a 48-week lead time.

"The diode should have a voltage rating >12 Volts and instantaneous current rating = the load current.
Most diodes current rating is much much less than their instantaneous current rating."

This seems to be the trickier spec to find.
Has anyone identified a diode that meets all of the requirements ?

I you have, it would be greatly appreciated if you would share that info.
Thanks very much !

If you're using it the way that @Just_in_time was, as a flyback diode, these are all roughly equivalent to the 1N5809 and should work.

https://www.digikey.com/en/products/filter/diodes-rectifiers-single/280?s=N4IgjCBcoJwGxVAYygMwIYBsDOBTANCAPZQDaIALAAwDsMAzABwgC6hADgC5QgDKnAJwCWAOwDmIAL6EYYCohApIGHAWJkQAJjhxtEQtt2MEbEFx79h4qYTCbZCpSryESkcvTA0wMKqw7ckHyCohLSWrQQ0GaBIACqIkKcAPKoALK46NgArgK4IITZPABqBSAAtqI8cH6E5egAHjwArFR+4QC0mo5QgtlqbuTN-jE8UpKSQA

Voltage rating should be more like 120V, BTW. 12V isn't nearly enough. Peak current rating is generally about 20x the steady state rating.

 

[2020Raptor]

almost a year wait time for a diode? what laser mind control device you building? as stated above, whats the purpose of this diode?

 

[2020Raptor]

Hey,

I think maybe two different diode use cases are being mixed in this thread. Is your question about:

1) Using a diode to protect against damage from the transient voltage created when power is turned off to a relay coil; OR

2) Using a diode to ensure current only flows in one direction

The thing you quoted doesn't have anything to do with case 1 and a diode doesn't have a "voltage rating" in the sense it is being used in that sentence. A diode has a forward and reverse bias voltage and the former is not a "rating" bur rather a characteristic of the semiconductor type and the latter is the breakdown voltage when the diode is reverse biased (the voltage at which the diode stops behaving like a one-way street and becomes two-way). The forward bias voltage drop (usually ~0.7V for silicon) has nothing to do with either case above (in terms of selecting which diode to use). The reverse bias (or peak inverse voltage) rating is relevant to the first case, but not in a way that is likely to matter for case 1 in automotive since the peak inverse voltage is pretty high for silicon diodes in general (above 12 V). And if you google this topic, you need to understand that there are different diodes not used in either of the two cases above where what I said won't apply in the same way (e.g., Zener diodes).

So tell me which case you are interested in and I'll tell you which diode to use (that doesn't have a 48 week lead time!).

Thanks very much for joining in.
I want to have two methods of turning on my light bar.

1. Upfitter switch
2. High beams (solid state relay, per a different thread)

My understanding is that use of the appropriate diode will prevent backfeeding the unpowered leg.
That is the goal, unless there are additional considerations of which I am not yet aware.

Which is what I discovered here while performing my due diligence.
Apparently, you cannot just use any diode labeled "12V".

Googling this topic will make me want to toss the light bar in the trash, ha ha.
Thus my desperate plea for assistance here on the forums.

 

[2020Raptor]

If you're using it the way that @Just_in_time was, as a flyback diode, these are all roughly equivalent to the 1N5809 and should work.

https://www.digikey.com/en/products/filter/diodes-rectifiers-single/280?s=N4IgjCBcoJwGxVAYygMwIYBsDOBTANCAPZQDaIALAAwDsMAzABwgC6hADgC5QgDKnAJwCWAOwDmIAL6EYYCohApIGHAWJkQAJjhxtEQtt2MEbEFx79h4qYTCbZCpSryESkcvTA0wMKqw7ckHyCohLSWrQQ0GaBIACqIkKcAPKoALK46NgArgK4IITZPABqBSAAtqI8cH6E5egAHjwArFR+4QC0mo5QgtlqbuTN-jE8UpKSQA

Voltage rating should be more like 120V, BTW. 12V isn't nearly enough. Peak current rating is generally about 20x the steady state rating.

Thanks very much.
A few questions:

Shouldn't the diode be 15 amp, rather than the 6 amp units shown ?
How to spec:
1. Voltage - Forward (Vf) (Max) @ If
2. Voltage - DC Reverse (Vr) (Max)
Is one of those aka "instantaneous voltage" ?

 

[GordoJay]

Thanks very much.
A few questions:

Shouldn't the diode be 15 amp, rather than the 6 amp units shown ?
How to spec:
1. Voltage - Forward (Vf) (Max) @ If
2. Voltage - DC Reverse (Vr) (Max)
Is one of those aka "instantaneous voltage" ?

You're not planning to use it in the same way as @Just_in_time, so you probably don't need such a robust diode. If you choose a diode with a forward current rating at least double the maximum coil current of your relay, you should be fine. I'd still want more than a 12V reverse breakdown voltage, but ~100V is common and cheap. Choose a relay with the lowest coil current possible so that you reduce the chances of bricking your BCM.

 

[2020Raptor]

Thanks for the clarification.

It should be 15+ amps, correct ?