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

[Oldfart]

When I hooked up my BD Squadron lights, I put a pair of Racer Spots on Aux 6, so I put in a 30 amp relay for those lights. I've recently run across some information about how it's a good idea to install a diode with a relay to protect your switch and your harness. This is news to me, and not something I've done before, or am familiar with. Is a diode needed? If it is, specs of what one is needed? Thanks.

 

[Muchmore]

Not really. Having one in line would be "like" having a fuse on both the positive and negative sides. A diode would prevent current flow from going in the wrong direction.

 

[The Car Stereo Company]

not necessary in this application. for more sensitive electronics, you should use one, but it wont hurt if you want to use one. a standard 1amp diode will work fine.

 

[Oldfart]

Not really. Having one in line would be "like" having a fuse on both the positive and negative sides. A diode would prevent current flow from going in the wrong direction.

not necessary in this application. for more sensitive electronics, you should use one, but it wont hurt if you want to use one. a standard 1amp diode will work fine.

Thanks, I just wanted to make sure I wasn't missing something that I should have included in my original install.

 

[GordoJay]

Why would you need a diode? I did a quick search and all I found was using a flyback diode to clamp voltage spikes across the coil. Since the only other thing in that circuit is a switch which can handle current flow in either direction, I'm wondering what you need one for. Is there some reason to put a diode in line with the power feed to the accessory?

 

[Oldfart]

Why would you need a diode? I did a quick search and all I found was using a flyback diode to clamp voltage spikes across the coil. Since the only other thing in that circuit is a switch which can handle current flow in either direction, I'm wondering what you need one for. Is there some reason to put a diode in line with the power feed to the accessory?

I had seen random semi-vague mentions of things like, "It will make the switch happier", or "It will extend the life of your switch" type of things, but nothing factual on why it would be needed. That's why I wanted to ask.

 

[GordoJay]

I had seen random semi-vague mentions of things like, "It will make the switch happier", or "It will extend the life of your switch" type of things, but nothing factual on why it would be needed. That's why I wanted to ask.

In theory you could pull an arc across the switch contacts as they start to pull apart. In practice, switches are really robust and the inductance from a small relay is very low, so it shouldn't be a problem. Now, if you're tapping into the electronics, like the high beam wire, then using a diode should be mandatory.

 

[Just_in_time]

When a switch/ relay is commanded off - the sudden stop of current will result in a spike of voltage if any inductance is in the load or wiring. You are guaranteed that the the wires will have inductance - so you are also guaranteed that you will have a spike. If your load has a inductive component to it the spike will be larger and the amount of energy will be larger as well.

The spike will result in pitting of the contacts in the the relay - increased resistance of the relay and eventually excessive voltage drop/heat on the relay.

it is common practice to place a diode on the load side of the relay from the output to ground. The anode on ground, the cathode on the output (assuming a positive voltage system). This diode is also known as a “catch” diode. When the relay opens - the current has a path to ground and the energy in the inductance is able to be dissipated.

This diode is reverse biased under normal conditions and hence dissipates no power except when the relay is commanded open. 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.

Finding a diode with leads for attachment so that you can physically connect it may be fun nowadays! My all time favorite diode is the 1N5809 but no idea if you can still buy them.

Relays also have inductance and some relays do not have diodes built in to suppress voltage spikes on their internal coils. If your relay does not have a diode built in, it can be a good idea to place one across the coil in the proper direction, this helps the relay closure and also serves to make the relay last longer. Off hand - my guess is the relay you may have picked has a diode built into it already.

So to be clear - any time a relay is used - their are typically 2 diodes in the discussion, one is built into the relay across the coil, the other is added to the circuit to the load side to allow current flow when the relay is opened.

I would recommend adding a diode to the load side of any relay circuit as standard practice to prevent pitting of the relay contacts. Again, anode to ground, cathode to positive output side such that it does not conduct under operating circumstances. The diode voltage rating needs to exceed the bus voltage (battery in this case I assume) and it need to handle a spike in current for a short duration....

Solid state relays don’t have a coil to worry about - but the diode on the load side is even more important as the voltage spike can/will result in excessive voltage across the switch/relay and catastrophic failure.

 

[bryanb]

Just a few more thoughts on the topic:

The textbook answer is that electromechanical relays should always have suppression (flyback) diodes because the collapsing magnetic field in the coil when power is removed generates a high voltage across the terminals of the relay and this can damage the solid state electronics driving the relay. Additionally, without the reverse biased diode to absorb the energy turning it into heat, there is significant electromagnetic interference produced and this can couple into nearby circuits not even connected to the one in question. Whether that causes a problem depends on how those other circuits are designed.

Automotive electronics are already designed for a high degree of noise immunity and a generally very harsh electrical environment so as a practical matter relays without a diode, as long as they are not connected to the unprotected output of a solid state device (transistor) probably won't need the diode. Properly designed automotive electronics will already protect the driving transistors on the output of the electronics module with transient voltage suppression devices and those will do the same job as the flyback diode. In other words, the diode is already there, but it may not be right at the relay (even though that's the best place for it if trying to prevent EMI).

In the case of the upfitter switches these already drive relays (through the BCM) so if you are connecting a relay to the upfitter "switch" you're really driving a relay with a relay and there's no silicon in the path to directly damage (though the EMI issue still exists). As some mentioned, this situation still theoretically poses a wear and tear situation on the driving relay's contacts, but it's not really worse than the relay switching its rated load. (Relays experience and are designed for arcing during normal operation, but even a small high voltage spike across the gate of a small transistor will punch a hole through it and kill it.)

The good news is that you can always add a reverse biased standard diode across the relay terminals without causing any problems. A 1N4007 will be suitable for any relay in an auto.

 

[sixshooter_45]

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.

Some vendors do this in their supplied wiring harness.

I think it stops the electrical path from backfeeding the uplifter switch when you throw into reverse or maybe it prevents the path to the OEM reverse lights when operating the uplifter switch, not really sure on this but I know one of the Vendors could let us know.