Flyback Diodes and inductor protection

Hey everyone,

The wiring courses don't really talk about inductive loads, and flyback protection, but as I continue to design my harness - I could see this possibly being a large problem. Maybe it's not that big of a deal with the modern electronics, and many ECU's / PMU's, but I want to make sure that I'm safe here before building the actual harness it's self.

For inductive loads like solenoids and other devices like pumps, do I need to use an external flyback diode? From my research, sometimes the reverse voltage spike can be 60-100v in a 12v automotive environment, which will certainly fry an internal in the ECU. The courses however did not really talk about inductive loads, mentioning that it was out of the scope for the course - so maybe the overall idea isn't too complicated.

What about relays and the coils used to energize that relay, should I be using a diode for relays? What about large current objects like fans and pumps, that require 30a+, I don't think I'd be able to find a 60a diode for my fans.

Any and all help is appreciated, thanks!

Most of the time it’s not something you need to worry about too much. For smaller loads like injectors and boost solenoids, the ECU hardware is designed to handle the inductive spike when the field collapses. For bigger loads like fans and fuel pumps, you wouldn’t drive those directly with the ECU anyway, they’ll be switched through a relay or a PDM. PDMs like the MoTeC units specifically state that their outputs are built to handle inductive loads and include clamping for that reason, so they’re safe to use on those kinds of devices.

When the ECU is only being used to trigger a relay, the only inductive spike is on the relay coil, and the ECU can cope with that as well. A lot of automotive relays also come with a built-in diode or resistor for extra protection, so in most cases you don’t need to add anything external, unless the manufacturer of that device specifically states it.

Thanks for the response Caleb, would putting a flyback diode across the inductor wires affect it's performance if not neccessary? That in, the ecu already handles the flyback protection. What about large fans like you were talking about, my fan is controlled lowside by ECU and high side by a relay, would I need any protection for either the relay or low side control line? Is there any general amperage or voltage draw that when higher, would deem a flyback diode necessary? In other words, does the amperage of an inductive load dictate the amount of flyback, and whether it should have a flyback diode? From my understanding, my ECU is only capable of 36v max for the outputs and inputs. Thank you!

The size of the current does affect how much energy is stored in the magnetic field, but the more important thing is the current rating of the ECU or PDM output you’re using. If you stay within that spec, and the manufacturer says the output has inductive load protection (clamping, flywheeling diodes, etc), then you don’t need to worry about adding anything external. The circuit will already be designed to handle the worst-case spike for the loads it’s rated to drive.

Manufacturers will almost always spell this out in their documentation. For example, in the Link ECU wiring manual they state, “Each auxiliary output consists of a low side driver and flywheeling diode” but also, “This does not apply to Auxiliary Ignition or Auxiliary Injection outputs as they do not have flywheeling diodes fitted. For that reason solenoids that require flywheeling can not be wired to these drives without an external flywheeling diode fitted. This will only usually apply to ISC solenoids."

The important thing here is not to assume injector or ignition outputs have no protection, they do, but it’s not a simple diode. Those outputs use dedicated high voltage tolerant driver circuits because injectors and coils actually need a fast, high-voltage collapse to work correctly. If you were to put a flyback diode across an injector, the magnetic field would collapse slowly through the diode and the injector would close more slowly. We’re only talking milliseconds, but that absolutely affects fueling. This is why ECUs do not clamp injector outputs with diodes.

The short answer is, read the manufacturer’s documentation. They will tell you exactly when a diode is required, and if you stick within the rated current and use the outputs for the devices they were intended to drive, you won’t need external flyback protection.

When you’re using an ECU output to trigger a relay for a high-current device, most automotive relays already have a resistor or diode inside. The only time I add a diode myself is when using a solid-state relay, because the internal electronics are much more sensitive and usually don’t include suppression.

If you’re unsure about a particular load or output, feel free to ask for peace of mind.