Diode, resistor, or both for inductive load protection - Page 1
Forum section: electronics / projects

author: drakejest
author_rank: Regular Contributor
timestamp: October 08, 2023, 05:51:44 pm
content: I have aautomotive relayand i would like to place some sort flyback protection on its coil pins just like how you would do on almost any circuits involving relays. I like placing a diode on the coils as it would also reduce the clicking sound of the relay.My go to solution is to just place a diode parallel to the load (in this case the coils) but i have never actually thought about how to properly part pick the diode , for example is there a preferred type? zener or schottkey? how do i calculate the amperage rating (i assume i will be using the peak current and not nominal current) and voltage rating.I have also noticed in the catalog of the relay, some relay has a built in resistor instead of a diode, and a quick read online says that a resistor is better than a diode in terms of being a flyback protection, but with a drawback of it consumes a bit of power during operation. Is it true that it is better than a diode in terms of voltage suppression? How do you choose the resistance and wattage value of the resistor?Also to solve the problems the resistor i have read that you can series the diode and resistor to get the best of both worlds. if it is indeed the perfect solution why do relay manufacturers dont include a resistor + diode installed in the coils in their catalog? it always just a resistor or just a diode.

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 08, 2023, 06:04:22 pm
content: Nor zener or schottky.Schottky are meant for pretty low voltages, rarely over 100V. Why would you want a zener?You only want to redirect the back-EMF into the coil itself to prevent the spike.99% of the relays use a 1N4148 (Small signal relays) or 1N4007 (Beefier relays).I'd just use the 4007, still cheap.A resistor or resistor + diode will also work. The spike will be there but much smaller, also the relay will turn off faster, as the stored energy will be dumped into the resistor, instead on the relay (Keeping the magnetic flux for longer).It's the engineer / designer's task to do this depending on the application.

author: srb1954
author_rank: Super Contributor
timestamp: October 08, 2023, 09:10:23 pm
content: Quote from: drakejest on October 08, 2023, 05:51:44 pmAn ordinary silicon diode is satisfactory, provided it is not a particularly slow type or high junction capacitance type e.g. 1N54xx series. There is no real advantage in using a Schottky diode as their greater speed is not beneficial for flyback suppression and their generally lower voltage ratings and higher reverse leakage currents are disadvantageous.The diode peak current rating should be greater than the maximum relay coil current and the diode reverse voltage rating should be greater than the supply voltage, taking into account worst case circuit conditions and any desired derating factor.QuoteA resistor isn't actually better than a diode for suppression of the flyback voltage but it is cheaper and it also allows the relay to drop out more quickly once the driver is switched off. The relay coil stored energy is dissipated in both the external resistor and the relay coil resistance and thus dissipates more quickly. The disadvantage is that the flyback voltage is much greater and puts more stress on the driver transistor during its switch-off transition. The driver transistor consequently has to have a higher voltage rating and particular attention should be paid to its safe operating area (SOAR) curve.The resistor's value can be determined from the peak relay current and how much extra flyback voltage can be tolerated. The resistor peak power rating can also be determined from its value and the peak relay current. Average additional resistor power dissipation due to the relay flyback is usually insignificant compared to the resistor's standing power dissipation when the relay is turned on.The same effect as a fixed resistor flyback suppression can be obtained by using a Zener diode, but across the driver transistor rather than across the coil. This allows a higher flyback voltage and faster relay drop out without the power wastage incurred by the resistor in the on state. The peak power dissipation in the Zener is quite high when the relay is first turned off so this has to be carefully looked at. The average power dissipation in the Zener is usually not a problem.QuoteIncorporating a fixed resistor, with or without the diode, into the relay is inflexible and puts additional constraints on the engineer attempting to optimise the overall circuit performance. Engineers generally prefer to tailor the flyback suppression characteristics themselves rather than use a fixed solution provided by the relay manufacturer.

author: Zero999
author_rank: Super Contributor
timestamp: October 08, 2023, 09:15:51 pm
content: If it's not possible to access the driver transistor, then the zener diode can be connected in series with a diode, so it doesn't forward conduct.Another way to reduce the power dissipation of the resistor is to put a capacitor in series with it, so the resistor doesn't draw and power at DC. The capacitor must be large enough to ensure it doesn't charge up to a high voltage, due to the back EMF from the relay coil.

author: johansen
author_rank: Super Contributor
timestamp: October 08, 2023, 09:55:24 pm
content: A simple diode across the coil has the problem of maximising the time to open the relay.A resistor in series with the diode, equal to the coil resistance will let the voltage across the coil increase to the operating voltage of the circuit, doubling the nominal voltage across the switch.Obviously an rc snubber can work, just choose the values correctly. Whats cheaper, a diode or a resistor?A slow diode like a 1n5401 or 4001 is fine. Its not like you are going to energize the coil while current is still flowing through the diode...

author: Benta
author_rank: Super Contributor
timestamp: October 08, 2023, 10:21:28 pm
content: Automotive electrics have been developed through more than 100 years, and your chances of "making it better" are close to 0.The idea of a flyback diode on the relays is certain to shorten the contact lifetime significantly due to arcing/burning.Another senseless "mod", sorry.

author: Circlotron
author_rank: Super Contributor
timestamp: October 08, 2023, 10:39:05 pm
content: Putting a diode across the relay coil makes the relay release slowly and much more likely for the contacts to stick. especially if they are switching a decent amount of current, and even more so if that current is DC, and probably even more so again if the load has a high inrush current like filament lamps. Best to let the relay release quick by clamping the coil voltage to whatever max the thing that is switching the coil will tolerate.

author: Chriss
author_rank: Frequent Contributor
timestamp: October 09, 2023, 09:27:04 am
content: Not to go in to the deep too much, put a diode something lin1N4007 or similar in paralel to the coil of the relay and work done.The reason why the manufacturers most in case put a res than a diode is maybe cos the res is much cheaper and if a diode is put then you have to take care how you connect the + and - pole to the coil.If you put the + and - on the relay in a improper way you probably will bur ne diode.With the res that is not the cas.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 09, 2023, 11:13:15 am
content: Quote from: drakejest on October 08, 2023, 05:51:44 pmWhich is a demonstration of the destruction the diode is causing. Relays need to click. Diode, with its low on-state voltage loss, keeps the coil current flowing until the coil itself has used inductance-stored energy, making it pull longer. The delay is not the problem, it's the slow and gradual turn-off as current ramps down.Thus, using a diode to slow down clicking increases sparking and significantly reduces contact life. This is why I strongly recommend a bidirectional TVS as a simple, one-component solution. Choose TVS nominal voltage a tad above worst-case high DC voltage, so that it never accidentally conducts in forward direction. It will then clamp the reverse voltage to higher value than standard diode, and dissipates larger part of the inductance-stored energy, burning that energy faster than the coil alone does.Other options to achieve the same are:(1) resistor in parallel with coil, which needs to be dimensioned carefully and dissipates power all the time while relay is turned on(2) diode + resistor in series(3) diode + unidirectional TVS / schottky in series

author: Terry Bites
author_rank: Super Contributor
timestamp: October 10, 2023, 05:08:52 pm
content: Ahhh. The voice of reason. Thankyou.

author: uer166
author_rank: Super Contributor
timestamp: October 10, 2023, 06:27:28 pm
content: Quote from: Chriss on October 09, 2023, 09:27:04 amIf by "work done" you mean shorten the contact lifespan and cause a relay to weld prematurely, then yes that would do it. Relay manufacturers aren't stupid, and a lot of research goes into relay design and operation. The reason they use a resistor is not cost. There are good reasons to use TVS/Zener/Resistor solutions that maximize the voltage across coil during turn-off, vs a recirculating diode as was said before.The higher power you're switching the more it matters.